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author | https://me.yahoo.com/a/g3Ccalpj0NhN566pHbUl6i9QF0QEkrhlfPM-#b1c14 <diana@web> | 2015-02-16 20:08:03 +0100 |
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committer | GNU Hurd web pages engine <web-hurd@gnu.org> | 2015-02-16 20:08:03 +0100 |
commit | 95878586ec7611791f4001a4ee17abf943fae3c1 (patch) | |
tree | 847cf658ab3c3208a296202194b16a6550b243cf /open_issues/memory_object_model_vs_block-level_cache.mdwn | |
parent | 8063426bf7848411b0ef3626d57be8cb4826715e (diff) |
rename open_issues.mdwn to service_solahart_jakarta_selatan__082122541663.mdwn
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diff --git a/open_issues/memory_object_model_vs_block-level_cache.mdwn b/open_issues/memory_object_model_vs_block-level_cache.mdwn deleted file mode 100644 index 22db9b86..00000000 --- a/open_issues/memory_object_model_vs_block-level_cache.mdwn +++ /dev/null @@ -1,514 +0,0 @@ -[[!meta copyright="Copyright © 2012, 2013 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_documentation open_issue_hurd open_issue_gnumach]] - -[[!toc]] - - -# IRC, freenode, #hurd, 2012-02-14 - - <slpz> Open question: what do you think about dropping the memory object - model and implementing a simple block-level cache? - -[[microkernel/mach/memory_object]]. - - <kilobug> slpz: AFAIK the memory object has more purpose than just cache, - it's allow used for passing chunk of data between processes, handling - swap (which similar to cache, but still slightly different), ... - <slpz> kilobug: user processes usually make their way to data with POSIX - operations, so memory objects are only needed for mmap'ed files - <slpz> kilobug: and swap can be replaced for an in-kernel system or even - could still use the memory object - <braunr> slpz: memory objects are used for the page cache - <kilobug> slpz: translators (especially diskfs based) make heavy use of - memory objects, and if "user processes" use POSIX semantics, Hurd process - (translators, pagers, ...) shouldn't be bound to POSIX - <slpz> braunr: and page cache could be moved to a lower level, near to the - devices - <braunr> not likely - <braunr> well, it could, but then you'd still have the file system overhead - <slpz> kilobug: but the use of memory objects it's not compulsory, you can - easily write a fs translator without implementing memory objects at all - (except to mmap) - <braunr> a unified buffer/VM cache as all modern systems have is probably - the most efficient approach - <slpz> braunr: I agree. I want to look at *BSD/Linux vfs systems to seem - how much cache policy depends on the filesystem - <slpz> braunr: Are you aware of any good papers on this matter? - <braunr> netbsd UVM, the linux virtual memory system - <braunr> both a bit old bit still relevant - <slpz> braunr: Thanks. - <slpz> the problem in our case is that having FS and cache information at - different contexts (kernel vs. translator), I find hard to coordinate - them. - <slpz> that's why I though about a block-level cache that GNU Mach could - manage by itself - <slpz> I wonder how QNX deals with this - <braunr> the point of having a simple page cache is explicitely about not - caring if those pages are blocks or files or whatever - <braunr> the kernel (at least, mach) normally has all the accounting - information it needs to implement its cache policy - <braunr> file system translators shouldn't cache much - <braunr> the pager interface could be refined, but it looks ok to me as it - is - <slpz> Mach has the accounting info, but it's not able to purge the cache - without coordination with translators - <braunr> which is normal - <slpz> And this is a big problem when memory pressure increases, as it - doesn't know for sure when memory is going to be freed - <braunr> Mach flushes its cache when it decides to, and sends back dirty - pages if needed by the pager - <braunr> that's the case with every paging implementation - <braunr> the main difference is security with untrusted pagers - <braunr> but that's another issue - <slpz> but in a monolithic implementation, the kernel is able for force a - chunk of cache memory to be freed without hoping for other process to do - the job - <braunr> that's not true - <braunr> they're not process, they're threads, but the timing issue is the - same - <braunr> see pdflush on linux - <slpz> no, it isn't. - <braunr> when memory is scarce, threads that request memory can either wait - or immediately fail, and if they wait, they're usually woken by one of - the vm threads once flushing is done - <slpz> a kernel thread can access all the information in the kernel, and - synchronization is pretty easy. - <braunr> on mach, synchronization is done with messages, that's even easier - than shared kernel locks - <slpz> with processes in different spaces, resource coordination becomes - really difficult - <braunr> and what kind of info would an external pager need when simply - asked to take back its dirty pages - <braunr> what resources ? - <slpz> just take a look at the thread storm problem when GNU Mach needs to - clean a bunch of pages - <braunr> Mach is big enough to correctly account memory - <braunr> there can be thread storms on monolithic systems - <braunr> that's a Mach issue, not a microkernel issue - <braunr> that's why linux limits the number of pdflush thread instances - <slpz> Mach can account memory, but can't assure when be freed by any - means, in a lesser degree than a monolithic system - <braunr> again i disagree - <braunr> no system can guarantee when memory will be freed with paging - <slpz> a block level cache can, for most situations - <braunr> slpz: why ? - <braunr> slpz: or how i mean ? - <slpz> braunr: with a block-level page cache, GNU Mach should be able to - flush dirty pages directly to the underlaying device without all the - complexity and resource cost involved in a m_o_data_return message. It - can also throttle the rate at which pages are being cleaned, and do all - this while blocking new page allocations to deal with memory exhaustion - cases. - <slpz> braunr: in the current state, when cleaning dirty pages, GNU Mach - sends a bunch on m_o_data_return to the corresponding pagers, hoping they - will do their job as soon and as fast as possible. - <slpz> memory is not really freed, but transformed from page cache to - anonymous memory pertaining to the corresponding translator - <slpz> and GNU Mach never knows for sure when this memory is released, if - it ever is. - <slpz> not being able to flush dirty pages synchronously is a big problem - when you need to throttle memory usage - <slpz> and needing allocating more memory when you're trying to free (which - is the case for the m_o_data_return mechanism) makes the problem even - worse - <braunr> your idea of a block level cache means in kernel block drivers - <braunr> that's not the direction we're taking - <braunr> i agree flushing should be a synchronous process, which was one of - the proposed improvements in the thread migration papers - <braunr> (they didn't achieve it but thought about it for future works, so - that the thread at the origin of the fault would handle it itself) - <braunr> but it should be possible to have kernel threads similar to - pdflush and throttle flush requests too - <braunr> again, i really think it's a mach bug, and having a buffer cache - would be stepping backward - <braunr> the real design issue is allocating memory while trying to free - it, yes - <slpz> braunr: thread migration doesn't apply to asynchronous IPC, and the - entire paging mechanism is implemented this way - <slpz> in fact, trying to do a synchronous m_o_data_return will trigger a - deadlock for sure - <slpz> to achieve synchronous flushing with translators, the entire paging - model must be redesigned - <slpz> It's true that I'm not very confident of the viability of user space - drivers - <slpz> at least, not for every device - <slpz> I know this is against the current ideas for most ukernel designs, - but if we want to achieve real work functionality, I think some - sacrifices must be done. Or at least a reasonable compromise. - <braunr> slpz: thread migration for paging requests implies synchronous - RPC, we don't care much about the IPC layer there - <braunr> and it requires large changes of the VM code in addition, yes - <braunr> let's not talk about this, we don't have thread migration anyway - :p - <braunr> except the allocation-on-free-path issue, i really don't see how - the current pager interface or the page cache creates problems wrt - flushing .. - <braunr> monolithic systems also have that problem, with lower impacts - though, but still - <slpz> braunr: because as it doesn't know when memory is really freed, 1) - it just blindly sends a bunch of m_o_data_return to the pagers, usually - overloading them (causing thread storms), and 2) it can't properly - throttle new page requests to deal with resource exhaustion - <braunr> it does know when memory is really freed - <braunr> and yes, it blindly sends a bunch of requests, they can and should - be trottled - <slpz> but dirty pages freed become indistinguishable from common anonymous - chunks released, so it doesn't really know if page flushes are really - working or not (i.e. doesn't know how fast a device is processing write - requests) - <braunr> memory is freed when the pager deallocates it - <braunr> the speed of the operation is irrelevant - <braunr> no system can rely on disk speed to guarantee correct page flushes - <braunr> disk or anything else - <slpz> requests can't be throttled if Mach doesn't know when they are being - processed - <braunr> it can easily know it - <braunr> they are processed as soon as the request is sent from the kernel - <braunr> and processing is done when the pager acknowledges the end of the - flush - <braunr> memory backing the flushed pages should be released before - acknowleding that to avoid starting new requests too soon - <slpz> AFAIK pagers doesn't acknowledge the end of the flush - <braunr> well that's where the interface should be refined - <slpz> Mach just sends the m_o_data_return and continues on its own - <braunr> that's why flushing should be synrhconous - <braunr> are you sure about that however ? - <slpz> so the entire paging system needs a new design... :) - <slpz> pretty sure - <braunr> not a new design .. - <braunr> there is m_o_supply_completed, i don't see how difficult it would - be to add m_o_data_return_completed - <braunr> it's not a small change, but not a difficult one either - <braunr> i'm more worried about the allocation problem - <braunr> the default pager should probably be wired in memory - <braunr> maybe others - <slpz> let's suppose a case in which Mach needs to free memory due to an - increase in its pressure. vm_pageout_daemon starts running, clean pages - are freed easily, but for each dirty one a m_o_data_return in sent. 1) - when should this daemon stop sending m_o_data_return and start waiting - for m_o_data_return_completed? 2) what happens if the translator needs to - read new blocks to fulfill a write request (pretty common in ext2fs)? - <braunr> it should stop after an arbitrary limit is reached - <braunr> a reasonable one - <braunr> linux limits the number of pdflush threads for that reason as i - mentioned (to 8 iirc) - <braunr> the problem of reading blocks while flushing is what i'm worried - about too, hence the need to wire that code - <braunr> well, i'm nto sure it's needed - <braunr> again, a reasonable about of free memory should be reserved for - that at all times - <slpz> but the work for pdflush seems to be a lot easier, as it only deals - directly with block devices (if I understood it correctly, I just started - looking at it). - <braunr> i don't know how other systems compute that, but this is how they - seem to do as well - <braunr> no, i don't think so - <slpz> well, I'll try to invest a few days understanding how pdflush work, - to see if some ideas can be borrowed for Hurd - <braunr> iirc, freebsd has thresholds in percent for each part of its cache - (active, inactive, free, dirty) - <slpz> but I still think simple solutions work better, and using the memory - object for page cache is tremendously complex. - <braunr> the amount of free cache pages is generally sufficient to - guarantee much memory can be released at once if needed, without flushing - anything - <braunr> yes but that's the whole point of the Mach VM - <braunr> and its greatest advance .. - <slpz> what, memory objects? - <braunr> yes - <braunr> using physical memory as a cache for anything, not just block - buffers - <slpz> memory objects work great as a way to provide a shared image of - objects between processes, but as page cache they are an overkill (IMHO). - <slpz> or, at least, in the way we're using them - <braunr> probably - <braunr> http://lwn.net/Articles/326552/ - <braunr> this can help udnerstand the problems we may have without better - knowledge of the underlying devices, yes - <braunr> (e.g. not being able to send multiple requests to pagers that - don't share the same disk) - <braunr> slpz: actually i'm not sure it's that overkill - <braunr> the linux vm uses struct vm_file to represent memory objects iirc - <braunr> there are many links between that structure and some vfs related - subsystems - <braunr> when a system very actively uses the page cache, the kernel has to - maintain a lot of objects to accurately describe the cache content - <braunr> you could consider this overkill at first too - <braunr> the mach way of doing it just implies some ipc messages instead of - function calls, it's not that overkill for me - <braunr> the main problems are recursion (allocation while freeing, - handling page faults in order to handle flushes, that sort of things) - <braunr> struct file and struct address_space actually - <braunr> slpz: see struct address_space, it contains a set of function - pointers that can help understanding the linux pager interface - <braunr> they probably sufferred from similar caveats and worked around - them, adjusting that interface on the way - <slpz> but their strategy makes them able to treat the relationship between - the page cache and the block devices in a really simple way, almost as a - traditional storage cache. - <slpz> meanwhile on Mach+pager scenario, the relationship between a block - in a file and its underlying storage becomes really blurry - <slpz> this is a huge advantage when flusing out data, specially when - resources are scarce - <slpz> I think the idea of using abstract objects for page cache, loses a - bit the point that we just want to avoid accessing constantly to a slow - device - <slpz> and breaking the tight relationship between the device and its - cache, makes things a lot harder - <slpz> this also manifest itself when flushing clean pages, as things like - having an static maximum for cached memory objects - <slpz> we shouldn't care about the number of objects, we just need to - control the number of pages - <slpz> but as we need the pager to flush pages, we need to keep alive a lot - of control ports to them - <mcsim> slpz: When mo_data_return is called, once the memory manager no - longer needs supplied data, it should be deallocated using - vm_deallocate. So this way pagers acknowledges the end of flush. - - -# IRC, freenode, #hurd, 2013-08-26 - - < Spyro> Ok, so - < Spyro> idiot question: in a nutshell, what is a memory object? - < Spyro> and how is swapping/paging handled? - < braunr> Spyro: a memory object is how the virtual memory system views a - file - < braunr> so it's a sequence of bytes with a length - < braunr> "swapping" is just a special case of paging that applies to - anonymous objects - < braunr> (which are named so because they're not associated with a file - and have no name) - < Spyro> Who creates a memory object, and when? - < braunr> pagers create memory objects when needed, e.g. when you open a - file - < Spyro> and this applies both to mmap opens as well as regular I/O opens - as in for read() and write()? - < braunr> basically, all file systems capable of handling mmap requests - and/or caching in physical memory are pagers - < braunr> yes - < braunr> read/write will go through the page cache when possible - < Spyro> and who owns the page cache? - < Spyro> also, who decides what pages ot evict to swap/file if physical - memory gets tight? - < braunr> the kernel - < braunr> that's one of the things that make mach a hybrid - < Spyro> so the kernel owns the page cage? - < Spyro> ...fml - < Spyro> cache! - < braunr> yes - - -## IRC, freenode, #hurd, 2013-08-27 - - < Spyro> so braunr: So, who creates the memory object, and how does it get - populated? - < Spyro> and how does a process accessing a file get hooked up to the - memory object? - < braunr> Spyro: i told you, pagers create memory objects - < braunr> memory objects are how the VM system views files, so they're - populated from the content of files - < braunr> either true files or virtual files such as in /proc - < braunr> Spyro: processes don't directly access memory objects unless - memory mapping them with vm_map() - < braunr> pagers (basically = file systems) do - <Spyro> ok, so how is a pager/fs involved in handling a fault? - - -## IRC, freenode, #hurd, 2013-08-28 - - <braunr> Spyro: each object is linked to a pager - <braunr> Spyro: when a fault occurs, the kernel looks up the VM map (kernel - or a user one), and the address in this map, then the map entry, checks - access and lots of other details - <Spyro> ok, so it's pager -> object -> vmem - <Spyro> ? - <braunr> Spyro: then finds the object mapped at that address (similar to - how a file is mapped with mmap) - <braunr> from the object, it finds the pager - <Spyro> ok - <braunr> and asks the pager about the data at the appropriate offset - <Spyro> so how does a user process do normal file I/O? is faulting just a - special case of it? - <braunr> it's completely separate - <Spyro> eww - <braunr> normal I/O is done with message passing - <braunr> the hurd io interface - <Spyro> ok - <Spyro> so who talks to who on a file I/O? - <braunr> a client (e.g. cat) talks to a file system server (e.g. ext2fs) - <Spyro> ok so - <Spyro> it's client to the pager for regular file I/O? - <braunr> Spyro: i don't understand the question - <braunr> Spyro: it's client to server, the server might not be a pager - <Spyro> ok - <Spyro> just trying to figure out the difference between paging/faulting - and regular I/O - <braunr> regular I/O is just message passing - <braunr> page fault handling is dealt with by pagers - <Spyro> and I have a hunch that the fs/pager is involved somehow in both, - because the server is the source of the data - <Spyro> I'm getting a headache - <braunr> nalaginrut: a server like ext2fs is both a file server and a pager - <Spyro> oh! - <Spyro> oh btw, does a file server make use of memory objects for caching? - <braunr> Spyro: yes - <Spyro> or rather, can it? - <Spyro> does it have to? - <braunr> memory objects are for caching, and thus for page faults - <braunr> Spyro: for caching, it's a requirement - <braunr> for I/O, it's not - <braunr> you could have I/O without memory objects - <Spyro> ok - <Spyro> so how does the pager/fileserver use memory objects for caching? - <Spyro> does it just map and write to them? - <braunr> basically yes but there is a complete protocol with the kernel for - that - <braunr> - http://www.gnu.org/software/hurd/gnumach-doc/External-Memory-Management.html#External-Memory-Management - <Spyro> heh, lucky guess - <Spyro> ty - <Spyro> I am in way over my head here btw - <Spyro> zero experience with micro kernels in practice - <braunr> it's not trivial - <braunr> that's not a microkernel thing at all - <braunr> that's how it works in monolithic kernels too - <braunr> i recommend netbsd uvm thesis - <braunr> there are nice pictures describing the vm system - <Spyro> derrr...preacious? - <Spyro> wow - <braunr> just ignore the anonymous memory handling part which is specific - to uvm - <Spyro> @_@ - <braunr> the rest is common to practically all VM systems out there - <Spyro> I know about the linux page cache - <braunr> well it's almost the same - <Spyro> with memory objects being the same thing as files in a page cache? - <braunr> memory objects are linux "address spaces" - <braunr> and address spaces are how the linux mm views a file, yes - <Spyro> derp - <Spyro> ... - <Spyro> um... - <braunr> struvt vm_page == struct page - * Spyro first must learn what an address_space is - <braunr> struct vm_map == struct mm_struct - <braunr> struct vm_map_entry == struct vm_area_struct - * Spyro isn't a linux kernel vm expert either - <braunr> struct vm_object == struct address_space - <braunr> roughly - <braunr> details vary a lot - <Spyro> and what's an address_space ? - <braunr> 11:41 < braunr> and address spaces are how the linux mm views a - file, yes - <Spyro> ok - <braunr> see include/linux/fs.h - <braunr> struct address_space_operations is the pager interface - * Spyro should look at the linux kernel sources perhaps, unless you have an - easier reference - <Spyro> embarrassingly, RVR hired me as an editor for the linux-mm wiki - <Spyro> I should know this stuff - <braunr> see - http://darnassus.sceen.net/~rbraun/design_and_implementation_of_the_uvm_virtual_memory_system.pdf - <braunr> page 42 - <braunr> page 66 for another nice view - <braunr> i wouldn't recommend using linux source as refernece - <braunr> it's very complicated, filled with a lot of code dealing with - details - <Spyro> lmao - <braunr> and linux guys have a habit of choosing crappy names - <Spyro> I was only going to - <Spyro> stoppit - <braunr> except for "linux" and "git" - <Spyro> ...make me laugh any more and I'll need rib surgery - <braunr> laugh ? - <Spyro> complicated and crappy - <braunr> seriously, "address space" for a file is very very confusing - <Spyro> oh I agree with that - <braunr> yes, names are crappy - <braunr> and the code is very complicated - <braunr> it took me half an hour to find where readahead is done once - <braunr> and i'm still not sure it was the right code - <Spyro> so in linkern, there is an address_space for each cached file? - <braunr> takes me 30 seconds in netbsd .. - <braunr> yes - <Spyro> eww - <Spyro> yeah, BAD name - <Spyro> but thanks for the explanation - <Spyro> now I finally know what an address space is - <braunr> many linux core developers admit they don't care much about names - <Spyro> so, in hurd, a memory object is to hurd, what an address_space is - to linux? - <braunr> yes - <braunr> notto hurd - <Spyro> ok - <braunr> to mach - <Spyro> you know what I mean - <Spyro> :P - <Spyro> easier than for linux I can tell you that much - <braunr> and the bsd vm system is a stripped version of the mach vm - <Spyro> ok - <braunr> that's why i think it's important to note it - <Spyro> good, I learned something abou tthe linux vm...from the mach guys - <Spyro> this is funny - <braunr> linux did too - <braunr> there is a paper about linux page eviction that directly borrows - the mach algorithm and improves it - <braunr> mach is the historic motivation behind mmap on posix - <Spyro> oh nice! - <Spyro> but yes, linux picked a shitty name - <braunr> is all that clearer to you ? - <Spyro> I think that address_space connection was a magic bolt of - understanding - <braunr> and do you see how I/O and paging are mostly unrelated ? - <Spyro> almost - <Spyro> but how does a file I/O take advantage of caching by a memory - object? - <Spyro> does the file server just nudge the core for a hint? - <braunr> the file system copies from the memory object - * Spyro noddles - <Spyro> I think I understand a bit better now - <braunr> it's message passing - <Spyro> but I havfe too much to digest already - <braunr> memory copying - <braunr> if the memory is already there, good, if not, the kernel will ask - the file system to bring the data - <braunr> if message passing uses zero copy, data retrieval can be deferred - until the client actually accesses it - <Spyro> which is a fancy way of saying demand paging? :P - <braunr> it's always demand paging - <braunr> what i mean is that the file system won't fetch data as soon as it - copies memory - <braunr> but when this data is actually needed by the client - <Spyro> uh... - <Spyro> whta's a precious page? - <braunr> let me check quickly - <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 - <braunr> basically, it's used when you want the kernel to keep cached data - in memory - <braunr> the cache becomes a lossless container for such pages - <braunr> the kernel may flush them, but not evict them - <Spyro> what's the difference? - <braunr> imagine a ramfs - <Spyro> point made - <braunr> ok - <Spyro> would be pretty hard to flush something that doesn't have a backing - store - <braunr> that was quick :) - <braunr> well - <braunr> the normal backing store for anonymous memory is the default pager - <braunr> aka swap - <Spyro> eww - <braunr> but if you want your data *either* in swap or in memory and never - in both - <braunr> it may be useful |