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diff --git a/open_issues/performance/io_system/read-ahead.mdwn b/open_issues/performance/io_system/read-ahead.mdwn new file mode 100644 index 00000000..59f22187 --- /dev/null +++ b/open_issues/performance/io_system/read-ahead.mdwn @@ -0,0 +1,3076 @@ +[[!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 |