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| author | Samuel Thibault <samuel.thibault@ens-lyon.org> | 2015-02-18 00:58:35 +0100 |
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| committer | Samuel Thibault <samuel.thibault@ens-lyon.org> | 2015-02-18 00:58:35 +0100 |
| commit | 49a086299e047b18280457b654790ef4a2e5abfa (patch) | |
| tree | c2b29e0734d560ce4f58c6945390650b5cac8a1b /service_solahart_jakarta_selatan__082122541663/performance/io_system | |
| parent | e2b3602ea241cd0f6bc3db88bf055bee459028b6 (diff) | |
Revert "rename open_issues.mdwn to service_solahart_jakarta_selatan__082122541663.mdwn"
This reverts commit 95878586ec7611791f4001a4ee17abf943fae3c1.
Diffstat (limited to 'service_solahart_jakarta_selatan__082122541663/performance/io_system')
3 files changed, 0 insertions, 3280 deletions
diff --git a/service_solahart_jakarta_selatan__082122541663/performance/io_system/binutils_ld_64ksec.mdwn b/service_solahart_jakarta_selatan__082122541663/performance/io_system/binutils_ld_64ksec.mdwn deleted file mode 100644 index 931fd0ee..00000000 --- a/service_solahart_jakarta_selatan__082122541663/performance/io_system/binutils_ld_64ksec.mdwn +++ /dev/null @@ -1,39 +0,0 @@ -[[!meta copyright="Copyright © 2010, 2011 Free Software Foundation, Inc."]] - -[[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable -id="license" text="Permission is granted to copy, distribute and/or modify this -document under the terms of the GNU Free Documentation License, Version 1.2 or -any later version published by the Free Software Foundation; with no Invariant -Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license -is included in the section entitled [[GNU Free Documentation -License|/fdl]]."]]"""]] - -[[!tag open_issue_hurd]] - -This one may be considered as a testcase for [[I/O system -optimization|community/gsoc/project_ideas/disk_io_performance]]. - -It is taken from the [[binutils testsuite|binutils]], -`ld/ld-elf/sec64k.exp`, where this -test may occasionally [[trigger a timeout|binutils#64ksec]]. It is -extracted from cdf7c161ebd4a934c9e705d33f5247fd52975612 sources, 2010-10-24. - - $ wget -O - http://www.gnu.org/software/hurd/open_issues/performance/io_system/binutils_ld_64ksec/test.tar.xz | xz -d | tar -x - $ cd test/ - $ \time ./ld-new.stripped -o dump dump?.o dump??.o - 0.00user 0.00system 2:46.11elapsed 0%CPU (0avgtext+0avgdata 0maxresident)k - 0inputs+0outputs (0major+0minor)pagefaults 0swaps - -On the idle grubber, this one repeatedly takes a few minutes wall time to -complete successfully, contrary to a few seconds on a GNU/Linux system. - -While processing the object files, there is heavy interaction with the relevant -[[hurd/translator/ext2fs]] process. Running [[hurd/debugging/rpctrace]] on -the testee shows that (primarily) an ever-repeating series of `io_seek` and -`io_read` is being processed. Running the testee on GNU/Linux with strace -shows the equivalent thing (`_llseek`, `read`) -- but Linux' I/O system isn't -as slow as the Hurd's. - -As Samuel figured out later, this slowness may in fact be due to a Xen-specific -issue, see [[Xen_lseek]]. After the latter has been addressed, we can -re-evaluate this issue here. diff --git a/service_solahart_jakarta_selatan__082122541663/performance/io_system/clustered_page_faults.mdwn b/service_solahart_jakarta_selatan__082122541663/performance/io_system/clustered_page_faults.mdwn deleted file mode 100644 index 8bd6ba72..00000000 --- a/service_solahart_jakarta_selatan__082122541663/performance/io_system/clustered_page_faults.mdwn +++ /dev/null @@ -1,165 +0,0 @@ -[[!meta copyright="Copyright © 2011, 2014 Free Software Foundation, Inc."]] - -[[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable -id="license" text="Permission is granted to copy, distribute and/or modify this -document under the terms of the GNU Free Documentation License, Version 1.2 or -any later version published by the Free Software Foundation; with no Invariant -Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license -is included in the section entitled [[GNU Free Documentation -License|/fdl]]."]]"""]] - -[[!tag open_issue_gnumach open_issue_hurd]] - -[[community/gsoc/project_ideas/disk_io_performance]]. - -[[!toc]] - - -# IRC, freenode, #hurd, 2011-02-16 - - <braunr> exceptfor the kernel, everything in an address space is - represented with a VM object - <braunr> those objects can represent anonymous memory (from malloc() or - because of a copy-on-write) - <braunr> or files - <braunr> on classic Unix systems, these are files - <braunr> on the Hurd, these are memory objects, backed by external pagers - (like ext2fs) - <braunr> so when you read a file - <braunr> the kernel maps it from ext2fs in your address space - <braunr> and when you access the memory, a fault occurs - <braunr> the kernel determines it's a region backed by ext2fs - <braunr> so it asks ext2fs to provide the data - <braunr> when the fault is resolved, your process goes on - <etenil> does the faul occur because Mach doesn't know how to access the - memory? - <braunr> it occurs because Mach intentionnaly didn't back the region with - physical memory - <braunr> the MMU is programmed not to know what is present in the memory - region - <braunr> or because it's read only - <braunr> (which is the case for COW faults) - <etenil> so that means this bit of memory is a buffer that ext2fs loads the - file into and then it is remapped to the application that asked for it - <braunr> more or less, yes - <braunr> ideally, it's directly written into the right pages - <braunr> there is no intermediate buffer - <etenil> I see - <etenil> and as you told me before, currently the page faults are handled - one at a time - <etenil> which wastes a lot of time - <braunr> a certain amount of time - <etenil> enough to bother the user :) - <etenil> I've seen pages have a fixed size - <braunr> yes - <braunr> use the PAGE_SIZE macro - <etenil> and when allocating memory, the size that's asked for is rounded - up to the page size - <etenil> so if I have this correctly, it means that a file ext2fs provides - could be split into a lot of pages - <braunr> yes - <braunr> once in memory, it is managed by the page cache - <braunr> so that pages more actively used are kept longer than others - <braunr> in order to minimize I/O - <etenil> ok - <braunr> so a better page cache code would also improve overall performance - <braunr> and more RAM would help a lot, since we are strongly limited by - the 768 MiB limit - <braunr> which reduces the page cache size a lot - <etenil> but the problem is that reading a whole file in means trigerring - many page faults just for one file - <braunr> if you want to stick to the page clustering thing, yes - <braunr> you want less page faults, so that there are less IPC between the - kernel and the pager - <etenil> so either I make pages bigger - <etenil> or I modify Mach so it can check up on a range of pages for faults - before actually processing - <braunr> you *don't* change the page size - <etenil> ah - <etenil> that's hardware isn't it? - <braunr> in Mach, yes - <etenil> ok - <braunr> and usually, you want the page size to be the CPU page size - <etenil> I see - <braunr> current CPU can support multiple page sizes, but it becomes quite - hard to correctly handle - <braunr> and bigger page sizes mean more fragmentation, so it only suits - machines with large amounts of RAM, which isn't the case for us - <etenil> ok - <etenil> so I'll try the second approach then - <braunr> that's what i'd recommand - <braunr> recommend* - <etenil> ok - - -# IRC, freenode, #hurd, 2011-02-16 - - <antrik> etenil: OSF Mach does have clustered paging BTW; so that's one - place to start looking... - <antrik> (KAM ported the OSF code to gnumach IIRC) - <antrik> there is also an existing patch for clustered paging in libpager, - which needs some adaptation - <antrik> the biggest part of the task is probably modifying the Hurd - servers to use the new interface - <antrik> but as I said, KAM's code should be available through google, and - can serve as a starting point - -<http://lists.gnu.org/archive/html/bug-hurd/2010-06/msg00023.html> - - -# IRC, freenode, #hurd, 2011-07-22 - - <braunr> but concerning clustered pagins/outs, i'm not sure it's a mach - interface limitation - <braunr> the external memory pager interface does allow multiple pages to - be transfered - <braunr> isn't it an internal Mach VM problem ? - <braunr> isn't it simply the page fault handler ? - <antrik> braunr: are you sure? I was under the impression that changing the - pager interface was among the requirements... - <antrik> hm... I wonder whether for pageins, it could actually be handled - in the pages instead of Mach... though this wouldn't work for pageouts, - so probably not very helpful - <antrik> err... in the pagers - <braunr> antrik: i'm almost sure - <braunr> but i've be proven wrong many times, so .. - <braunr> there are two main facts that lead me to think this - <braunr> 1/ - http://www.gnu.org/software/hurd/gnumach-doc/Memory-Objects-and-Data.html#Memory-Objects-and-Data - says lengths are provided and doesn't mention the limitation - <braunr> 2/ when reading about UVM, one of the major improvements (between - 10 and 30% of global performance depending on the benchmarks) was - implementing the madvise semantics - <braunr> and this didn't involve a new pager interface, but rather a new - page fault handler - <antrik> braunr: hm... the interface indeed looks like it can handle - multiple pages in both directions... perhaps it was at the Hurd level - where the pager interface needs to be modified, not the Mach one?... - <braunr> antrik: would be nice wouldn't it ? :) - <braunr> antrik: more probably the page fault handler - - -# IRC, freenode, #hurd, 2011-09-28 - - <slpz> antrik: I've just recovered part of my old multipage I/O work - <slpz> antrik: I intend to clean and submit it after finishing the changes - to the pageout system. - <antrik> slpz: oh, great! - <antrik> didn't know you worked on multipage I/O - <antrik> slpz: BTW, have you checked whether any of the work done for GSoC - last year is any good?... - <antrik> (apart from missing copyright assignments, which would be a - serious problem for the Hurd parts...) - <slpz> antrik: It was seven years ago, but I did: - http://www.mail-archive.com/bug-hurd@gnu.org/msg10285.html :-) - <slpz> antrik: Sincerely, I don't think the quality of that code is good - enough to be considered... but I think it was my fault as his mentor for - not correcting him soon enough... - <antrik> slpz: I see - <antrik> TBH, I feel guilty myself, for not asking about the situation - immediately when he stopped attending meetings... - <antrik> slpz: oh, you even already looked into vm_pageout_scan() back then - :-) - - -# [[Read-Ahead]] diff --git a/service_solahart_jakarta_selatan__082122541663/performance/io_system/read-ahead.mdwn b/service_solahart_jakarta_selatan__082122541663/performance/io_system/read-ahead.mdwn deleted file mode 100644 index 59f22187..00000000 --- a/service_solahart_jakarta_selatan__082122541663/performance/io_system/read-ahead.mdwn +++ /dev/null @@ -1,3076 +0,0 @@ -[[!meta copyright="Copyright © 2011, 2012, 2013, 2014 Free Software Foundation, -Inc."]] - -[[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable -id="license" text="Permission is granted to copy, distribute and/or modify this -document under the terms of the GNU Free Documentation License, Version 1.2 or -any later version published by the Free Software Foundation; with no Invariant -Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license -is included in the section entitled [[GNU Free Documentation -License|/fdl]]."]]"""]] - -[[!tag open_issue_gnumach open_issue_hurd]] - -[[!toc]] - - -# [[community/gsoc/project_ideas/disk_io_performance]] - - -# [[gnumach_page_cache_policy]] - - -# 2011-02 - -[[Etenil]] has been working in this area. - - -## IRC, freenode, #hurd, 2011-02-13 - - <etenil> youpi: Would libdiskfs/diskfs.h be in the right place to make - readahead functions? - <youpi> etenil: no, it'd rather be at the memory management layer, - i.e. mach, unfortunately - <youpi> because that's where you see the page faults - <etenil> youpi: Linux also provides a readahead() function for higher level - applications. I'll probably have to add the same thing in a place that's - higher level than mach - <youpi> well, that should just be hooked to the same common implementation - <etenil> the man page for readahead() also states that portable - applications should avoid it, but it could be benefic to have it for - portability - <youpi> it's not in posix indeed - - -## IRC, freenode, #hurd, 2011-02-14 - - <etenil> youpi: I've investigated prefetching (readahead) techniques. One - called DiskSeen seems really efficient. I can't tell yet if it's patented - etc. but I'll keep you informed - <youpi> don't bother with complicated techniques, even the most simple ones - will be plenty :) - <etenil> it's not complicated really - <youpi> the matter is more about how to plug it into mach - <etenil> ok - <youpi> then don't bother with potential pattents - <antrik> etenil: please take a look at the work KAM did for last year's - GSoC - <youpi> just use a trivial technique :) - <etenil> ok, i'll just go the easy way then - - <braunr> antrik: what was etenil referring to when talking about - prefetching ? - <braunr> oh, madvise() stuff - <braunr> i could help him with that - - -## IRC, freenode, #hurd, 2011-02-15 - - <etenil> oh, I'm looking into prefetching/readahead to improve I/O - performance - <braunr> etenil: ok - <braunr> etenil: that's actually a VM improvement, like samuel told you - <etenil> yes - <braunr> a true I/O improvement would be I/O scheduling - <braunr> and how to implement it in a hurdish way - <braunr> (or if it makes sense to have it in the kernel) - <etenil> that's what I've been wondering too lately - <braunr> concerning the VM, you should look at madvise() - <etenil> my understanding is that Mach considers devices without really - knowing what they are - <braunr> that's roughly the interface used both at the syscall() and the - kernel levels in BSD, which made it in many other unix systems - <etenil> whereas I/O optimisations are often hard disk drives specific - <braunr> that's true for almost any kernel - <braunr> the device knowledge is at the driver level - <etenil> yes - <braunr> (here, I separate kernels from their drivers ofc) - <etenil> but Mach also contains some drivers, so I'm going through the code - to find the apropriate place for these improvements - <braunr> you shouldn't tough the drivers at all - <braunr> touch - <etenil> true, but I need to understand how it works before fiddling around - <braunr> hm - <braunr> not at all - <braunr> the VM improvement is about pagein clustering - <braunr> you don't need to know how pages are fetched - <braunr> well, not at the device level - <braunr> you need to know about the protocol between the kernel and - external pagers - <etenil> ok - <braunr> you could also implement pageout clustering - <etenil> if I understand you well, you say that what I'd need to do is a - queuing system for the paging in the VM? - <braunr> no - <braunr> i'm saying that, when a page fault occurs, the kernel should - (depending on what was configured through madvise()) transfer pages in - multiple blocks rather than one at a time - <braunr> communication with external pagers is already async, made through - regular ports - <braunr> which already implement message queuing - <braunr> you would just need to make the mapped regions larger - <braunr> and maybe change the interface so that this size is passed - <etenil> mmh - <braunr> (also don't forget that page clustering can include pages *before* - the page which caused the fault, so you may have to pass the start of - that region too) - <etenil> I'm not sure I understand the page fault thing - <etenil> is it like a segmentation error? - <etenil> I can't find a clear definition in Mach's manual - <braunr> ah - <braunr> it's a fundamental operating system concept - <braunr> http://en.wikipedia.org/wiki/Page_fault - <etenil> ah ok - <etenil> I understand now - <etenil> so what's currently happening is that when a page fault occurs, - Mach is transfering pages one at a time and wastes time - <braunr> sometimes, transferring just one page is what you want - <braunr> it depends on the application, which is why there is madvise() - <braunr> our rootfs, on the other hand, would benefit much from such an - improvement - <braunr> in UVM, this optimization is account for around 10% global - performance improvement - <braunr> accounted* - <etenil> not bad - <braunr> well, with an improved page cache, I'm sure I/O would matter less - on systems with more RAM - <braunr> (and another improvement would make mach support more RAM in the - first place !) - <braunr> an I/O scheduler outside the kernel would be a very good project - IMO - <braunr> in e.g. libstore/storeio - <etenil> yes - <braunr> but as i stated in my thesis, a resource scheduler should be as - close to its resource as it can - <braunr> and since mach can host several operating systems, I/O schedulers - should reside near device drivers - <braunr> and since current drivers are in the kernel, it makes sens to have - it in the kernel too - <braunr> so there must be some discussion about this - <etenil> doesn't this mean that we'll have to get some optimizations in - Mach and have the same outside of Mach for translators that access the - hardware directly? - <braunr> etenil: why ? - <etenil> well as you said Mach contains some drivers, but in principle, it - shouldn't, translators should do disk access etc, yes? - <braunr> etenil: ok - <braunr> etenil: so ? - <etenil> well, let's say if one were to introduce SATA support in Hurd, - nothing would stop him/her to do so with a translator rather than in Mach - <braunr> you should avoid the term translator here - <braunr> it's really hurd specific - <braunr> let's just say a user space task would be responsible for that - job, maybe multiple instances of it, yes - <etenil> ok, so in this case, let's say we have some I/O optimization - techniques like readahead and I/O scheduling within Mach, would these - also apply to the user-space task, or would they need to be - reimplemented? - <braunr> if you have user space drivers, there is no point having I/O - scheduling in the kernel - <etenil> but we also have drivers within the kernel - <braunr> what you call readahead, and I call pagein/out clustering, is - really tied to the VM, so it must be in Mach in any case - <braunr> well - <braunr> you either have one or the other - <braunr> currently we have them in the kernel - <braunr> if we switch to DDE, we should have all of them outside - <braunr> that's why such things must be discussed - <etenil> ok so if I follow you, then future I/O device drivers will need to - be implemented for Mach - <braunr> currently, yes - <braunr> but preferrably, someone should continue the work that has been - done on DDe so that drivers are outside the kernel - <etenil> so for the time being, I will try and improve I/O in Mach, and if - drivers ever get out, then some of the I/O optimizations will need to be - moved out of Mach - <braunr> let me remind you one of the things i said - <braunr> i said I/O scheduling should be close to their resource, because - we can host several operating systems - <braunr> now, the Hurd is the only system running on top of Mach - <braunr> so we could just have I/O scheduling outside too - <braunr> then you should consider neighbor hurds - <braunr> which can use different partitions, but on the same device - <braunr> currently, partitions are managed in the kernel, so file systems - (and storeio) can't make good scheduling decisions if it remains that way - <braunr> but that can change too - <braunr> a single storeio representing a whole disk could be shared by - several hurd instances, just as if it were a high level driver - <braunr> then you could implement I/O scheduling in storeio, which would be - an improvement for the current implementation, and reusable for future - work - <etenil> yes, that was my first instinct - <braunr> and you would be mostly free of the kernel internals that make it - a nightmare - <etenil> but youpi said that it would be better to modify Mach instead - <braunr> he mentioned the page clustering thing - <braunr> not I/O scheduling - <braunr> theseare really two different things - <etenil> ok - <braunr> you *can't* implement page clustering outside Mach because Mach - implements virtual memory - <braunr> both policies and mechanisms - <etenil> well, I'd rather think of one thing at a time if that's alright - <etenil> so what I'm busy with right now is setting up clustered page-in - <etenil> which need to be done within Mach - <braunr> keep clustered page-outs in mind too - <braunr> although there are more constraints on those - <etenil> yes - <etenil> I've looked up madvise(). There's a lot of documentation about it - in Linux but I couldn't find references to it in Mach (nor Hurd), does it - exist? - <braunr> well, if it did, you wouldn't be caring about clustered page - transfers, would you ? - <braunr> be careful about linux specific stuff - <etenil> I suppose not - <braunr> you should implement at least posix options, and if there are - more, consider the bsd variants - <braunr> (the Mach VM is the ancestor of all modern BSD VMs) - <etenil> madvise() seems to be posix - <braunr> there are system specific extensions - <braunr> be careful - <braunr> CONFORMING TO POSIX.1b. POSIX.1-2001 describes posix_madvise(3) - with constants POSIX_MADV_NORMAL, etc., with a behav‐ ior close to that - described here. There is a similar posix_fadvise(2) for file access. - <braunr> MADV_REMOVE, MADV_DONTFORK, MADV_DOFORK, MADV_HWPOISON, - MADV_MERGEABLE, and MADV_UNMERGEABLE are Linux- specific. - <etenil> I was about to post these - <etenil> ok, so basically madvise() allows tasks etc. to specify a usage - type for a chunk of memory, then I could apply the relevant I/O - optimization based on this - <braunr> that's it - <etenil> cool, then I don't need to worry about knowing what the I/O is - operating on, I just need to apply the optimizations as advised - <etenil> that's convenient - <etenil> ok I'll start working on this tonight - <etenil> making a basic readahead shouldn't be too hard - <braunr> readahead is a misleading name - <etenil> is pagein better? - <braunr> applies to too many things, doesn't include the case where - previous elements could be prefetched - <braunr> clustered page transfers is what i would use - <braunr> page prefetching maybe - <etenil> ok - <braunr> you should stick to something that's already used in the - literature since you're not inventing something new - <etenil> yes I've read a paper about prefetching - <etenil> ok - <etenil> thanks for your help braunr - <braunr> sure - <braunr> you're welcome - <antrik> braunr: madvise() is really the least important part of the - picture... - <antrik> very few applications actually use it. but pretty much all - applications will profit from clustered paging - <antrik> I would consider madvise() an optional goody, not an integral part - of the implementation - <antrik> etenil: you can find some stuff about KAM's work on - http://www.gnu.org/software/hurd/user/kam.html - <antrik> not much specific though - <etenil> thanks - <antrik> I don't remember exactly, but I guess there is also some - information on the mailing list. check the archives for last summer - <antrik> look for Karim Allah Ahmed - <etenil> antrik: I disagree, madvise gives me a good starting point, even - if eventually the optimisations should run even without it - <antrik> the code he wrote should be available from Google's summer of code - page somewhere... - <braunr> antrik: right, i was mentioning madvise() because the kernel (VM) - interface is pretty similar to the syscall - <braunr> but even a default policy would be nice - <antrik> etenil: I fear that many bits were discussed only on IRC... so - you'd better look through the IRC logs from last April onwards... - <etenil> ok - - <etenil> at the beginning I thought I could put that into libstore - <etenil> which would have been fine - - <antrik> BTW, I remembered now that KAM's GSoC application should have a - pretty good description of the necessary changes... unfortunately, these - are not publicly visible IIRC :-( - - -## IRC, freenode, #hurd, 2011-02-16 - - <etenil> braunr: I've looked in the kernel to see where prefetching would - fit best. We talked of the VM yesterday, but I'm not sure about it. It - seems to me that the device part of the kernel makes more sense since - it's logically what manages devices, am I wrong? - <braunr> etenil: you are - <braunr> etenil: well - <braunr> etenil: drivers should already support clustered sector - read/writes - <etenil> ah - <braunr> but yes, there must be support in the drivers too - <braunr> what would really benefit the Hurd mostly concerns page faults, so - the right place is the VM subsystem - -[[clustered_page_faults]] - - -# 2012-03 - - -## IRC, freenode, #hurd, 2012-03-21 - - <mcsim> I thought that readahead should have some heuristics, like - accounting size of object and last access time, but i didn't find any in - kam's patch. Are heuristics needed or it will be overhead for - microkernel? - <youpi> size of object and last access time are not necessarily useful to - take into account - <youpi> what would usually typically be kept is the amount of contiguous - data that has been read lately - <youpi> to know whether it's random or sequential, and how much is read - <youpi> (the whole size of the object does not necessarily give any - indication of how much of it will be read) - <mcsim> if big object is accessed often, performance could be increased if - frame that will be read ahead will be increased too. - <youpi> yes, but the size of the object really does not matter - <youpi> you can just observe how much data is read and realize that it's - read a lot - <youpi> all the more so with userland fs translators - <youpi> it's not because you mount a CD image that you need to read it all - <mcsim> youpi: indeed. this will be better. But on other hand there is - principle about policy and mechanism. And kernel should implement - mechanism, but heuristics seems to be policy. Or in this case moving - readahead policy to user level would be overhead? - <antrik> mcsim: paging policy is all in kernel anyways; so it makes perfect - sense to put the readahead policy there as well - <antrik> (of course it can be argued -- probably rightly -- that all of - this should go into userspace instead...) - <mcsim> antrik: probably defpager partly could do that. AFAIR, it is - possible for defpager to return more memory than was asked. - <mcsim> antrik: I want to outline what should be done during gsoc. First, - kernel should support simple readahead for specified number of pages - (regarding direction of access) + simple heuristic for changing frame - size. Also default pager could make some analysis, for instance if it has - many data located consequentially it could return more data then was - asked. For other pagers I won't do anything. Is it suitable? - <antrik> mcsim: I think we actually had the same discussion already with - KAM ;-) - <antrik> for clustered pageout, the kernel *has* to make the decision. I'm - really not convinced it makes sense to leave the decision for clustered - pagein to the individual pagers - <antrik> especially as this will actually complicate matters because a) it - will require work in *every* pager, and b) it will probably make handling - of MADVISE & friends more complex - <antrik> implementing readahead only for the default pager would actually - be rather unrewarding. I'm pretty sure it's the one giving the *least* - benefit - <antrik> it's much, much more important for ext2 - <youpi> mcsim: maybe try to dig in the irc logs, we discussed about it with - neal. the current natural place would be the kernel, because it's the - piece that gets the traps and thus knows what happens with each - projection, while the backend just provides the pages without knowing - which projection wants it. Moving to userland would not only be overhead, - but quite difficult - <mcsim> antrik: OK, but I'm not sure that I could do it for ext2. - <mcsim> OK, I'll dig. - - -## IRC, freenode, #hurd, 2012-04-01 - - <mcsim> as part of implementing of readahead project I have to add - interface for setting appropriate behaviour for memory range. This - interface than should be compatible with madvise call, that has a lot of - possible advises, but most part of them are specific for Linux (according - to man page). Should mach also support these Linux-specific values? - <mcsim> p.s. these Linux-specific values shouldn't affect readahead - algorithm. - <youpi> the interface shouldn't prevent from adding them some day - <youpi> so that we don't have to add them yet - <mcsim> ok. And what behaviour with value MADV_NORMAL should be look like? - Seems that it should be synonym to MADV_SEQUENTIAL, isn't it? - <youpi> no, it just means "no idea what it is" - <youpi> in the linux implementation, that means some given readahead value - <youpi> while SEQUENTIAL means twice as much - <youpi> and RANDOM means zero - <mcsim> youpi: thank you. - <mcsim> youpi: Than, it seems to be better that kernel interface for - setting behaviour will accept readahead value, without hiding it behind - such constants, like VM_BEHAVIOR_DEFAULT (like it was in kam's - patch). And than implementation of madvise will call vm_behaviour_set - with appropriate frame size. Is that right? - <youpi> question of taste, better ask on the list - <mcsim> ok - - -## IRC, freenode, #hurd, 2012-06-09 - - <mcsim> hello. What fictitious pages in gnumach are needed for? - <mcsim> I mean why real page couldn't be grabbed straight, but in sometimes - fictitious page is grabbed first and than converted to real? - <braunr> mcsim: iirc, fictitious pages are needed by device pagers which - must comply with the vm pager interface - <braunr> mcsim: specifically, they must return a vm_page structure, but - this vm_page describes device memory - <braunr> mcsim: and then, it must not be treated like normal vm_page, which - can be added to page queues (e.g. page cache) - - -## IRC, freenode, #hurd, 2012-06-22 - - <mcsim> braunr: Ah. Patch for large storages introduced new callback - pager_notify_evict. User had to define this callback on his own as - pager_dropweak, for instance. But neal's patch change this. Now all - callbacks could have any name, but user defines structure with pager ops - and supplies it in pager_create. - <mcsim> So, I just changed notify_evict to confirm it to new style. - <mcsim> braunr: I want to changed interface of mo_change_attributes and - test my changes with real partitions. For both these I have to update - ext2fs translator, but both partitions I have are bigger than 2Gb, that's - why I need apply this patch.z - <mcsim> But what to do with mo_change_attributes? I need somehow inform - kernel about page fault policy. - <mcsim> When I change mo_ interface in kernel I have to update all programs - that use this interface and ext2fs is one of them. - - <mcsim> braunr: Who do you think better to inform kernel about fault - policy? At the moment I've added fault_strategy parameter that accepts - following strategies: randow, sequential with single page cluster, - sequential with double page cluster and sequential with quad page - cluster. OSF/mach has completely another interface of - mo_change_attributes. In OSF/mach mo_change_attributes accepts structure - of parameter. This structure could have different formats depending o - <mcsim> This rpc could be useful because it is not very handy to update - mo_change_attributes for kernel, for hurd libs and for glibc. Instead of - this kernel will accept just one more structure format. - <braunr> well, like i wrote on the mailing list several weeks ago, i don't - think the policy selection is of concern currently - <braunr> you should focus on the implementation of page clustering and - readahead - <braunr> concerning the interface, i don't think it's very important - <braunr> also, i really don't like the fact that the policy is per object - <braunr> it should be per map entry - <braunr> i think it mentioned that in my mail too - <braunr> i really think you're wasting time on this - <braunr> http://lists.gnu.org/archive/html/bug-hurd/2012-04/msg00064.html - <braunr> http://lists.gnu.org/archive/html/bug-hurd/2012-04/msg00029.html - <braunr> mcsim: any reason you completely ignored those ? - <mcsim> braunr: Ok. I'll do clustering for map entries. - <braunr> no it's not about that either :/ - <braunr> clustering is grouping several pages in the same transfer between - kernel and pager - <braunr> the *policy* is held in map entries - <antrik> mcsim: I'm not sure I properly understand your question about the - policy interface... but if I do, it's IMHO usually better to expose - individual parameters as RPC arguments explicitly, rather than hiding - them in an opaque structure... - <antrik> (there was quite some discussion about that with libburn guy) - <mcsim> antrik: Following will be ok? kern_return_t vm_advice(map, address, - length, advice, cluster_size) - <mcsim> Where advice will be either random or sequential - <antrik> looks fine to me... but then, I'm not an expert on this stuff :-) - <antrik> perhaps "policy" would be clearer than "advice"? - <mcsim> madvise has following prototype: int madvise(void *addr, size_t - len, int advice); - <mcsim> hmm... looks like I made a typo. Or advi_c_e is ok too? - <antrik> advise is a verb; advice a noun... there is a reason why both - forms show up in the madvise prototype :-) - <mcsim> so final variant should be kern_return_t vm_advise(map, address, - length, policy, cluster_size)? - <antrik> mcsim: nah, you are probably right that its better to keep - consistency with madvise, even if the name of the "advice" parameter - there might not be ideal... - <antrik> BTW, where does cluster_size come from? from the filesystem? - <antrik> I see merits both to naming the parameter "policy" (clearer) or - "advice" (more consistent) -- you decide :-) - <mcsim> antrik: also there is variant strategy, like with inheritance :) - I'll choose advice for now. - <mcsim> What do you mean under "where does cluster_size come from"? - <antrik> well, madvise doesn't have this parameter; so the value must come - from a different source? - <mcsim> in madvise implementation it could fixed value or somehow - calculated basing on size of memory range. In OSF/mach cluster size is - supplied too (via mo_change_attributes). - <antrik> ah, so you don't really know either :-) - <antrik> well, my guess is that it is derived from the cluster size used by - the filesystem in question - <antrik> so for us it would always be 4k for now - <antrik> (and thus you can probably leave it out alltogether...) - <antrik> well, fatfs can use larger clusters - <antrik> I would say, implement it only if it's very easy to do... if it's - extra effort, it's probably not worth it - <mcsim> There is sense to make cluster size bigger for ext2 too, since most - likely consecutive clusters will be within same group. - <mcsim> But anyway I'll handle this later. - <antrik> well, I don't know what cluster_size does exactly; but by the - sound of it, I'd guess it makes an assumption that it's *always* better - to read in this cluster size, even for random access -- which would be - simply wrong for 4k filesystem clusters... - <antrik> BTW, I agree with braunr that madvice() is optional -- it is way - way more important to get readahead working as a default policy first - - -## IRC, freenode, #hurd, 2012-07-01 - - <mcsim> youpi: Do you think you could review my code? - <youpi> sure, just post it to the list - <youpi> make sure to break it down into logical pieces - <mcsim> youpi: I pushed it my branch at gnumach repository - <mcsim> youpi: or it is still better to post changes to list? - <youpi> posting to the list would permit feedback from other people too - <youpi> mcsim: posix distinguishes normal, sequential and random - <youpi> we should probably too - <youpi> the system call should probably be named "vm_advise", to be a verb - like allocate etc. - <mcsim> youpi: ok. A have a talk with antrik regarding naming, I'll change - this later because compiling of glibc take a lot of time. - <youpi> mcsim: I find it odd that vm_for_every_page allocates non-existing - pages - <youpi> there should probably be at least a flag to request it or not - <mcsim> youpi: normal policy is synonym to default. And this could be - treated as either random or sequential, isn't it? - <braunr> mcsim: normally, no - <youpi> yes, the normal policy would be the default - <youpi> it doesn't mean random or sequential - <youpi> it's just to be a compromise between both - <youpi> random is meant to make no read-ahead, since that'd be spurious - anyway - <youpi> while by default we should make readahead - <braunr> and sequential makes even more aggressive readahead, which usually - implies a greater number of pages to fetch - <braunr> that's all - <youpi> yes - <youpi> well, that part is handled by the cluster_size parameter actually - <braunr> what about reading pages preceding the faulted paged ? - <mcsim> Shouldn't sequential clean some pages (if they, for example, are - not precious) that are placed before fault page? - <braunr> ? - <youpi> that could make sense, yes - <braunr> you lost me - <youpi> and something that you wouldn't to with the normal policy - <youpi> braunr: clear what has been read previously - <braunr> ? - <youpi> since the access is supposed to be sequential - <braunr> oh - <youpi> the application will proabably not re-read what was already read - <braunr> you mean to avoid caching it ? - <youpi> yes - <braunr> inactive memory is there for that - <youpi> while with the normal policy you'd assume that the application - might want to go back etc. - <youpi> yes, but you can help it - <braunr> yes - <youpi> instead of making other pages compete with it - <braunr> but then, it's for precious pages - <youpi> I have to say I don't know what a precious page it - <youpi> s - <youpi> does it mean dirty pages? - <braunr> no - <braunr> precious means cached pages - <braunr> "If precious is FALSE, the kernel treats the data as a temporary - and may throw it away if it hasn't been changed. If the precious value is - TRUE, the kernel treats its copy as a data repository and promises to - return it to the manager; the manager may tell the kernel to throw it - away instead by flushing and not cleaning the data" - <braunr> hm no - <braunr> precious means the kernel must keep it - <mcsim> youpi: According to vm_for_every_page. What kind of flag do you - suppose? If object is internal, I suppose not to cross the bound of - object, setting in_end appropriately in vm_calculate_clusters. - <mcsim> If object is external we don't know its actual size, so we should - make mo request first. And for this we should create fictitious pages. - <braunr> mcsim: but how would you implement this "cleaning" with sequential - ? - <youpi> mcsim: ah, ok, I thought you were allocating memory, but it's just - fictitious pages - <youpi> comment "Allocate a new page" should be fixed :) - <mcsim> braunr: I don't now how I will implement this specifically (haven't - tried yet), but I don't think that this is impossible - <youpi> braunr: anyway it's useful as an example where normal and - sequential would be different - <braunr> if it can be done simply - <braunr> because i can see more trouble than gains in there :) - <mcsim> braunr: ok :) - <braunr> mcsim: hm also, why fictitious pages ? - <braunr> fictitious pages should normally be used only when dealing with - memory mapped physically which is not real physical memory, e.g. device - memory - <mcsim> but vm_fault could occur when object represent some device memory. - <braunr> that's exactly why there are fictitious pages - <mcsim> at the moment of allocating of fictitious page it is not know what - backing store of object is. - <braunr> really ? - <braunr> damn, i've got used to UVM too much :/ - <mcsim> braunr: I said something wrong? - <braunr> no no - <braunr> it's just that sometimes, i'm confusing details about the various - BSD implementations i've studied - <braunr> out-of-gsoc-topic question: besides network drivers, do you think - we'll have other drivers that will run in userspace and have to implement - memory mapping ? like framebuffers ? - <braunr> or will there be a translation layer such as storeio that will - handle mapping ? - <youpi> framebuffers typically will, yes - <youpi> that'd be antrik's work on drm - <braunr> hmm - <braunr> ok - <youpi> mcsim: so does the implementation work, and do you see performance - improvement? - <mcsim> youpi: I haven't tested it yet with large ext2 :/ - <mcsim> youpi: I'm going to finish now moving of ext2 to new interface, - than other translators in hurd repository and than finish memory policies - in gnumach. Is it ok? - <youpi> which new interface? - <mcsim> Written by neal. I wrote some temporary code to make ext2 work with - it, but I'm going to change this now. - <youpi> you mean the old unapplied patch? - <mcsim> yes - <youpi> did you have a look at Karim's work? - <youpi> (I have to say I never found the time to check how it related with - neal's patch) - <mcsim> I found only his work in kernel. I didn't see his work in applying - of neal's patch. - <youpi> ok - <youpi> how do they relate with each other? - <youpi> (I have never actually looked at either of them :/) - <mcsim> his work in kernel and neal's patch? - <youpi> yes - <mcsim> They do not correlate with each other. - <youpi> ah, I must be misremembering what each of them do - <mcsim> in kam's patch was changes to support sequential reading in reverse - order (as in OSF/Mach), but posix does not support such behavior, so I - didn't implement this either. - <youpi> I can't find the pointer to neal's patch, do you have it off-hand? - <mcsim> http://comments.gmane.org/gmane.os.hurd.bugs/351 - <youpi> thx - <youpi> I think we are not talking about the same patch from Karim - <youpi> I mean lists.gnu.org/archive/html/bug-hurd/2010-06/msg00023.html - <mcsim> I mean this patch: - http://lists.gnu.org/archive/html/bug-hurd/2010-06/msg00024.html - <mcsim> Oh. - <youpi> ok - <mcsim> seems, this is just the same - <youpi> yes - <youpi> from a non-expert view, I would have thought these patches play - hand in hand, do they really? - <mcsim> this patch is completely for kernel and neal's one is completely - for libpager. - <youpi> i.e. neal's fixes libpager, and karim's fixes the kernel - <mcsim> yes - <youpi> ending up with fixing the whole path? - <youpi> AIUI, karim's patch will be needed so that your increased readahead - will end up with clustered page request? - <mcsim> I will not use kam's patch - <youpi> is it not needed to actually get pages in together? - <youpi> how do you tell libpager to fetch pages together? - <youpi> about the cluster size, I'd say it shouldn't be specified at - vm_advise() level - <youpi> in other OSes, it is usually automatically tuned - <youpi> by ramping it up to a maximum readahead size (which, however, could - be specified) - <youpi> that's important for the normal policy, where there are typically - successive periods of sequential reads, but you don't know in advance for - how long - <mcsim> braunr said that there are legal issues with his code, so I cannot - use it. - <braunr> did i ? - <braunr> mcsim: can you give me a link to the code again please ? - <youpi> see above :) - <braunr> which one ? - <youpi> both - <youpi> they only differ by a typo - <braunr> mcsim: i don't remember saying that, do you have any link ? - <braunr> or log ? - <mcsim> sorry, can you rephrase "ending up with fixing the whole path"? - <mcsim> cluster_size in vm_advise also could be considered as advise - <braunr> no - <braunr> it must be the third time we're talking about this - <youpi> mcsim: I mean both parts would be needed to actually achieve - clustered i/o - <braunr> again, why make cluster_size a per object attribute ? :( - <youpi> wouldn't some objects benefit from bigger cluster sizes, while - others wouldn't? - <youpi> but again, I believe it should rather be autotuned - <youpi> (for each object) - <braunr> if we merely want posix compatibility (and for a first attempt, - it's quite enough), vm_advise is good, and the kernel selects the - implementation (and thus the cluster sizes) - <braunr> if we want finer grained control, perhaps a per pager cluster_size - would be good, although its efficiency depends on several parameters - <braunr> (e.g. where the page is in this cluster) - <braunr> but a per object cluster size is a large waste of memory - considering very few applications (if not none) would use the "feature" - .. - <braunr> (if any*) - <youpi> there must be a misunderstanding - <youpi> why would it be a waste of memory? - <braunr> "per object" - <youpi> so? - <braunr> there can be many memory objects in the kernel - <youpi> so? - <braunr> so such an overhead must be useful to accept it - <youpi> in my understanding, a cluster size per object is just a mere - integer for each object - <youpi> what overhead? - <braunr> yes - <youpi> don't we have just thousands of objects? - <braunr> for now - <braunr> remember we're trying to remove the page cache limit :) - <youpi> that still won't be more than tens of thousands of objects - <youpi> times an integer - <youpi> that's completely neglectible - <mcsim> braunr: Strange, Can't find in logs. Weird things are happening in - my memory :/ Sorry. - <braunr> mcsim: i'm almost sure i never said that :/ - <braunr> but i don't trust my memory too much either - <braunr> youpi: depends - <youpi> mcsim: I mean both parts would be needed to actually achieve - clustered i/o - <mcsim> braunr: I made I call vm_advise that applies policy to memory range - (vm_map_entry to be specific) - <braunr> mcsim: good - <youpi> actually the cluster size should even be per memory range - <mcsim> youpi: In this sense, yes - <youpi> k - <mcsim> sorry, Internet connection lags - <braunr> when changing a structure used to create many objects, keep in - mind one thing - <braunr> if its size gets larger than a threshold (currently, powers of - two), the cache used by the slab allocator will allocate twice the - necessary amount - <youpi> sure - <braunr> this is the case with most object caching allocators, although - some can have specific caches for common sizes such as 96k which aren't - powers of two - <braunr> anyway, an integer is negligible, but the final structure size - must be checked - <braunr> (for both 32 and 64 bits) - <mcsim> braunr: ok. - <mcsim> But I didn't understand what should be done with cluster size in - vm_advise? Should I delete it? - <braunr> to me, the cluster size is a pager property - <youpi> to me, the cluster size is a map property - <braunr> whereas vm_advise indicates what applications want - <youpi> you could have several process accessing the same file in different - ways - <braunr> youpi: that's why there is a policy - <youpi> isn't cluster_size part of the policy? - <braunr> but if the pager abilities are limited, it won't change much - <braunr> i'm not sure - <youpi> cluster_size is the amount of readahead, isn't it? - <braunr> no, it's the amount of data in a single transfer - <mcsim> Yes, it is. - <braunr> ok, i'll have to check your code - <youpi> shouldn't transfers permit unbound amounts of data? - <mcsim> braunr: than I misunderstand what readahead is - <braunr> well then cluster size is per policy :) - <braunr> e.g. random => 0, normal => 3, sequential => 15 - <braunr> why make it per map entry ? - <youpi> because it depends on what the application doezs - <braunr> let me check the code - <youpi> if it's accessing randomly, no need for big transfers - <youpi> just page transfers will be fine - <youpi> if accessing sequentially, rather use whole MiB of transfers - <youpi> and these behavior can be for the same file - <braunr> mcsim: the call is vm_advi*s*e - <braunr> mcsim: the call is vm_advi_s_e - <braunr> not advice - <youpi> yes, he agreed earlier - <braunr> ok - <mcsim> cluster_size is the amount of data that I try to read at one time. - <mcsim> at singe mo_data_request - <mcsim> *single - <youpi> which, to me, will depend on the actual map - <braunr> ok so it is the transfer size - <youpi> and should be autotuned, especially for normal behavior - <braunr> youpi: it makes no sense to have both the advice and the actual - size per map entry - <youpi> to get big readahead with all apps - <youpi> braunr: the size is not only dependent on the advice, but also on - the application behavior - <braunr> youpi: how does this application tell this ? - <youpi> even for sequential, you shouldn't necessarily use very big amounts - of transfers - <braunr> there is no need for the advice if there is a cluster size - <youpi> there can be, in the case of sequential, as we said, to clear - previous pages - <youpi> but otherwise, indeed - <youpi> but for me it's the converse - <youpi> the cluster size should be tuned anyway - <braunr> and i'm against giving the cluster size in the advise call, as we - may want to prefetch previous data as well - <youpi> I don't see how that collides - <braunr> well, if you consider it's the transfer size, it doesn't - <youpi> to me cluster size is just the size of a window - <braunr> if you consider it's the amount of pages following a faulted page, - it will - <braunr> also, if your policy says e.g. "3 pages before, 10 after", and - your cluster size is 2, what happens ? - <braunr> i would find it much simpler to do what other VM variants do: - compute the I/O sizes directly from the policy - <youpi> don't they autotune, and use the policy as a maximum ? - <braunr> depends on the implementations - <youpi> ok, but yes I agree - <youpi> although casting the size into stone in the policy looks bogus to - me - <braunr> but making cluster_size part of the kernel interface looks way too - messy - <braunr> it is - <braunr> that's why i would have thought it as part of the pager properties - <braunr> the pager is the true component besides the kernel that is - actually involved in paging ... - <youpi> well, for me the flexibility should still be per application - <youpi> by pager you mean the whole pager, not each file, right? - <braunr> if a pager can page more because e.g. it's a file system with big - block sizes, why not fetch more ? - <braunr> yes - <braunr> it could be each file - <braunr> but only if we have use for it - <braunr> and i don't see that currently - <youpi> well, posix currently doesn't provide a way to set it - <youpi> so it would be useless atm - <braunr> i was thinking about our hurd pagers - <youpi> could we perhaps say that the policy maximum could be a fraction of - available memory? - <braunr> why would we want that ? - <youpi> (total memory, I mean) - <youpi> to make it not completely cast into stone - <youpi> as have been in the past in gnumach - <braunr> i fail to understand :/ - <youpi> there must be a misunderstanding then - <youpi> (pun not intended) - <braunr> why do you want to limit the policy maximum ? - <youpi> how to decide it? - <braunr> the pager sets it - <youpi> actually I don't see how a pager could decide it - <youpi> on what ground does it make the decision? - <youpi> readahead should ideally be as much as 1MiB - <braunr> 02:02 < braunr> if a pager can page more because e.g. it's a file - system with big block sizes, why not fetch more ? - <braunr> is the example i have in mind - <braunr> otherwise some default values - <youpi> that's way smaller than 1MiB, isn't it? - <braunr> yes - <braunr> and 1 MiB seems a lot to me :) - <youpi> for readahead, not really - <braunr> maybe for sequential - <youpi> that's what we care about! - <braunr> ah, i thought we cared about normal - <youpi> "as much as 1MiB", I said - <youpi> I don't mean normal :) - <braunr> right - <braunr> but again, why limit ? - <braunr> we could have 2 or more ? - <youpi> at some point you don't get more efficiency - <youpi> but eat more memory - <braunr> having the pager set the amount allows us to easily adjust it over - time - <mcsim> braunr: Do you think that readahead should be implemented in - libpager? - <youpi> than needed - <braunr> mcsim: no - <braunr> mcsim: err - <braunr> mcsim: can't answer - <youpi> mcsim: do you read the log of what you have missed during - disconnection? - <braunr> i'm not sure about what libpager does actually - <mcsim> yes - <braunr> for me it's just mutualisation of code used by pagers - <braunr> i don't know the details - <braunr> youpi: yes - <braunr> youpi: that's why we want these values not hardcoded in the kernel - <braunr> youpi: so that they can be adjusted by our shiny user space OS - <youpi> (btw apparently linux uses minimum 16k, maximum 128 or 256k) - <braunr> that's more reasonable - <youpi> that's just 4 times less :) - <mcsim> braunr: You say that pager should decide how much data should be - read ahead, but each pager can't implement it on it's own as there will - be too much overhead. So the only way is to implement this in libpager. - <braunr> mcsim: gni ? - <braunr> why couldn't they ? - <youpi> mcsim: he means the size, not the actual implementation - <youpi> the maximum size, actually - <braunr> actually, i would imagine it as the pager giving per policy - parameters - <youpi> right - <braunr> like how many before and after - <youpi> I agree, then - <braunr> the kernel could limit, sure, to avoid letting pagers use - completely insane values - <youpi> (and that's just a max, the kernel autotunes below that) - <braunr> why not - <youpi> that kernel limit could be a fraction of memory, then? - <braunr> it could, yes - <braunr> i see what you mean now - <youpi> mcsim: did you understand our discussion? - <youpi> don't hesitate to ask for clarification - <mcsim> I supposed cluster_size to be such parameter. And advice will help - to interpret this parameter (whether data should be read after fault page - or some data should be cleaned before) - <youpi> mcsim: we however believe that it's rather the pager than the - application that would tell that - <youpi> at least for the default values - <youpi> posix doesn't have a way to specify it, and I don't think it will - in the future - <braunr> and i don't think our own hurd-specific programs will need more - than that - <braunr> if they do, we can slightly change the interface to make it a per - object property - <braunr> i've checked the slab properties, and it seems we can safely add - it per object - <braunr> cf http://www.sceen.net/~rbraun/slabinfo.out - <braunr> so it would still be set by the pager, but if depending on the - object, the pager could set different values - <braunr> youpi: do you think the pager should just provide one maximum size - ? or per policy sizes ? - <youpi> I'd say per policy size - <youpi> so people can increase sequential size like crazy when they know - their sequential applications need it, without disturbing the normal - behavior - <braunr> right - <braunr> so the last decision is per pager or per object - <braunr> mcsim: i'd say whatever makes your implementation simpler :) - <mcsim> braunr: how kernel knows that object are created by specific pager? - <braunr> that's the kind of things i'm referring to with "whatever makes - your implementation simpler" - <braunr> but usually, vm_objects have an ipc port and some properties - relatedto their pagers - <braunr> -usually - <braunr> the problem i had in mind was the locking protocol but our spin - locks are noops, so it will be difficult to detect deadlocks - <mcsim> braunr: and for every policy there should be variable in vm_object - structure with appropriate cluster_size? - <braunr> if you want it per object, yes - <braunr> although i really don't think we want it - <youpi> better keep it per pager for now - <braunr> let's imagine youpi finishes his 64-bits support, and i can - successfully remove the page cache limit - <braunr> we'd jump from 1.8 GiB at most to potentially dozens of GiB of RAM - <braunr> and 1.8, mostly unused - <braunr> to dozens almost completely used, almost all the times for the - most interesting use cases - <braunr> we may have lots and lots of objects to keep around - <braunr> so if noone really uses the feature ... there is no point - <youpi> but also lots and lots of memory to spend on it :) - <youpi> a lot of objects are just one page, but a lof of them are not - <braunr> sure - <braunr> we wouldn't be doing that otherwise :) - <braunr> i'm just saying there is no reason to add the overhead of several - integers for each object if they're simply not used at all - <braunr> hmm, 64-bits, better page cache, clustered paging I/O :> - <braunr> (and readahead included in the last ofc) - <braunr> good night ! - <mcsim> than, probably, make system-global max-cluster_size? This will save - some memory. Also there is usually no sense in reading really huge chunks - at once. - <youpi> but that'd be tedious to set - <youpi> there are only a few pagers, that's no wasted memory - <youpi> the user being able to set it for his own pager is however a very - nice feature, which can be very useful for databases, image processing, - etc. - <mcsim> In conclusion I have to implement following: 3 memory policies per - object and per vm_map_entry. Max cluster size for every policy should be - set per pager. - <mcsim> So, there should be 2 system calls for setting memory policy and - one for setting cluster sizes. - <mcsim> Also amount of data to transfer should be tuned automatically by - every page fault. - <mcsim> youpi: Correct me, please, if I'm wrong. - <youpi> I believe that's what we ended up to decide, yes - - -## IRC, freenode, #hurd, 2012-07-02 - - <braunr> is it safe to say that all memory objects implemented by external - pagers have "file" semantics ? - <braunr> i wonder if the current memory manager interface is suitable for - device pagers - <mcsim> braunr: What does "file" semantics mean? - <braunr> mcsim: anonymous memory doesn't have the same semantics as a file - for example - <braunr> anonymous memory that is discontiguous in physical memory can be - contiguous in swap - <braunr> and its location can change with time - <braunr> whereas with a memory object, the data exchanged with pagers is - identified with its offset - <braunr> in (probably) all other systems, this way of specifying data is - common to all files, whatever the file system - <braunr> linux uses the struct vm_file name, while in BSD/Solaris they are - called vnodes (the link between a file system inode and virtual memory) - <braunr> my question is : can we implement external device pagers with the - current interface, or is this interface really meant for files ? - <braunr> also - <braunr> mcsim: something about what you said yesterday - <braunr> 02:39 < mcsim> In conclusion I have to implement following: 3 - memory policies per object and per vm_map_entry. Max cluster size for - every policy should be set per pager. - <braunr> not per object - <braunr> one policy per map entry - <braunr> transfer parameters (pages before and after the faulted page) per - policy, defined by pagers - <braunr> 02:39 < mcsim> So, there should be 2 system calls for setting - memory policy and one for setting cluster sizes. - <braunr> adding one call for vm_advise is good because it mirrors the posix - call - <braunr> but for the parameters, i'd suggest changing an already existing - call - <braunr> not sure which one though - <mcsim> braunr: do you know how mo_change_attributes implemented in - OSF/Mach? - <braunr> after a quick reading of the reference manual, i think i - understand why they made it per object - <braunr> mcsim: no - <braunr> did they change the call to include those paging parameters ? - <mcsim> it accept two parameters: flavor and pointer to structure with - parameters. - <mcsim> flavor determines semantics of structure with parameters. - <mcsim> - http://www.darwin-development.org/cgi-bin/cvsweb/osfmk/src/mach_kernel/vm/memory_object.c?rev=1.1 - <mcsim> structure can have 3 different views and what exect view will be is - determined by value of flavor - <mcsim> So, I thought about implementing similar call that could be used - for various purposes. - <mcsim> like ioctl - <braunr> "pointer to structure with parameters" <= which one ? - <braunr> mcsim: don't model anything anywhere like ioctl please - <mcsim> memory_object_info_t attributes - <braunr> ioctl is the very thing we want NOT to have on the hurd - <braunr> ok attributes - <braunr> and what are the possible values of flavour, and what kinds of - attributes ? - <mcsim> and then appears something like this on each case: behave = - (old_memory_object_behave_info_t) attributes; - <braunr> ok i see - <mcsim> flavor could be OLD_MEMORY_OBJECT_BEHAVIOR_INFO, - MEMORY_OBJECT_BEHAVIOR_INFO, MEMORY_OBJECT_PERFORMANCE_INFO etc - <braunr> i don't really see the point of flavour here, other than - compatibility - <braunr> having attributes is nice, but you should probably add it as a - call parameter, not inside a structure - <braunr> as a general rule, we don't like passing structures too much - to/from the kernel, because handling them with mig isn't very clean - <mcsim> ok - <mcsim> What policy parameters should be defined by pager? - <braunr> i'd say number of pages to page-in before and after the faulted - page - <mcsim> Only pages before and after the faulted page? - <braunr> for me yes - <braunr> youpi might have different things in mind - <braunr> the page cleaning in sequential mode is something i wouldn't do - <braunr> 1/ applications might want data read sequentially to remain in the - cache, for other sequential accesses - <braunr> 2/ applications that really don't want to cache anything should - use O_DIRECT - <braunr> 3/ it's complicated, and we're in july - <braunr> i'd rather have a correct and stable result than too many unused - features - <mcsim> braunr: MADV_SEQUENTIAL Expect page references in sequential order. - (Hence, pages in the given range can be aggressively read ahead, and may - be freed soon after they are accessed.) - <mcsim> this is from linux man - <mcsim> braunr: Can I at least make keeping in mind that it could be - implemented? - <mcsim> I mean future rpc interface - <mcsim> braunr: From behalf of kernel pager is just a port. - <mcsim> That's why it is not clear for me how I can make in kernel - per-pager policy - <braunr> mcsim: you can't - <braunr> 15:19 < braunr> after a quick reading of the reference manual, i - think i understand why they made it per object - <braunr> - http://pubs.opengroup.org/onlinepubs/009695399/functions/posix_madvise.html - <braunr> POSIX_MADV_SEQUENTIAL - <braunr> Specifies that the application expects to access the specified - range sequentially from lower addresses to higher addresses. - <braunr> linux might free pages after their access, why not, but this is - entirely up to the implementation - <mcsim> I know, when but applications might want data read sequentially to - remain in the cache, for other sequential accesses this kind of access - could be treated rather normal or random - <braunr> we can do differently - <braunr> mcsim: no - <braunr> sequential means the access will be sequential - <braunr> so aggressive readahead (e.g. 0 pages before, many after), should - be used - <braunr> for better performance - <braunr> from my pov, it has nothing to do with caching - <braunr> i actually sometimes expect data to remain in cache - <braunr> e.g. before playing a movie from sshfs, i sometimes prefetch it - using dd - <braunr> then i use mplayer - <braunr> i'd be very disappointed if my data didn't remain in the cache :) - <mcsim> At least these pages could be placed into inactive list to be first - candidates for pageout. - <braunr> that's what will happen by default - <braunr> mcsim: if we need more properties for memory objects, we'll adjust - the call later, when we actually implement them - <mcsim> so, first call is vm_advise and second is changed - mo_change_attributes? - <braunr> yes - <mcsim> there will appear 3 new parameters in mo_c_a: policy, pages before - and pages after? - <mcsim> braunr: With vm_advise I didn't understand one thing. This call is - defined in defs file, so that should mean that vm_advise is ordinal rpc - call. But on the same time it is defined as syscall in mach internals (in - mach_trap_table). - <braunr> mcsim: what ? - <braunr> were is it "defined" ? (it doesn't exit in gnumach currently) - <mcsim> Ok, let consider vm_map - <mcsim> I define it both in mach_trap_table and in defs file. - <mcsim> But why? - <braunr> uh ? - <braunr> let me see - <mcsim> Why defining in defs file is not enough? - <mcsim> and previous question: there will appear 3 new parameters in - mo_c_a: policy, pages before and pages after? - <braunr> mcsim: give me the exact file paths please - <braunr> mcsim: we'll discuss the new parameters after - <mcsim> kern/syscall_sw.c - <braunr> right i see - <mcsim> here mach_trap_table in defined - <braunr> i think they're not used - <braunr> they were probably introduced for performance - <mcsim> and ./include/mach/mach.defs - <braunr> don't bother adding vm_advise as a syscall - <braunr> about the parameters, it's a bit more complicated - <braunr> you should add 6 parameters - <braunr> before and after, for the 3 policies - <braunr> but - <braunr> as seen in the posix page, there could be more policies .. - <braunr> ok forget what i said, it's stupid - <braunr> yes, the 3 parameters you had in mind are correct - <braunr> don't forget a "don't change" value for the policy though, so the - kernel ignores the before/after values if we don't want to change that - <mcsim> ok - <braunr> mcsim: another reason i asked about "file semantics" is the way we - handle the cache - <braunr> mcsim: file semantics imply data is cached, whereas anonymous and - device memory usually isn't - <braunr> (although having the cache at the vm layer instead of the pager - layer allows nice things like the swap cache) - <mcsim> But this shouldn't affect possibility of implementing of device - pager. - <braunr> yes it may - <braunr> consider how a fault is actually handled by a device - <braunr> mach must use weird fictitious pages for that - <braunr> whereas it would be better to simply let the pager handle the - fault as it sees fit - <mcsim> setting may_cache to false should resolve the issue - <braunr> for the caching problem, yes - <braunr> which is why i still think it's better to handle the cache at the - vm layer, unlike UVM which lets the vnode pager handle its own cache, and - removes the vm cache completely - <mcsim> The only issue with pager interface I see is implementing of - scatter-gather DMA (as current interface does not support non-consecutive - access) - <braunr> right - <braunr> but that's a performance issue - <braunr> my problem with device pagers is correctness - <braunr> currently, i think the kernel just asks pagers for "data" - <braunr> whereas a device pager should really map its device memory where - the fault happen - <mcsim> braunr: You mean that every access to memory should cause page - fault? - <mcsim> I mean mapping of device memory - <braunr> no - <braunr> i mean a fault on device mapped memory should directly access a - shared region - <braunr> whereas file pagers only implement backing store - <braunr> let me explain a bit more - <braunr> here is what happens with file mapped memory - <braunr> you map it, access it (some I/O is done to get the page content in - physical memory), then later it's flushed back - <braunr> whereas with device memory, there shouldn't be any I/O, the device - memory should directly be mapped (well, some devices need the same - caching behaviour, while others provide direct access) - <braunr> one of the obvious consequences is that, when you map device - memory (e.g. a framebuffer), you expect changes in your mapped memory to - be effective right away - <braunr> while with file mapped memory, you need to msync() it - <braunr> (some framebuffers also need to be synced, which suggests greater - control is needed for external pagers) - <mcsim> Seems that I understand you. But how it is implemented in other - OS'es? Do they set something in mmu? - <braunr> mcsim: in netbsd, pagers have a fault operatin in addition to get - and put - <braunr> the device pager sets get and put to null and implements fault - only - <braunr> the fault callback then calls the d_mmap callback of the specific - driver - <braunr> which usually results in the mmu being programmed directly - <braunr> (e.g. pmap_enter or similar) - <braunr> in linux, i think raw device drivers, being implemented as - character device files, must provide raw read/write/mmap/etc.. functions - <braunr> so it looks pretty much similar - <braunr> i'd say our current external pager interface is insufficient for - device pagers - <braunr> but antrik may know more since he worked on ggi - <braunr> antrik: ^ - <mcsim> braunr: Seems he used io_map - <braunr> mcsim: where ar eyou looking at ? the incubator ? - <mcsim> his master's thesis - <braunr> ah the thesis - <braunr> but where ? :) - <mcsim> I'll give you a link - <mcsim> http://dl.dropbox.com/u/36519904/kgi_on_hurd.pdf - <braunr> thanks - <mcsim> see p 158 - <braunr> arg, more than 200 pages, and he says he's lazy :/ - <braunr> mcsim: btw, have a look at m_o_ready - <mcsim> braunr: This is old form of mo_change attributes - <mcsim> I'm not going to change it - <braunr> mcsim: these are actually the default object parameters right ? - <braunr> mcsim: if you don't change it, it means the kernel must set - default values until the pager changes them, if it does - <mcsim> yes. - <antrik> mcsim: madvise() on Linux has a separate flag to indicate that - pages won't be reused. thus I think it would *not* be a good idea to - imply it in SEQUENTIAL - <antrik> braunr: yes, my KMS code relies on mapping memory objects for the - framebuffer - <antrik> (it should be noted though that on "modern" hardware, mapping - graphics memory directly usually gives very poor performance, and drivers - tend to avoid it...) - <antrik> mcsim: BTW, it was most likely me who warned about legal issues - with KAM's work. AFAIK he never managed to get the copyright assignment - done :-( - <antrik> (that's not really mandatory for the gnumach work though... only - for the Hurd userspace parts) - <antrik> also I'd like to point out again that the cluster_size argument - from OSF Mach was probably *not* meant for advice from application - programs, but rather was supposed to reflect the cluster size of the - filesystem in question. at least that sounds much more plausible to me... - <antrik> braunr: I have no idea whay you mean by "device pager". device - memory is mapped once when the VM mapping is established; there is no - need for any fault handling... - <antrik> mcsim: to be clear, I think the cluster_size parameter is mostly - orthogonal to policy... and probably not very useful at all, as ext2 - almost always uses page-sized clusters. I'm strongly advise against - bothering with it in the initial implementation - <antrik> mcsim: to avoid confusion, better use a completely different name - for the policy-decided readahead size - <mcsim> antrik: ok - <antrik> braunr: well, yes, the thesis report turned out HUGE; but the - actual work I did on the KGI port is fairly tiny (not more than a few - weeks of actual hacking... everything else was just brooding) - <antrik> braunr: more importantly, it's pretty much the last (and only - non-trivial) work I did on the Hurd :-( - <antrik> (also, I don't think I used the word "lazy"... my problem is not - laziness per se; but rather inability to motivate myself to do anything - not providing near-instant gratification...) - <braunr> antrik: right - <braunr> antrik: i shouldn't consider myself lazy either - <braunr> mcsim: i agree with antrik, as i told you weeks ago - <braunr> about - <braunr> 21:45 < antrik> mcsim: to be clear, I think the cluster_size - parameter is mostly orthogonal to policy... and probably not very useful - at all, as ext2 almost always uses page-sized clusters. I'm strongly - advise against bothering with it - <braunr> in the initial implementation - <braunr> antrik: but how do you actually map device memory ? - <braunr> also, strangely enough, here is the comment in dragonflys - madvise(2) - <braunr> 21:45 < antrik> mcsim: to be clear, I think the cluster_size - parameter is mostly orthogonal to policy... and probably not very useful - at all, as ext2 almost always uses page-sized clusters. I'm strongly - advise against bothering with it - <braunr> in the initial implementation - <braunr> arg - <braunr> MADV_SEQUENTIAL Causes the VM system to depress the priority of - pages immediately preceding a given page when it is faulted in. - <antrik> braunr: interesting... - <antrik> (about SEQUENTIAL on dragonfly) - <antrik> as for mapping device memory, I just use to device_map() on the - mem device to map the physical address space into a memory object, and - then through vm_map into the driver (and sometimes application) address - space - <antrik> formally, there *is* a pager involved of course (implemented - in-kernel by the mem device), but it doesn't really do anything - interesting - <antrik> thinking about it, there *might* actually be page faults involved - when the address ranges are first accessed... but even then, the handling - is really trivial and not terribly interesting - <braunr> antrik: it does the most interesting part, create the physical - mapping - <braunr> and as trivial as it is, it requires a special interface - <braunr> i'll read about device_map again - <braunr> but yes, the fact that it's in-kernel is what solves the problem - here - <braunr> what i'm interested in is to do it outside the kernel :) - <antrik> why would you want to do that? - <antrik> there is no policy involved in doing an MMIO mapping - <antrik> you ask for the pysical memory region you are interested in, and - that's it - <antrik> whether the kernel adds the page table entries immediately or on - faults is really an implementation detail - <antrik> braunr: ^ - <braunr> yes it's a detail - <braunr> but do we currently have the interface to make such mappings from - userspace ? - <braunr> and i want to do that because i'd like as many drivers as possible - outside the kernel of course - <antrik> again, the userspace driver asks the kernel to establish the - mapping (through device_map() and then vm_map() on the resulting memory - object) - <braunr> hm i'm missing something - <braunr> - http://www.gnu.org/software/hurd/gnumach-doc/Device-Map.html#Device-Map - <= this one ? - <antrik> yes, this one - <braunr> but this implies the device is implemented by the kernel - <antrik> the mem device is, yes - <antrik> but that's not a driver - <braunr> ah - <antrik> it's just the interface for doing MMIO - <antrik> (well, any physical mapping... but MMIO is probably the only real - use case for that) - <braunr> ok - <braunr> i was thinking about completely removing the device interface from - the kernel actually - <braunr> but it makes sense to have such devices there - <antrik> well, in theory, specific kernel drivers can expose their own - device_map() -- but IIRC the only one that does (besides mem of course) - is maptime -- which is not a real driver either... - -[[Mapped-time_interface|microkernel/mach/gnumach/interface/device/time]]. - - <braunr> oh btw, i didn't know you had a blog :) - <antrik> well, it would be possible to replace the device interface by - specific interfaces for the generic pseudo devices... I'm not sure how - useful that would be - <braunr> there are lots of interesting stuff there - <antrik> hehe... another failure ;-) - <braunr> failure ? - <antrik> well, when I realized that I'm speding a lot of time pondering - things, and never can get myself to actually impelemnt any of them, I had - the idea that if I write them down, there might at least be *some* good - from it... - <antrik> unfortunately it turned out that I need so much effort to write - things down, that most of the time I can't get myself to do that either - :-( - <braunr> i see - <braunr> well it's still nice to have it - <antrik> (notice that the latest entry is two years old... and I haven't - even started describing most of my central ideas :-( ) - <braunr> antrik: i tried to create a blog once, and found what i wrote so - stupid i immediately removed it - <antrik> hehe - <antrik> actually some of my entries seem silly in retrospect as well... - <antrik> but I guess that's just the way it is ;-) - <braunr> :) - <braunr> i'm almost sure other people would be interested in what i had to - say - <antrik> BTW, I'm actually not sure whether the Mach interfaces are - sufficient to implement GEM/TTM... we would certainly need kernel support - for GART (as for any other kind IOMMU in fact); but beyond that it's not - clear to me - <braunr> GEM ? TTM ? GART ? - <antrik> GEM = Graphics Execution Manager. part of the "new" DRM interface, - closely tied with KMS - <antrik> TTM = Translation Table Manager. does part of the background work - for most of the GEM drivers - <braunr> "The Graphics Execution Manager (GEM) is a computer software - system developed by Intel to do memory management for device drivers for - graphics chipsets." hmm - <antrik> (in fact it was originally meant to provide the actual interface; - but the Inter folks decided that it's not useful for their UMA graphics) - <antrik> GART = Graphics Aperture - <antrik> kind of an IOMMU for graphics cards - <antrik> allowing the graphics card to work with virtual mappings of main - memory - <antrik> (i.e. allowing safe DMA) - <braunr> ok - <braunr> all this graphics stuff looks so complex :/ - <antrik> it is - <antrik> I have a whole big chapter on that in my thesis... and I'm not - even sure I got everything right - <braunr> what is nvidia using/doing (except for getting the finger) ? - <antrik> flushing out all the details for KMS, GEM etc. took the developers - like two years (even longer if counting the history of TTM) - <antrik> Nvidia's proprietary stuff uses a completely own kernel interface, - which is of course not exposed or docuemented in any way... but I guess - it's actually similar in what it does) - <braunr> ok - <antrik> (you could ask the nouveau guys if you are truly - interested... they are doing most of their reverse engineering at the - kernel interface level) - <braunr> it seems graphics have very special needs, and a lot of them - <braunr> and the interfaces are changing often - <braunr> so it's not that much interesting currently - <braunr> it just means we'll probably have to change the mach interface too - <braunr> like you said - <braunr> so the answer to my question, which was something like "do mach - external pagers only implement files ?", is likely yes - <antrik> well, KMS/GEM had reached some stability; but now there are - further changes ahead with the embedded folks coming in with all their - dedicated hardware, calling for unified buffer management across the - whole pipeline (from capture to output) - <antrik> and yes: graphics hardware tends to be much more complex regarding - the interface than any other hardware. that's because it's a combination - of actual I/O (like most other devices) with a very powerful coprocessor - <antrik> and the coprocessor part is pretty much unique amongst peripherial - devices - <antrik> (actually, the I/O part is also much more complex than most other - hardware... but that alone would only require a more complex driver, not - special interfaces) - <antrik> embedded hardware makes it more interesting in that the I/O - part(s) are separate from the coprocessor ones; and that there are often - several separate specialised ones of each... the DRM/KMS stuff is not - prepared to deal with this - <antrik> v4l over time has evolved to cover such things; but it's not - really the right place to implement graphics drivers... which is why - there are not efforts to unify these frameworks. funny times... - - -## IRC, freenode, #hurd, 2012-07-03 - - <braunr> mcsim: vm_for_every_page should be static - <mcsim> braunr: ok - <braunr> mcsim: see http://gcc.gnu.org/onlinedocs/gcc/Inline.html - <braunr> and it looks big enough that you shouldn't make it inline - <braunr> let the compiler decide for you (which is possible only if the - function is static) - <braunr> (otherwise a global symbol needs to exist) - <braunr> mcsim: i don't know where you copied that comment from, but you - should review the description of the vm_advice call in mach.Defs - <mcsim> braunr: I see - <mcsim> braunr: It was vm_inherit :) - <braunr> mcsim: why isn't NORMAL defined in vm_advise.h ? - <braunr> mcsim: i figured actually ;) - <mcsim> braunr: I was going to do it later when. - <braunr> mcsim: for more info on inline, see - http://www.kernel.org/doc/Documentation/CodingStyle - <braunr> arg that's an old one - <mcsim> braunr: I know that I do not follow coding style - <braunr> mcsim: this one is about linux :p - <braunr> mcsim: http://lxr.linux.no/linux/Documentation/CodingStyle should - have it - <braunr> mcsim: "Chapter 15: The inline disease" - <mcsim> I was going to fix it later during refactoring when I'll merge - mplaneta/gsoc12/working to mplaneta/gsoc12/master - <braunr> be sure not to forget :p - <braunr> and the best not to forget is to do it asap - <braunr> +way - <mcsim> As to inline. I thought that even if I specify function as inline - gcc makes final decision about it. - <mcsim> There was a specifier that made function always inline, AFAIR. - <braunr> gcc can force a function not to be inline, yes - <braunr> but inline is still considered as a strong hint - - -## IRC, freenode, #hurd, 2012-07-05 - - <mcsim1> braunr: hello. You've said that pager has to supply 2 values to - kernel to give it an advice how execute page fault. These two values - should be number of pages before and after the page where fault - occurred. But for sequential policy number of pager before makes no - sense. For random policy too. For normal policy it would be sane to make - readahead symmetric. Probably it would be sane to make pager supply - cluster_size (if it is necessary to supply any) that w - <mcsim1> *that will be advice for kernel of least sane value? And maximal - value will be f(free_memory, map_entry_size)? - <antrik> mcsim1: I doubt symmetric readahead would be a good default - policy... while it's hard to estimate an optimum over all typical use - cases, I'm pretty sure most situtations will benefit almost exclusively - from reading following pages, not preceeding ones - <antrik> I'm not even sure it's useful to read preceding pages at all in - the default policy -- the use cases are probably so rare that the penalty - in all other use cases is not justified. I might be wrong on that - though... - <antrik> I wonder how other systems handle that - <LarstiQ> antrik: if there is a mismatch between pages and the underlying - store, like why changing small bits of data on an ssd is slow? - <braunr> mcsim1: i don't see why not - <braunr> antrik: netbsd reads a few pages before too - <braunr> actually, what netbsd does vary on the version, some only mapped - in resident pages, later versions started asynchronous transfers in the - hope those pages would be there - <antrik> LarstiQ: not sure what you are trying to say - <braunr> in linux : - <braunr> 321 * MADV_NORMAL - the default behavior is to read clusters. - This - <braunr> 322 * results in some read-ahead and read-behind. - <braunr> not sure if it's actually what the implementation does - <antrik> well, right -- it's probably always useful to read whole clusters - at a time, especially if they are the same size as pages... that doesn't - mean it always reads preceding pages; only if the read is in the middle - of the cluster AIUI - <LarstiQ> antrik: basically what braunr just pasted - <antrik> and in most cases, we will want to read some *following* clusters - as well, but probably not preceding ones - * LarstiQ nods - <braunr> antrik: the default policy is usually rather sequential - <braunr> here are the numbers for netbsd - <braunr> 166 static struct uvm_advice uvmadvice[] = { - <braunr> 167 { MADV_NORMAL, 3, 4 }, - <braunr> 168 { MADV_RANDOM, 0, 0 }, - <braunr> 169 { MADV_SEQUENTIAL, 8, 7}, - <braunr> 170 }; - <braunr> struct uvm_advice { - <braunr> int advice; - <braunr> int nback; - <braunr> int nforw; - <braunr> }; - <braunr> surprising isn't it ? - <braunr> they may suggest sequential may be backwards too - <braunr> makes sense - <antrik> braunr: what are these numbers? pages? - <braunr> yes - <antrik> braunr: I suspect the idea behind SEQUENTIAL is that with typical - sequential access patterns, you will start at one end of the file, and - then go towards the other end -- so the extra clusters in the "wrong" - direction do not actually come into play - <antrik> only situation where some extra clusters are actually read is when - you start in the middle of a file, and thus do not know yet in which - direction the sequential read will go... - <braunr> yes, there are similar comments in the linux code - <braunr> mcsim1: so having before and after numbers seems both - straightforward and in par with other implementations - <antrik> I'm still surprised about the almost symmetrical policy for NORMAL - though - <antrik> BTW, is it common to use heuristics for automatically recognizing - random and sequential patterns in the absence of explicit madise? - <braunr> i don't know - <braunr> netbsd doesn't use any, linux seems to have different behaviours - for anonymous and file memory - <antrik> when KAM was working on this stuff, someone suggested that... - <braunr> there is a file_ra_state struct in linux, for per file read-ahead - policy - <braunr> now the structure is of course per file system, since they all use - the same address - <braunr> (which is why i wanted it to be per pager in the first place) - <antrik> mcsim1: as I said before, it might be useful for the pager to - supply cluster size, if it's different than page size. but right now I - don't think this is something worth bothering with... - <antrik> I seriously doubt it would be useful for the pager to supply any - other kind of policy - <antrik> braunr: I don't understand your remark about using the same - address... - <antrik> braunr: pre-mapping seems the obvious way to implement readahead - policy - <antrik> err... per-mapping - <braunr> the ra_state (read ahead state) isn't the policy - <braunr> the policy is per mapping, parts of the implementation of the - policy is per file system - <mcsim1> braunr: How do you look at following implementation of NORMAL - policy: We have fault page that is current. Than we have maximal size of - readahead block. First we find first absent pages before and after - current. Than we try to fit block that will be readahead into this - range. Here could be following situations: in range RBS/2 (RBS -- size of - readahead block) there is no any page, so readahead will be symmetric; if - current page is first absent page than all - <mcsim1> RBS block will consist of pages that are after current; on the - contrary if current page is last absent than readahead will go backwards. - <mcsim1> Additionally if current page is approximately in the middle of the - range we can decrease RBS, supposing that access is random. - <braunr> mcsim1: i think your gsoc project is about readahead, we're in - july, and you need to get the job done - <braunr> mcsim1: grab one policy that works, pages before and after are - good enough - <braunr> use sane default values, let the pagers decide if they want - something else - <braunr> and concentrate on the real work now - <antrik> braunr: I still don't see why pagers should mess with that... only - complicates matters IMHO - <braunr> antrik: probably, since they almost all use the default - implementation - <braunr> mcsim1: just use sane values inside the kernel :p - <braunr> this simplifies things by only adding the new vm_advise call and - not change the existing external pager interface - - -## IRC, freenode, #hurd, 2012-07-12 - - <braunr> mcsim: so, to begin with, tell us what state you've reached please - <mcsim> braunr: I'm writing code for hurd and gnumach. For gnumach I'm - implementing memory policies now. RANDOM and NORMAL seems work, but in - hurd I found error that I made during editing ext2fs. So for now ext2fs - does not work - <braunr> policies ? - <braunr> what about mechanism ? - <mcsim> also I moved some translators to new interface. - <mcsim> It works too - <braunr> well that's impressive - <mcsim> braunr: I'm not sure yet that everything works - <braunr> right, but that's already a very good step - <braunr> i thought you were still working on the interfaces to be honest - <mcsim> And with mechanism I didn't implement moving pages to inactive - queue - <braunr> what do you mean ? - <braunr> ah you mean with the sequential policy ? - <mcsim> yes - <braunr> you can consider this a secondary goal - <mcsim> sequential I was going to implement like you've said, but I still - want to support moving pages to inactive queue - <braunr> i think you shouldn't - <braunr> first get to a state where clustered transfers do work fine - <mcsim> policies are implemented in function calculate_clusters - <braunr> then, you can try, and measure the difference - <mcsim> ok. I'm now working on fixing ext2fs - <braunr> so, except from bug squashing, what's left to do ? - <mcsim> finish policies and ext2fs; move fatfs, ufs, isofs to new - interface; test this all; edit patches from debian repository, that - conflict with my changes; rearrange commits and fix code indentation; - update documentation; - <braunr> think about measurements too - <tschwinge> mcsim: Please don't spend a lot of time on ufs. No testing - required for that one. - <braunr> and keep us informed about your progress on bug fixing, so we can - test soon - <mcsim> Forgot about moving system to new interfaces (I mean determine form - of vm_advise and memory_object_change_attributes) - <braunr> s/determine/final/ - <mcsim> braunr: ok. - <braunr> what do you mean "moving system to new interfaces" ? - <mcsim> braunr: I also pushed code changes to gnumach and hurd git - repositories - <mcsim> I met an issue with memory_object_change_attributes when I tried to - use it as I have to update all applications that use it. This includes - libc and translators that are not in hurd repository or use debian - patches. So I will not be able to run system with new - memory_object_change_attributes interface, until I update all software - that use this rpc - <braunr> this is a bit like the problem i had with my change - <braunr> the solution is : don't do it - <braunr> i mean, don't change the interface in an incompatible way - <braunr> if you can't change an existing call, add a new one - <mcsim> temporary I changed memory_object_set_attributes as it isn't used - any more. - <mcsim> braunr: ok. Adding new call is a good idea :) - - -## IRC, freenode, #hurd, 2012-07-16 - - <braunr> mcsim: how did you deal with multiple page transfers towards the - default pager ? - <mcsim> braunr: hello. Didn't handle this yet, but AFAIR default pager - supports multiple page transfers. - <braunr> mcsim: i'm almost sure it doesn't - <mcsim> braunr: indeed - <mcsim> braunr: So, I'll update it just other translators. - <braunr> like other translators you mean ? - <mcsim> *just as - <mcsim> braunr: yes - <braunr> ok - <braunr> be aware also that it may need some support in vm_pageout.c in - gnumach - <mcsim> braunr: thank you - <braunr> if you see anything strange in the default pager, don't hesitate - to talk about it - <mcsim> braunr: ok. I didn't finish with ext2fs yet. - <braunr> so it's a good thing you're aware of it now, before you begin - working on it :) - <mcsim> braunr: I'm working on ext2 now. - <braunr> yes i understand - <braunr> i meant "before beginning work on the default pager" - <mcsim> ok - - <antrik> mcsim: BTW, we were mostly talking about readahead (pagein) over - the past weeks, so I wonder what the status on clustered page*out* is?... - <mcsim> antrik: I don't work on this, but following, I think, is an example - of *clustered* pageout: _pager_seqnos_memory_object_data_return: object = - 113, seqno = 4, control = 120, start_address = 0, length = 8192, dirty = - 1. This is an example of debugging printout that shows that pageout - manipulates with chunks bigger than page sized. - <mcsim> antrik: Another one with bigger length - _pager_seqnos_memory_object_data_return: object = 125, seqno = 124, - control = 132, start_address = 131072, length = 126976, dirty = 1, kcopy - <antrik> mcsim: that's odd -- I didn't know the functionality for that even - exists in our codebase... - <antrik> my understanding was that Mach always sends individual pageout - requests for ever single page it wants cleaned... - <antrik> (and this being the reason for the dreadful thread storms we are - facing...) - <braunr> antrik: ok - <braunr> antrik: yes that's what is happening - <braunr> the thread storms aren't that much of a problem now - <braunr> (by carefully throttling pageouts, which is a task i intend to - work on during the following months, this won't be an issue any more) - - -## IRC, freenode, #hurd, 2012-07-19 - - <mcsim> I moved fatfs, ufs, isofs to new interface, corrected some errors - in other that I already moved, moved kernel to new interface (renamed - vm_advice to vm_advise and added rpcs memory_object_set_advice and - memory_object_get_advice). Made some changes in mechanism and tried to - finish ext2 translator. - <mcsim> braunr: I've got an issue with fictitious pages... - <mcsim> When I determine bounds of cluster in external object I never know - its actual size. So, mo_data_request call could ask data that are behind - object bounds. The problem is that pager returns data that it has and - because of this fictitious pages that were allocated are not freed. - <braunr> why don't you know the size ? - <mcsim> I see 2 solutions. First one is do not allocate fictitious pages at - all (but I think that there could be issues). Another lies in allocating - fictitious pages, but then freeing them with mo_data_lock. - <mcsim> braunr: Because pages does not inform kernel about object size. - <braunr> i don't understand what you mean - <mcsim> I think that second way is better. - <braunr> so how does it happen ? - <braunr> you get a page fault - <mcsim> Don't you understand problem or solutions? - <braunr> then a lookup in the map finds the map entry - <braunr> and the map entry gives you the link to the underlying object - <mcsim> from vm_object.h: vm_size_t size; /* - Object size (only valid if internal) */ - <braunr> mcsim: ugh - <mcsim> For external they are either 0x8000 or 0x20000... - <braunr> and for internal ? - <braunr> i'm very surprised to learn that - <mcsim> braunr: for internal size is actual - <braunr> right sorry, wrong question - <braunr> did you find what 0x8000 and 0x20000 are ? - <mcsim> for external I met only these 2 magic numbers when printed out - arguments of functions _pager_seqno_memory_object_... when they were - called. - <braunr> yes but did you try to find out where they come from ? - <mcsim> braunr: no. I think that 0x2000(many zeros) is maximal possible - object size. - <braunr> what's the exact value ? - <mcsim> can't tell exactly :/ My hurd box has broken again. - <braunr> mcsim: how does the vm find the backing content then ? - <mcsim> braunr: Do you know if it is guaranteed that map_entry size will be - not bigger than external object size? - <braunr> mcsim: i know it's not - <braunr> but you can use the map entry boundaries though - <mcsim> braunr: vm asks pager - <braunr> but if the page is already present - <braunr> how does it know ? - <braunr> it must be inside a vm_object .. - <mcsim> If I can use these boundaries than the problem, I described is not - actual. - <braunr> good - <braunr> it makes sense to use these boundaries, as the application can't - use data outside the mapping - <mcsim> I ask page with vm_page_lookup - <braunr> it would matter for shared objects, but then they have their own - faults :p - <braunr> ok - <braunr> so the size is actually completely ignord - <mcsim> if it is present than I stop expansion of cluster. - <braunr> which makes sense - <mcsim> braunr: yes, for external. - <braunr> all right - <braunr> use the mapping boundaries, it will do - <braunr> mcsim: i have only one comment about what i could see - <braunr> mcsim: there are 'advice' fields in both vm_map_entry and - vm_object - <braunr> there should be something else in vm_object - <braunr> i told you about pages before and after - <braunr> mcsim: how are you using this per object "advice" currently ? - <braunr> (in addition, using the same name twice for both mechanism and - policy is very sonfusing) - <braunr> confusing* - <mcsim> braunr: I try to expand cluster as much as it possible, but not - much than limit - <mcsim> they both determine policy, but advice for entry has bigger - priority - <braunr> that's wrong - <braunr> mapping and content shouldn't compete for policy - <braunr> the mapping tells the policy (=the advice) while the content tells - how to implement (e.g. how much content) - <braunr> IMO, you could simply get rid of the per object "advice" field and - use default values for now - <mcsim> braunr: What sense these values for number of pages before and - after should have? - <braunr> or use something well known, easy, and effective like preceding - and following pages - <braunr> they give the vm the amount of content to ask the backing pager - <mcsim> braunr: maximal amount, minimal amount or exact amount? - <braunr> neither - <braunr> that's why i recommend you forget it for now - <braunr> but - <braunr> imagine you implement the three standard policies (normal, random, - sequential) - <braunr> then the pager assigns preceding and following numbers for each of - them, say [5;5], [0;0], [15;15] respectively - <braunr> these numbers would tell the vm how many pages to ask the pagers - in a single request and from where - <mcsim> braunr: but in fact there could be much more policies. - <braunr> yes - <mcsim> also in kernel context there is no such unit as pager. - <braunr> so there should be a call like memory_object_set_advice(int - advice, int preceding, int following); - <braunr> for example - <braunr> what ? - <braunr> the pager is the memory manager - <braunr> it does exist in kernel context - <braunr> (or i don't understand what you mean) - <mcsim> there is only port, but port could be either pager or something - else - <braunr> no, it's a pager - <braunr> it's a port whose receive right is hold by a task implementing the - pager interface - <braunr> either the default pager or an untrusted task - <braunr> (or null if the object is anonymous memory not yet sent to the - default pager) - <mcsim> port is always pager? - <braunr> the object port is, yes - <braunr> struct ipc_port *pager; /* Where to get - data */ - <mcsim> So, you suggest to keep set of advices for each object? - <braunr> i suggest you don't change anything in objects for now - <braunr> keep the advice in the mappings only, and implement default - behaviour for the known policies - <braunr> mcsim: if you understand this point, then i have nothing more to - say, and we should let nowhere_man present his work - <mcsim> braunr: ok. I'll implement only default behaviors for know policies - for now. - <braunr> (actually, using the mapping boundaries is slightly unoptimal, as - we could have several mappings for the same content, e.g. a program with - read only executable mapping, then ro only) - <braunr> mcsim: another way to know the "size" is to actually lookup for - pages in objects - <braunr> hm no, that's not true - <mcsim> braunr: But if there is no page we have to ask it - <mcsim> and I don't understand why using mappings boundaries is unoptimal - <braunr> here is bash - <braunr> 0000000000400000 868K r-x-- /bin/bash - <braunr> 00000000006d9000 36K rw--- /bin/bash - <braunr> two entries, same file - <braunr> (there is the anonymous memory layer for the second, but it would - matter for the first cow faults) - - -## IRC, freenode, #hurd, 2012-08-02 - - <mcsim> braunr: You said that I probably need some support in vm_pageout.c - to make defpager work with clustered page transfers, but TBH I thought - that I have to implement only pagein. Do you expect from me implementing - pageout either? Or I misunderstand role of vm_pageout.c? - <braunr> no - <braunr> you're expected to implement only pagins for now - <braunr> pageins - <mcsim> well, I'm finishing merging of ext2fs patch for large stores and - work on defpager in parallel. - <mcsim> braunr: Also I didn't get your idea about configuring of paging - mechanism on behalf of pagers. - <braunr> which one ? - <mcsim> braunr: You said that pager has somehow pass size of desired - clusters for different paging policies. - <braunr> mcsim: i said not to care about that - <braunr> and the wording isn't correct, it's not "on behalf of pagers" - <mcsim> servers? - <braunr> pagers could tell the kernel what size (before and after a faulted - page) they prefer for each existing policy - <braunr> but that's one way to do it - <braunr> defaults work well too - <braunr> as shown in other implementations - - -## IRC, freenode, #hurd, 2012-08-09 - - <mcsim> braunr: I'm still debugging ext2 with large storage patch - <braunr> mcsim: tough problems ? - <mcsim> braunr: The same issues as I always meet when do debugging, but it - takes time. - <braunr> mcsim: so nothing blocking so far ? - <mcsim> braunr: I can't tell you for sure that I will finish up to 13th of - August and this is unofficial pencil down date. - <braunr> all right, but are you blocked ? - <mcsim> braunr: If you mean the issues that I can not even imagine how to - solve than there is no ones. - <braunr> good - <braunr> mcsim: i'll try to review your code again this week end - <braunr> mcsim: make sure to commit everything even if it's messy - <mcsim> braunr: ok - <mcsim> braunr: I made changes to defpager, but I haven't tried - them. Commit them too? - <braunr> mcsim: sure - <braunr> mcsim: does it work fine without the large storage patch ? - <mcsim> braunr: looks fine, but TBH I can't even run such things like fsx, - because even without my changes it failed mightily at once. - -[[file_system_exerciser]]. - - <braunr> mcsim: right, well, that will be part of another task :) - - -## IRC, freenode, #hurd, 2012-08-13 - - <mcsim> braunr: hello. Seems ext2fs with large store patch works. - - -## IRC, freenode, #hurd, 2012-08-19 - - <mcsim> hello. Consider such situation. There is a page fault and kernel - decided to request pager for several pages, but at the moment pager is - able to provide only first pages, the rest ones are not know yet. Is it - possible to supply only one page and regarding rest ones tell the kernel - something like: "Rest pages try again later"? - <mcsim> I tried pager_data_unavailable && pager_flush_some, but this seems - does not work. - <mcsim> Or I have to supply something anyway? - <braunr> mcsim: better not provide them - <braunr> the kernel only really needs one page - <braunr> don't try to implement "try again later", the kernel will do that - if other page faults occur for those pages - <mcsim> braunr: No, translator just hangs - <braunr> ? - <mcsim> braunr: And I even can't deattach it without reboot - <braunr> hangs when what - <braunr> ? - <braunr> i mean, what happens when it hangs ? - <mcsim> If kernel request 2 pages and I provide one, than when page fault - occurs in second page translator hangs. - <braunr> well that's a bug - <braunr> clustered pager transfer is a mere optimization, you shouldn't - transfer more than you can just to satisfy some requested size - <mcsim> I think that it because I create fictitious pages before calling - mo_data_request - <braunr> as placeholders ? - <mcsim> Yes. Is it correct if I will not grab fictitious pages? - <braunr> no - <braunr> i don't know the details well enough about fictitious pages - unfortunately, but it really feels wrong to use them where real physical - pages should be used instead - <braunr> normally, an in-transfer page is simply marked busy - <mcsim> But If page is already marked busy kernel will not ask it another - time. - <braunr> when the pager replies, you unbusy them - <braunr> your bug may be that you incorrectly use pmap - <braunr> you shouldn't create mmu mappings for pages you didn't receive - from the pagers - <mcsim> I don't create them - <braunr> ok so you correctly get the second page fault - <mcsim> If pager supplies only first pages, when asked were two, than - second page will not become un-busy. - <braunr> that's a bug - <braunr> your code shouldn't assume the pager will provide all the pages it - was asked for - <braunr> only the main one - <mcsim> Will it be ok if I will provide special attribute that will keep - information that page has been advised? - <braunr> what for ? - <braunr> i don't understand "page has been advised" - <mcsim> Advised page is page that is asked in cluster, but there wasn't a - page fault in it. - <mcsim> I need this attribute because if I don't inform kernel about this - page anyhow, than kernel will not change attributes of this page. - <braunr> why would it change its attributes ? - <mcsim> But if page fault will occur in page that was asked than page will - be already busy by the moment. - <braunr> and what attribute ? - <mcsim> advised - <braunr> i'm lost - <braunr> 08:53 < mcsim> I need this attribute because if I don't inform - kernel about this page anyhow, than kernel will not change attributes of - this page. - <braunr> you need the advised attribute because if you don't inform the - kernel about this page, the kernel will not change the advised attribute - of this page ? - <mcsim> Not only advised, but busy as well. - <mcsim> And if page fault will occur in this page, kernel will not ask it - second time. Kernel will just block. - <braunr> well that's normal - <mcsim> But if kernel will block and pager is not going to report somehow - about this page, than translator will hang. - <braunr> but the pager is going to report - <braunr> and in this report, there can be less pages then requested - <mcsim> braunr: You told not to report - <braunr> the kernel can deduce it didn't receive all the pages, and mark - them unbusy anyway - <braunr> i told not to transfer more than requested - <braunr> but not sending data can be a form of communication - <braunr> i mean, sending a message in which data is missing - <braunr> it simply means its not there, but this info is sufficient for the - kernel - <mcsim> hmmm... Seems I understood you. Let me try something. - <mcsim> braunr: I informed kernel about missing page as follows: - pager_data_supply (pager, precious, writelock, i, 1, NULL, 0); Am I - right? - <braunr> i don't know the interface well - <braunr> what does it mean - <braunr> ? - <braunr> are you passing NULL as the data for a missing page ? - <mcsim> yes - <braunr> i see - <braunr> you shouldn't need a request for that though, avoiding useless ipc - is a good thing - <mcsim> i is number of page, 1 is quantity - <braunr> but if you can't find a better way for now, it will do - <mcsim> But this does not work :( - <braunr> that's a bug - <braunr> in your code probably - <mcsim> braunr: supplying NULL as data returns MACH_SEND_INVALID_MEMORY - <braunr> but why would it work ? - <braunr> mach expects something - <braunr> you have to change that - <mcsim> It's mig who refuses data. Mach does not even get the call. - <braunr> hum - <mcsim> That's why I propose to provide new attribute, that will keep - information regarding whether the page was asked as advice or not. - <braunr> i still don't understand why - <braunr> why don't you fix mig so you can your null message instead ? - <braunr> +send - <mcsim> braunr: because usually this is an error - <braunr> the kernel will decide if it's an erro - <braunr> r - <braunr> what kinf of reply do you intend to send the kernel with for these - "advised" pages ? - <mcsim> no reply. But when page fault will occur in busy page and it will - be also advised, kernel will not block, but ask this page another time. - <mcsim> And how kernel will know that this is an error or not? - <braunr> why ask another time ?! - <braunr> you really don't want to flood pagers with useless messages - <braunr> here is how it should be - <braunr> 1/ the kernel requests pages from the pager - <braunr> it know the range - <braunr> 2/ the pager replies what it can, full range, subset of it, even - only one page - <braunr> 3/ the kernel uses what the pager replied, and unbusies the other - pages - <mcsim> First time page was asked because page fault occurred in - neighborhood. And second time because PF occurred in page. - <braunr> well it shouldn't - <braunr> or it should, but then you have a segfault - <mcsim> But kernel does not keep bound of range, that it asked. - <braunr> if the kernel can't find the main page, the one it needs to make - progress, it's a segfault - <mcsim> And this range could be supplied in several messages. - <braunr> absolutely not - <braunr> you defeat the purpose of clustered pageins if you use several - messages - <mcsim> But interface supports it - <braunr> interface supported single page transfers, doesn't mean it's good - <braunr> well, you could use several messages - <braunr> as what we really want is less I/O - <mcsim> Noone keeps bounds of requested range, so it couldn't be checked - that range was split - <braunr> but it would be so much better to do it all with as few messages - as possible - <braunr> does the kernel knows the main page ? - <braunr> know* - <mcsim> Splitting range is not optimal, but it's not an error. - <braunr> i assume it does - <braunr> doesn't it ? - <mcsim> no, that's why I want to provide new attribute. - <braunr> i'm sorry i'm lost again - <braunr> how does the kernel knows a page fault has been serviced ? - <braunr> know* - <mcsim> It receives an interrupt - <braunr> ? - <braunr> let's not mix terms - <mcsim> oh.. I read as received. Sorry - <mcsim> It get mo_data_supply message. Than it replaces fictitious pages - with real ones. - <braunr> so you get a message - <braunr> and you kept track of the range using fictitious pages - <braunr> use the busy flag instead, and another way to retain the range - <mcsim> I allocate fictitious pages to reserve place. Than if page fault - will occur in this page fictitious page kernel will not send another - mo_data_request call, it will wait until fictitious page unblocks. - <braunr> i'll have to check the code but it looks unoptimal to me - <braunr> we really don't want to allocate useless objects when a simple - busy flag would do - <mcsim> busy flag for what? There is no page yet - <braunr> we're talking about mo_data_supply - <braunr> actually we're talking about the whole page fault process - <mcsim> We can't mark nothing as busy, that's why kernel allocates - fictitious page and marks it as busy until real page would be supplied. - <braunr> what do you mean "nothing" ? - <mcsim> VM_PAGE_NULL - <braunr> uh ? - <braunr> when are physical pages allocated ? - <braunr> on request or on reply from the pager ? - <braunr> i'm reading mo_data_supply, and it looks like the page is already - busy at that time - <mcsim> they are allocated by pager and than supplied in reply - <mcsim> Yes, but these pages are fictitious - <braunr> show me please - <braunr> in the master branch, not yours - <mcsim> that page is fictitious? - <braunr> yes - <braunr> i'm referring to the way mach currently does things - <mcsim> vm/vm_fault.c:582 - <braunr> that's memory_object_lock_page - <braunr> hm wait - <braunr> my bad - <braunr> ah that damn object chaining :/ - <braunr> ok - <braunr> the original code is stupid enough to use fictitious pages all the - time, you probably have to do the same - <mcsim> hm... Attributes will be useless, pager should tell something about - pages, that it is not going to supply. - <braunr> yes - <braunr> that's what null is for - <mcsim> Not null, null is error. - <braunr> one problem i can think of is making sure the kernel doesn't - interpret missing as error - <braunr> right - <mcsim> I think better have special value for mo_data_error - <braunr> probably - - -### IRC, freenode, #hurd, 2012-08-20 - - <antrik> braunr: I think it's useful to allow supplying the data in several - batches. the kernel should *not* assume that any data missing in the - first batch won't be supplied later. - <braunr> antrik: it really depends - <braunr> i personally prefer synchronous approaches - <antrik> demanding that all data is supplied at once could actually turn - readahead into a performace killer - <mcsim> antrik: Why? The only drawback I see is higher response time for - page fault, but it also leads to reduced overhead. - <braunr> that's why "it depends" - <braunr> mcsim: it brings benefit only if enough preloaded pages are - actually used to compensate for the time it took the pager to provide - them - <braunr> which is the case for many workloads (including sequential access, - which is the common case we want to optimize here) - <antrik> mcsim: the overhead of an extra RPC is negligible compared to - increased latencies when dealing with slow backing stores (such as disk - or network) - <mcsim> antrik: also many replies lead to fragmentation, while in one reply - all data is gathered in one bunch. If all data is placed consecutively, - than it may be transferred next time faster. - <braunr> mcsim: what kind of fragmentation ? - <antrik> I really really don't think it's a good idea for the page to hold - back the first page (which is usually the one actually blocking) while - it's still loading some other pages (which will probably be needed only - in the future anyways, if at all) - <antrik> err... for the pager to hold back - <braunr> antrik: then all pagers should be changed to handle asynchronous - data supply - <braunr> it's a bit late to change that now - <mcsim> there could be two cases of data placement in backing store: 1/ all - asked data is placed consecutively; 2/ it is spread among backing - store. If pager gets data in one message it more like place it - consecutively. So to have data consecutive in each pager, each pager has - to try send data in one message. Having data placed consecutive is - important, since reading of such data is much more faster. - <braunr> mcsim: you're confusing things .. - <braunr> or you're not telling them properly - <mcsim> Ok. Let me try one more time - <braunr> since you're working *only* on pagein, not pageout, how do you - expect spread pages being sent in a single message be better than - multiple messages ? - <mcsim> braunr: I think about future :) - <braunr> ok - <braunr> but antrik is right, paging in too much can reduce performance - <braunr> so the default policy should be adjusted for both the worst case - (one page) and the average/best (some/mane contiguous pages) - <braunr> through measurement ideally - <antrik> mcsim: BTW, I still think implementing clustered pageout has - higher priority than implementing madvise()... but if the latter is less - work, it might still make sense to do it first of course :-) - <braunr> many* - <braunr> there aren't many users of madvise, true - <mcsim> antrik: Implementing madvise I expect to be very simple. It should - just translate call to vm_advise - <antrik> well, that part is easy of course :-) so you already implemented - vm_advise itself I take it? - <mcsim> antrik: Yes, that was also quite easy. - <antrik> great :-) - <antrik> in that case it would be silly of course to postpone implementing - the madvise() wrapper. in other words: never mind my remark about - priorities :-) - - -## IRC, freenode, #hurd, 2012-09-03 - - <mcsim> I try a test with ext2fs. It works, than I just recompile ext2fs - and it stops working, than I recompile it again several times and each - time the result is unpredictable. - <braunr> sounds like a concurrency issue - <mcsim> I can run the same test several times and ext2 works until I - recompile it. That's the problem. Could that be concurrency too? - <braunr> mcsim: without bad luck, yes, unless "several times" is a lot - <braunr> like several dozens of tries - - -## IRC, freenode, #hurd, 2012-09-04 - - <mcsim> hello. I want to tell that ext2fs translator, that I work on, - replaced for my system old variant that processed only single pages - requests. And it works with partitions bigger than 2 Gb. - <mcsim> Probably I'm not for from the end. - <mcsim> But it's worth to mention that I didn't fix that nasty bug that I - told yesterday about. - <mcsim> braunr: That bug sometimes appears after recompilation of ext2fs - and always disappears after sync or reboot. Now I'm going to finish - defpager and test other translators. - - -## IRC, freenode, #hurd, 2012-09-17 - - <mcsim> braunr: hello. Do you remember that you said that pager has to - inform kernel about appropriate cluster size for readahead? - <mcsim> I don't understand how kernel store this information, because it - does not know about such unit as "pager". - <mcsim> Can you give me an advice about how this could be implemented? - <youpi> mcsim: it can store it in the object - <mcsim> youpi: It too big overhead - <mcsim> youpi: at least from my pow - <mcsim> *pov - <braunr> mcsim: we discussed this already - <braunr> mcsim: there is no "pager" entity in the kernel, which is a defect - from my PoV - <braunr> mcsim: the best you can do is follow what the kernel already does - <braunr> that is, store this property per object$ - <braunr> we don't care much about the overhead for now - <braunr> my guess is there is already some padding, so the overhead is - likely to be amortized by this - <braunr> like youpi said - <mcsim> I remember that discussion, but I didn't get than whether there - should be only one or two values for all policies. Or each policy should - have its own values? - <mcsim> braunr: ^ - <braunr> each policy should have its own values, which means it can be - implemented with a simple static array somewhere - <braunr> the information in each object is a policy selector, such as an - index in this static array - <mcsim> ok - <braunr> mcsim: if you want to minimize the overhead, you can make this - selector a char, and place it near another char member, so that you use - space that was previously used as padding by the compiler - <braunr> mcsim: do you see what i mean ? - <mcsim> yes - <braunr> good - - -## IRC, freenode, #hurd, 2012-09-17 - - <mcsim> hello. May I add function krealloc to slab.c? - <braunr> mcsim: what for ? - <mcsim> braunr: It is quite useful for creating dynamic arrays - <braunr> you don't want dynamic arrays - <mcsim> why? - <braunr> they're expensive - <braunr> try other data structures - <mcsim> more expensive than linked lists? - <braunr> depends - <braunr> but linked lists aren't the only other alternative - <braunr> that's why btrees and radix trees (basically trees of arrays) - exist - <braunr> the best general purpose data structure we have in mach is the red - black tree currently - <braunr> but always think about what you want to do with it - <mcsim> I want to store there sets of sizes for different memory - policies. I don't expect this array to be big. But for sure I can use - rbtree for it. - <braunr> why not a static array ? - <braunr> arrays are perfect for known data sizes - <mcsim> I expect from pager to supply its own sizes. So at the beginning in - this array is only default policy. When pager wants to supply it own - policy kernel lookups table of advice. If this policy is new set of sizes - then kernel creates new entry in table of advice. - <braunr> that would mean one set of sizes for each object - <braunr> why don't you make things simple first ? - <mcsim> Object stores only pointer to entry in this table. - <braunr> but there is no pager object shared by memory objects in the - kernel - <mcsim> I mean struct vm_object - <braunr> so that's what i'm saying, one set per object - <braunr> it's useless overhead - <braunr> i would really suggest using a global set of policies for now - <mcsim> Probably, I don't understand you. Where do you want to store this - static array? - <braunr> it's a global one - <mcsim> "for now"? It is not a problem to implement a table for local - advice, using either rbtree or dynamic array. - <braunr> it's useless overhead - <braunr> and it's not a single integer, you want a whole container per - object - <braunr> don't do anything fancy unless you know you really want it - <braunr> i'll link the netbsd code again as a very good example of how to - implement global policies that work more than decently for every file - system in this OS - <braunr> - http://cvsweb.netbsd.org/bsdweb.cgi/src/sys/uvm/uvm_fault.c?rev=1.194&content-type=text/x-cvsweb-markup&only_with_tag=MAIN - <braunr> look for uvmadvice - <mcsim> But different translators have different demands. Thus changing of - global policy for one translator would have impact on behavior of another - one. - <braunr> i understand - <braunr> this isn't l4, or anything experimental - <braunr> we want something that works well for us - <mcsim> And this is acceptable? - <braunr> until you're able to demonstrate we need different policies, i'd - recommend not making things more complicated than they already are and - need to be - <braunr> why wouldn't it ? - <braunr> we've been discussing this a long time :/ - <mcsim> because every process runs in isolated environment and the fact - that there is something outside this environment, that has no rights to - do that, does it surprises me. - <braunr> ? - <mcsim> ok. let me dip in uvm code. Probably my questions disappear - <braunr> i don't think it will - <braunr> you're asking about the system design here, not implementation - details - <braunr> with l4, there are as you'd expect well defined components - handling policies for address space allocation, or paging, or whatever - <braunr> but this is mach - <braunr> mach has a big shared global vm server with in kernel policies for - it - <braunr> so it's ok to implement a global policy for this - <braunr> and let's be pragmatic, if we don't need complicated stuff, why - would we waste time on this ? - <mcsim> It is not complicated. - <braunr> retaining a whole container for each object, whereas they're all - going to contain exactly the same stuff for years to come seems overly - complicated for me - <mcsim> I'm not going to create separate container for each object. - <braunr> i'm not following you then - <braunr> how can pagers upload their sizes in the kernel ? - <mcsim> I'm going to create a new container only for combination of cluster - sizes that are not present in table of advice. - <braunr> that's equivalent - <braunr> you're ruling out the default set, but that's just an optimization - <braunr> whenever a file system decides to use other sizes, the problem - will arise - <mcsim> Before creating a container I'm going to lookup a table. And only - than create - <braunr> a table ? - <mcsim> But there will be the same container for a huge bunch of objects - <braunr> how do you select it ? - <braunr> if it's a per pager container, remember there is no shared pager - object in the kernel, only ports to external programs - <mcsim> I'll give an example - <mcsim> Suppose there are only two policies. At the beginning we have table - {{random = 4096, sequential = 8096}}. Than pager 1 wants to add new - policy where random cluster size is 8192. He asks kernel to create it and - after this table will be following: {{random = 4096, sequential = 8192}, - {random = 8192, sequential = 8192}}. If pager 2 wants to create the same - policy as pager 1, kernel will lockup table and will not create new - entry. So the table will be the same. - <mcsim> And each object has link to appropriate table entry - <braunr> i'm not sure how this can work - <braunr> how can pagers 1 and 2 know the sizes are the same for the same - policy ? - <braunr> (and actually they shouldn't) - <mcsim> For faster lookup there will be create hash keys for each entry - <braunr> what's the lookup key ? - <mcsim> They do not know - <mcsim> The kernel knows - <braunr> then i really don't understand - <braunr> and how do you select sizes based on the policy ? - <braunr> and how do you remove unused entries ? - <braunr> (ok this can be implemented with a simple ref counter) - <mcsim> "and how do you select sizes based on the policy ?" you mean at - page fault? - <braunr> yes - <mcsim> entry or object keeps pointer to appropriate entry in the table - <braunr> ok your per object data is a pointer to the table entry and the - policy is the index inside - <braunr> so you really need a ref counter there - <mcsim> yes - <braunr> and you need to maintain this table - <braunr> for me it's uselessly complicated - <mcsim> but this keeps design clear - <braunr> not for me - <braunr> i don't see how this is clearer - <braunr> it's just more powerful - <braunr> a power we clearly don't need now - <braunr> and in the following years - <braunr> in addition, i'm very worried about the potential problems this - can introduce - <mcsim> In fact I don't feel comfortable from the thought that one - translator can impact on behavior of another. - <braunr> simple example: the table is shared, it needs a lock, other data - structures you may have added in your patch may also need a lock - <braunr> but our locks are noop for now, so you just can't be sure there is - no deadlock or other issues - <braunr> and adding smp is a *lot* more important than being able to select - precisely policy sizes that we're very likely not to change a lot - <braunr> what do you mean by "one translator can impact another" ? - <mcsim> As I understand your idea (I haven't read uvm code yet) that there - is a global table of cluster sizes for different policies. And every - translator can change values in this table. That is what I mean under one - translator will have an impact on another one. - <braunr> absolutely not - <braunr> translators *can't* change sizes - <braunr> the sizes are completely static, assumed to be fit all - <braunr> -be - <braunr> it's not optimial but it's very simple and effective in practice - <braunr> optimal* - <braunr> and it's not a table of cluster sizes - <braunr> it's a table of pages before/after the faulted one - <braunr> this reflects the fact tha in mach, virtual memory (implementation - and policy) is in the kernel - <braunr> translators must not be able to change that - <braunr> let's talk about pagers here, not translators - <mcsim> Finally I got you. This is an acceptable tradeoff. - <braunr> it took some time :) - <braunr> just to clear something - <braunr> 20:12 < mcsim> For faster lookup there will be create hash keys - for each entry - <braunr> i'm not sure i understand you here - <mcsim> To found out if there is such policy (set of sizes) in the table we - can lookup every entry and compare each value. But it is better to create - a hash value for set and thus find equal policies. - <braunr> first, i'm really not comfortable with hash tables - <braunr> they really need careful configuration - <braunr> next, as we don't expect many entries in this table, there is - probably no need for this overhead - <braunr> remember that one property of tables is locality of reference - <braunr> you access the first entry, the processor automatically fills a - whole cache line - <braunr> so if your table fits on just a few, it's probably faster to - compare entries completely than to jump around in memory - <mcsim> But we can sort hash keys, and in this way find policies quickly. - <braunr> cache misses are way slower than computation - <braunr> so unless you have massive amounts of data, don't use an optimized - container - <mcsim> (20:38:53) braunr: that's why btrees and radix trees (basically - trees of arrays) exist - <mcsim> and what will be the key? - <braunr> i'm not saying to use a tree instead of a hash table - <braunr> i'm saying, unless you have many entries, just use a simple table - <braunr> and since pagers don't add and remove entries from this table - often, it's on case reallocation is ok - <braunr> one* - <mcsim> So here dynamic arrays fit the most? - <braunr> probably - <braunr> it really depends on the number of entries and the write ratio - <braunr> keep in mind current processors have 32-bits or (more commonly) - 64-bits cache line sizes - <mcsim> bytes probably? - <braunr> yes bytes - <braunr> but i'm not willing to add a realloc like call to our general - purpose kernel allocator - <braunr> i don't want to make it easy for people to rely on it, and i hope - the lack of it will make them think about other solutions instead :) - <braunr> and if they really want to, they can just use alloc/free - <mcsim> Under "other solutions" you mean trees? - <braunr> i mean anything else :) - <braunr> lists are simple, trees are elegant (but add non negligible - overhead) - <braunr> i like trees because they truely "gracefully" scale - <braunr> but they're still O(log n) - <braunr> a good hash table is O(1), but must be carefully measured and - adjusted - <braunr> there are many other data structures, many of them you can find in - linux - <braunr> but in mach we don't need a lot of them - <mcsim> Your favorite data structures are lists and trees. Next, what - should you claim, is that lisp is your favorite language :) - <braunr> functional programming should eventually rule the world, yes - <braunr> i wouldn't count lists are my favorite, which are really trees - <braunr> as* - <braunr> there is a reason why red black trees back higher level data - structures like vectors or maps in many common libraries ;) - <braunr> mcsim: hum but just to make it clear, i asked this question about - hashing because i was curious about what you had in mind, i still think - it's best to use static predetermined values for policies - <mcsim> braunr: I understand this. - <braunr> :) - <mcsim> braunr: Yeah. You should be cautious with me :) - - -## IRC, freenode, #hurd, 2012-09-21 - - <antrik> mcsim: there is only one cluster size per object -- it depends on - the properties of the backing store, nothing else. - <antrik> (while the readahead policies depend on the use pattern of the - application, and thus should be selected per mapping) - <antrik> but I'm still not convinced it's worthwhile to bother with cluster - size at all. do other systems even do that?... - - -## IRC, freenode, #hurd, 2012-09-23 - - <braunr> mcsim: how long do you think it will take you to polish your gsoc - work ? - <braunr> (and when before you begin that part actually, because we'll to - review the whole stuff prior to polishing it) - <mcsim> braunr: I think about 2 weeks - <mcsim> But you may already start review it, if you're intended to do it - before I'll rearrange commits. - <mcsim> Gnumach, ext2fs and defpager are ready. I just have to polish the - code. - <braunr> mcsim: i don't know when i'll be able to do that - <braunr> so expect a few weeks on my (our) side too - <mcsim> ok - <braunr> sorry for being slow, that's how hurd development is :) - <mcsim> What should I do with libc patch that adds madvise support? - <mcsim> Post it to bug-hurd? - <braunr> hm probably the same i did for pthreads, create a topic branch in - glibc.git - <mcsim> there is only one commit - <braunr> yes - <braunr> (mine was a one liner :p) - <mcsim> ok - <braunr> it will probably be a debian patch before going into glibc anyway, - just for making sure it works - <mcsim> But according to term. I expect that my study begins in a week and - I'll have to do some stuff then, so actually probably I'll need a week - more. - <braunr> don't worry, that's expected - <braunr> and that's the reason why we're slow - <mcsim> And what should I do with large store patch? - <braunr> hm good question - <braunr> what did you do for now ? - <braunr> include it in your work ? - <braunr> that's what i saw iirc - <mcsim> Yes. It consists of two parts. - <braunr> the original part and the modificaionts ? - <braunr> modifications* - <braunr> i think youpi would know better about that - <mcsim> First (small) adds notification to libpager interface and second - one adds support for large stores. - <braunr> i suppose we'll probably merge the large store patch at some point - anyway - <mcsim> Yes both original and modifications - <braunr> good - <mcsim> I'll split these parts to different commits and I'll try to make - support for large stores independent from other work. - <braunr> that would be best - <braunr> if you can make it so that, by ommitting (or including) one patch, - we can add your patches to the debian package, it would be great - <braunr> (only with regard to the large store change, not other potential - smaller conflicts) - <mcsim> braunr: I also found several bugs in defpager, that I haven't fixed - since winter. - <braunr> oh - <mcsim> seems nobody hasn't expect them. - <braunr> i'm very interested in those actually (not too soon because it - concerns my work on pageout, which is postponed after pthreads and - select) - <mcsim> ok. than I'll do it first. - - -## IRC, freenode, #hurd, 2012-09-24 - - <braunr> mcsim: what is vm_get_advice_info ? - <mcsim> braunr: hello. It should supply some machine specific parameters - regarding clustered reading. At the moment it supplies only maximal - possible size of cluster. - <braunr> mcsim: why such a need ? - <mcsim> It is used by defpager, as it can't allocate memory dynamically and - every thread has to allocate maximal size beforehand - <braunr> mcsim: i see - - -## IRC, freenode, #hurd, 2012-10-05 - - <mcsim> braunr: I think it's not worth to separate large store patch for - ext2 and patch for moving it to new libpager interface. Am I right? - <braunr> mcsim: it's worth separating, but not creating two versions - <braunr> i'm not sure what you mean here - <mcsim> First, I applied large store patch, and than I was changing patched - code, to make it work with new libpager interface. So changes to make - ext2 work with new interface depend on large store patch. - <mcsim> braunr: ^ - <braunr> mcsim: you're not forced to make each version resulting from a new - commit work - <braunr> but don't make big commits - <braunr> so if changing an interface requires its users to be updated - twice, it doesn't make sense to do that - <braunr> just update the interface cleanly, you'll have one or more commits - that produce intermediate version that don't build, that's ok - <braunr> then in another, separate commit, adjust the users - <mcsim> braunr: The only user now is ext2. And the problem with ext2 is - that I updated not the version from git repository, but the version, that - I've got after applying the large store patch. So in other words my - question is follows: should I make a commit that moves to new interface - version of ext2fs without large store patch? - <braunr> you're asking if you can include the large store patch in your - work, and by extension, in the main branch - <braunr> i would say yes, but this must be discussed with others - - -## IRC, freenode, #hurd, 2013-02-18 - - <braunr> mcsim: so, currently reviewing gnumach - <mcsim> braunr: hello - <braunr> mcsim: the review branch, right ? - <mcsim> braunr: yes - <mcsim> braunr: What do you start with? - <braunr> memory refreshing - <braunr> i see you added the advice twice, to vm_object and vm_map_entry - <braunr> iirc, we agreed to only add it to map entries - <braunr> am i wrong ? - <mcsim> let me see - <braunr> the real question being: what do you use the object advice for ? - <mcsim> >iirc, we agreed to only add it to map entries - <mcsim> braunr: TBH, do not remember that. At some point we came to - conclusion that there should be only one advice. But I'm not sure if it - was final point. - <braunr> maybe it wasn't, yes - <braunr> that's why i've just reformulated the question - <mcsim> if (map_entry && (map_entry->advice != VM_ADVICE_DEFAULT)) - <mcsim> advice = map_entry->advice; - <mcsim> else - <mcsim> advice = object->advice; - <braunr> ok - <mcsim> It just participates in determining actual advice - <braunr> ok that's not a bad thing - <braunr> let's keep it - <braunr> please document VM_ADVICE_KEEP - <braunr> and rephrase "How to handle page faults" in vm_object.h to - something like 'How to tune page fault handling" - <braunr> mcsim: what's the point of VM_ADVICE_KEEP btw ? - <mcsim> braunr: Probably it is better to remove it? - <braunr> well if it doesn't do anything, probably - <mcsim> braunr: advising was part of mo_set_attributes before - <mcsim> no it is redudant - <braunr> i see - <braunr> so yes, remove it - <mcsim> s/no/now - <braunr> (don't waste time on a gcs-like changelog format for now) - <braunr> i also suggest creating _vX branches - <braunr> so we can compare the changes between each of your review branches - <braunr> hm, minor coding style issues like switch(...) instead of switch - (...) - <braunr> why does syscall_vm_advise return MACH_SEND_INTERRUPTED if the - target map is NULL ? - <braunr> is it modelled after an existing behaviour ? - <braunr> ah, it's the syscall version - <mcsim> braunr: every syscall does so - <braunr> and the error is supposed to be used by user stubs to switch to - the rpc version - <braunr> ok - <braunr> hm - <braunr> you've replaced obsolete port_set_select and port_set_backup calls - with your own - <braunr> don't do that - <braunr> instead, add your calls to the new gnumach interface - <braunr> mcsim: out of curiosity, have you actually tried the syscall - version ? - <mcsim> braunr: Isn't it called by default? - <braunr> i don't think so, no - <mcsim> than no - <braunr> ok - <braunr> you could name vm_get_advice_info vm_advice_info - <mcsim> regarding obsolete calls, did you say that only in regard of - port_set_* or all other calls too? - <braunr> all of the - <braunr> m - <braunr> i missed one, yes - <braunr> the idea is: don't change the existing interface - <mcsim> >you could name vm_get_advice_info vm_advice_info - <mcsim> could or should? i.e. rename? - <braunr> i'd say should, to remain consistent with the existing similar - calls - <mcsim> ok - <braunr> can you explain KERN_NO_DATA a bit more ? - <braunr> i suppose it's what servers should answer for neighbour pages that - don't exist in the backend, right ? - <mcsim> kernel can ask server for some data to read them beforehand, but - server can be in situation when it does not know what data should be - prefetched - <mcsim> yes - <braunr> ok - <mcsim> it is used by ext2 server - <mcsim> with large store patch - <braunr> so its purpose is to allow the kernel to free the preallocated - pages that won't be used - <braunr> do i get it right ? - <mcsim> no. - <mcsim> ext2 server has a buffer for pages and when kernel asks to read - pages ahead it specifies region of that buffer - <braunr> ah ok - <mcsim> but consecutive pages in buffer does not correspond to consecutive - pages on disk - <braunr> so, the kernel can only prefetch pages that were already read by - the server ? - <mcsim> no, it can ask a server to prefetch pages that were not read by - server - <braunr> hum - <braunr> ok - <mcsim> but in case with buffer, if buffer page is empty, server does not - know what to prefetch - <braunr> i'm not sure i'm following - <braunr> well, i'm sure i'm not following - <braunr> what happens when the kernel requests data from a server, right - after a page fault ? - <braunr> what does the message afk for ? - <mcsim> kernel is unaware regarding actual size of file where was page - fault because of buffer indirection, right? - <braunr> i don't know what "buffer" refers to here - <mcsim> this is buffer in memory where ext2 server reads pages - <mcsim> with large store patch ext2 server does not map the whole disk, but - some of its pages - <mcsim> and it maps these pages in special buffer - <mcsim> that means that constructiveness of pages in memory does not mean - that they are consecutive on disk or logically (belong to the same file) - <braunr> ok so it's a page pool - <braunr> with unordered pages - <braunr> but what do you mean when you say "server does not know what to - prefetch" - <braunr> it normally has everything to determine that - <mcsim> For instance, page fault occurs that leads to reading of - 4k-file. But kernel does not know actual size of file and asks to - prefetch 16K bytes - <braunr> yes - <mcsim> There is no sense to prefetch something that does not belong to - this file - <braunr> yes but the server *knows* that - <mcsim> and server answers with KERN_NO_DATA - <mcsim> server should always say something about every page that was asked - <braunr> then, again, isn't the purpose of KERN_NO_DATA to notify the - kernel it can release the preallocated pages meant for the non existing - data ? - <braunr> (non existing or more generally non prefetchable) - <mcsim> yes - <braunr> then - <braunr> why did you answer no to - <braunr> 15:46 < braunr> so its purpose is to allow the kernel to free the - preallocated pages that won't be used - <braunr> is there something missing ? - <braunr> (well obviously, notify the kernel it can go on with page fault - handling) - <mcsim> braunr: sorry, misunderstoo/misread - <braunr> ok - <braunr> so good, i got this right :) - <braunr> i wonder if KERN_NO_DATA may be a bit too vague - <braunr> people might confuse it with ENODATA - <mcsim> Actually, this is transformation of ENODATA - <mcsim> I was looking among POSIX error codes and thought that this is the - most appropriate - <braunr> i'm not sure it is - <braunr> first, it's about STREAMS, a commonly unused feature - <braunr> and second, the code is obsolete - <mcsim> braunr: AFAIR purpose of KERN_NO_DATA is not only free - pages. Without this call something should hang - <braunr> 15:59 < braunr> (well obviously, notify the kernel it can go on - with page fault handling) - <mcsim> yes - <braunr> hm - <mcsim> sorry again - <braunr> i don't see anything better for the error name for now - <braunr> and it's really minor so let's keep it as it is - <braunr> actually, ENODATA being obsolete helps here - <braunr> ok, done for now, work calling - <braunr> we'll continue later or tomorrow - <mcsim> braunr: ok - <braunr> other than that, this looks ok on the kernel side for now - <braunr> the next change is a bit larger so i'd like to take the time to - read it - <mcsim> braunr: ok - <mcsim> regarding moving calls in mach.defs, can I put them elsewhere? - <braunr> gnumach.defs - <braunr> you'll probably need to rebase your changes to get it - <mcsim> braunr: I'll rebase this later, when we finish with review - <braunr> ok - <braunr> keep the comments in a list then, not to forget - <braunr> (logging irc is also useful) - - -## IRC, freenode, #hurd, 2013-02-20 - - <braunr> mcsim: why does VM_ADVICE_DEFAULT have its own entry ? - <mcsim> braunr: this kind of fallback mode - <mcsim> i suppose that even random strategy could even read several pages - at once - <braunr> yes - <braunr> but then, why did you name it "default" ? - <mcsim> because it is assigned by default - <braunr> ah - <braunr> so you expect pagers to set something else - <braunr> for all objects they create - <mcsim> yes - <braunr> ok - <braunr> why not, but add a comment please - <mcsim> at least until all pagers will support clustered reading - <mcsim> ok - <braunr> even after that, it's ok - <braunr> just say it's there to keep the previous behaviour by default - <braunr> so people don't get the idea of changing it too easily - <mcsim> comment in vm_advice.h? - <braunr> no, in vm_fault.C - <braunr> right above the array - <braunr> why does vm_calculate_clusters return two ranges ? - <braunr> also, "Function PAGE_IS_NOT_ELIGIBLE is used to determine if", - PAGE_IS_NOT_ELIGIBLE doesn't look like a function - <mcsim> I thought make it possible not only prefetch range, but also free - some memory that is not used already - <mcsim> braunr: ^ - <mcsim> but didn't implement it :/ - <braunr> don't overengineer it - <braunr> reduce to what's needed - <mcsim> braunr: ok - <mcsim> braunr: do you think it's worth to implement? - <braunr> no - <mcsim> braunr: it could be useful for sequential policy - <braunr> describe what you have in mind a bit more please, i think i don't - have the complete picture - <mcsim> with sequential policy user supposed to read strictly in sequential - order, so pages that user is not supposed to read could be put in unused - list - <braunr> what pages the user isn't supposed to read ? - <mcsim> if user read pages in increasing order than it is not supposed to - read pages that are right before the page where page fault occured - <braunr> right ? - <braunr> do you mean higher ? - <mcsim> that are before - <braunr> before would be lower then - <braunr> oh - <braunr> "right before" - <mcsim> yes :) - <braunr> why not ? - <braunr> the initial assumption, that MADV_SEQUENTIAL expects *strict* - sequential access, looks wrong - <braunr> remember it's just a hint - <braunr> a user could just acces pages that are closer to one another and - still use MADV_SEQUENTIAL, expecting a speedup because pages are close - <braunr> well ok, this wouldn't be wise - <braunr> MADV_SEQUENTIAL should be optimized for true sequential access, - agreed - <braunr> but i'm not sure i'm following you - <mcsim> but I'm not going to page these pages out. Just put in unused - list, and if they will be used later they will be move to active list - <braunr> your optimization seem to be about freeing pages that were - prefetched and not actually accessed - <braunr> what's the unused list ? - <mcsim> inactive list - <braunr> ok - <braunr> so that they're freed sooner - <mcsim> yes - <braunr> well, i guess all neighbour pages should first be put in the - inactive list - <braunr> iirc, pages in the inactive list aren't mapped - <braunr> this would force another page fault, with a quick resolution, to - tell the vm system the page was actually used, and must become active, - and paged out later than other inactive pages - <braunr> but i really think it's not worth doing it now - <braunr> clustered pagins is about improving I/O - <braunr> page faults without I/O are orders of magnitude faster than I/O - <braunr> it wouldn't bring much right now - <mcsim> ok, I remove this, but put in TODO - <mcsim> I'm not sure that right list is inactive list, but the list that is - scanned to pageout pages to swap partition. There should be such list - <braunr> both the active and inactive are - <braunr> the active one is scanned when the inactive isn't large enough - <braunr> (the current ratio of active pages is limited to 1/3) - <braunr> (btw, we could try increasing it to 1/2) - <braunr> iirc, linux uses 1/2 - <braunr> your comment about unlock_request isn't obvious, i'll have to - reread again - <braunr> i mean, the problem isn't obvious - <braunr> ew, functions with so many indentation levels :/ - <braunr> i forgot how ugly some parts of the mach vm were - <braunr> mcsim: basically it's ok, i'll wait for the simplified version for - another pass - <mcsim> simplified? - <braunr> 22:11 < braunr> reduce to what's needed - <mcsim> ok - <mcsim> and what comment? - <braunr> your XXX in vm_fault.c - <braunr> when calling vm_calculate_clusters - <mcsim> is m->unlock_request the same for all cluster or I should - recalculate it for every page? - <mcsim> s/all/whole - <braunr> that's what i say, i'll have to come back to that later - <braunr> after i have reviewed the userspace code i think - <braunr> so i understand the interactions better - <mcsim> braunr: pushed v1 branch - <mcsim> braunr: "Move new calls to gnumach.defs file" and "Implement - putting pages in inactive list with sequential policy" are in my TODO - <braunr> mcsim: ok - - -## IRC, freenode, #hurd, 2013-02-24 - - <braunr> mcsim: where does the commit from neal (reworking libpager) come - from ? - <braunr> (ok the question looks a little weird semantically but i think you - get my point) - <mcsim> braunr: you want me to give you a link to mail with this commit? - <braunr> why not, yes - <mcsim> http://permalink.gmane.org/gmane.os.hurd.bugs/446 - <braunr> ok so - http://lists.gnu.org/archive/html/bug-hurd/2012-06/msg00001.html - <braunr> ok so, we actually have three things to review here - <braunr> that libpager patch, the ext2fs large store one, and your work - <braunr> mcsim: i suppose something in your work depends on neal's patch, - right ? - <braunr> i mean, why did you work on top of it ? - <mcsim> Yes - <mcsim> All user level code - <braunr> i see it adds some notifications - <mcsim> no - <mcsim> notifacations are for large store - <braunr> ok - <mcsim> but the rest is for my work - <braunr> but what does it do that you require ? - <mcsim> braunr: this patch adds support for multipage work. There were just - stubs that returned errors for chunks longer than one page before. - <braunr> ok - <braunr> for now, i'll just consider that it's ok, as well as the large - store patch - <braunr> ok i've skipped all patches up to "Make mach-defpager process - multipage requests in m_o_data_request." since they're obvious - <braunr> but this one isn't - <braunr> mcsim: why is the offset member a vm_size_t in struct block ? - <braunr> (these things matter for large file support on 32-bit systems) - <mcsim> braunr: It should be vm_offset_t, right? - <braunr> yes - <braunr> well - <braunr> it seems so but - <braunr> im not sure what offset is here - <braunr> vm_offset is normally the offset inside a vm_object - <braunr> and if we want large file support, it could become a 64-bit - integer - <braunr> while vm_size_t is a size inside an address space, so it's either - 32 or 64-bit, depending on the address space size - <braunr> but here, if offset is an offset inside an address space, - vm_size_t is fine - <braunr> same question for send_range_parameters - <mcsim> braunr: TBH, I do not differ vm_size_t and vm_offset_t well - <braunr> they can be easily confused yes - <braunr> they're both offsets and sizes actually - <braunr> they're integers - <mcsim> so here I used vm_offset_t because field name is offset - <braunr> but vm_size_t is an offset/size inside an address space (a - vm_map), while vm_offset_t is an offset/size inside an object - <mcsim> braunr: I didn't know that - <braunr> it's not clear at all - <braunr> and it may not have been that clear in mach either - <braunr> but i think it's best to consider them this way from now on - <braunr> well, it's not that important anyway since we don't have large - file support, but we should some day :/ - <braunr> i'm afraid we'll have it as a side effect of the 64-bit port - <braunr> mcsim: just name them vm_offset_t when they're offsets for - consistency - <mcsim> but seems that I guessed, because I use vm_offset_t variables in - mo_ functions - <braunr> well ok, but my question was about struct block - <braunr> where you use vm_size_t - <mcsim> braunr: I consider this like a mistake - <braunr> ok - <braunr> moving on - <braunr> in upload_range, there are two XXX comments - <braunr> i'm not sure to understand - <mcsim> Second XXX I put because at the moment when I wrote this not all - hurd libraries and servers supported size different from vm_page_size - <mcsim> But then I fixed this and replaced vm_page_size with size in - page_read_file_direct - <braunr> ok then update the comment accordingly - <mcsim> When I was adding third XXX, I tried to check everything. But I - still had felling that I forgot something. - <mcsim> No it is better to remove second and third XXX, since I didn't find - what I missed - <braunr> well, that's what i mean by "update" :) - <mcsim> ok - <mcsim> and first XXX just an optimisation. Its idea is that there is no - case when the whole structure is used in one function. - <braunr> ok - <mcsim> But I was not sure if was worth to do, because if there will appear - some bug in future it could be hard to find it. - <mcsim> I mean that maintainability decreases because of using union - <mcsim> So, I'd rather keep it like it is - <braunr> how is struct send_range_parameters used ? - <braunr> it doesn't looked to be something stored long - <braunr> also, you're allowed to use GNU extensions - <mcsim> It is used to pass parameters from one function to another - <mcsim> which of them? - <braunr> see - http://gcc.gnu.org/onlinedocs/gcc-4.4.7/gcc/Unnamed-Fields.html#Unnamed-Fields - <braunr> mcsim: if it's used to pass parameters, it's likely always on the - stack - <mcsim> braunr: I use it when necessary - <braunr> we really don't care much about a few extra words on the stack - <braunr> the difference in size would - <mcsim> agree - <braunr> matter - <braunr> oops - <braunr> the difference in size would matter if a lot of those were stored - in memory for long durations - <braunr> that's not the case, so the size isn't a problem, and you should - remove the comment - <mcsim> ok - <braunr> mcsim: if i get it right, the libpager rework patch changes some - parameters from byte offset to page frame numbers - <mcsim> braunr: yes - <braunr> why don't you check errors in send_range ? - <mcsim> braunr: it was absent in original code, but you're right, I should - do this - <braunr> i'm not sure how to handle any error there, but at least an assert - <mcsim> I found a place where pager just panics - <braunr> for now it's ok - <braunr> your work isn't about avoiding panics, but there must be a check, - so if we can debug it and reach that point, we'll know what went wrong - <braunr> i don't understand the prototype change of default_read :/ - <braunr> it looks like it doesn't return anything any more - <braunr> has it become asynchronous ? - <mcsim> It was returning some status before, but now it handles this status - on its own - <braunr> hum - <braunr> how ? - <braunr> how do you deal with errors ? - <mcsim> in old code default_read returned kr and this kr was used to - determine what m_o_ function will be used - <mcsim> now default_read calls m_o_ on its own - <braunr> ok - - -## IRC, freenode, #hurd, 2013-03-06 - - <mcsim> braunr: hi, regarding memory policies. Should I create separate - policy that will do pageout or VM_ADVICE_SEQUENTIAL is good enough? - <mcsim> braunr: at the moment it is exactly like NORMAL - <braunr> mcsim: i thought you only did pageins - <mcsim> braunr: yes, but I'm doing pageouts now - <braunr> oh - <braunr> i'd prefer you didn't :/ - <braunr> if you want to improve paging, i have a suggestion i believe is a - lot better - <braunr> and we have 3 patches concerning libpager that we need to review, - polish, and merge in - <mcsim> braunr: That's not hard, and I think I know what to do - <braunr> yes i understand that - <braunr> but it may change the interface and conflict with the pending - changes - <mcsim> braunr: What changes? - <braunr> the large store patch, neal's libpager rework patch on top of - which you made your changes, and your changes - <braunr> the idea i have in mind was writeback throttling - -[[hurd/translator/ext2fs]], [[hurd/libpager]]. - - <braunr> i was planning on doing it myself but if you want to work on it, - feel free to - <braunr> it would be a much better improvement at this time than clustered - pageouts - <braunr> (which can then immediately follow - <braunr> ) - <mcsim> braunr: ok - <mcsim> braunr: but this looks much more bigger task for me - <braunr> we'll talk about the strategy i had in mind tomorrow - <braunr> i hope you find it simple enough - <braunr> on the other hand, clustered pageouts are very similar to pageins - <braunr> and we have enough paging related changes to review that adding - another wouldn't be such a problem actually - <mcsim> so, add? - <braunr> if that's what you want to do, ok - <braunr> i'll think about your initial question tomorrow - - -## IRC, freenode, #hurd, 2013-09-30 - - <antrik> talking about which... did the clustered I/O work ever get - concluded? - <braunr> antrik: yes, mcsim was able to finish clustered pageins, and it's - still on my TODO list - <braunr> it will get merged eventually, now that the large store patch has - also been applied - - -## IRC, freenode, #hurd, 2013-12-31 - - <braunr> mcsim: do you think you'll have time during january to work out - your clustered pagein work again ? :) - <mcsim> braunr: hello. yes, I think. Depends how much time :) - <braunr> shouldn't be much i guess - <mcsim> what exactly should be done there? - <braunr> probably a rebase, and once the review and tests have been - completed, writing the full changelogs - <mcsim> ok - <braunr> the libpager notification on eviction patch has been pushed in as - part of the merge of the ext2fs large store patch - <braunr> i have to review neal's rework patch again, and merge it - <braunr> and then i'll test your work and make debian packages for - darnassus - <braunr> play with it a bit, see how itgoes - <braunr> mcsim: i guess you could start with - 62004794b01e9e712af4943e02d889157ea9163f (Fix bugs and warnings in - mach-defpager) - <braunr> rebase it, send it as a patch on bug-hurd, it should be - straightforward and short - - -## IRC, freenode, #hurd, 2014-03-04 - - <teythoon> btw, has mcsim worked on vectorized i/o ? there was someting you - wanted to integrate - <teythoon> not sure what - <braunr> clustered pageins - <braunr> but he seems busy - <teythoon> oh, pageins |