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diff --git a/open_issues/libpager_deadlock.mdwn b/open_issues/libpager_deadlock.mdwn deleted file mode 100644 index 017ecff6..00000000 --- a/open_issues/libpager_deadlock.mdwn +++ /dev/null @@ -1,165 +0,0 @@ -[[!meta copyright="Copyright © 2010, 2012 Free Software Foundation, Inc."]] - -[[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable -id="license" text="Permission is granted to copy, distribute and/or modify this -document under the terms of the GNU Free Documentation License, Version 1.2 or -any later version published by the Free Software Foundation; with no Invariant -Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license -is included in the section entitled [[GNU Free Documentation -License|/fdl]]."]]"""]] - -[[!tag open_issue_hurd]] - -Deadlocks in libpager/periodic sync have been found. - - -# [[gnumach_page_cache_policy]] - - -## IRC, freenode, #hurd, 2012-07-12 - - <braunr> ah great, a paper about the mach pageout daemon ! - <mcsim> braunr: Where is paper about the mach pageout daemon? - <braunr> ftp://ftp.cs.cmu.edu/project/mach/doc/published/defaultmm.ps - <braunr> might give us a clue about the swap deadlock (although i still - have a few ideas to check) - <braunr> - http://www.sceen.net/~rbraun/moving_the_default_memory_manager_out_of_the_mach_kernel.pdf - <braunr> we should more seriously consider sergio's advisory pageout branch - some day - <braunr> i'll try to get in touch with him about that before he completely - looses interest - <braunr> i'll include it in my "make that page cache as decent as possible" - task - <braunr> many of his comments match what i've seen - <braunr> and we both did a few optimizations the same way - <braunr> (like not deactivating pages when they enter the cache) - - -## IRC, freenode, #hurd, 2012-07-13 - - <braunr> antrik: i'm able to consistently reproduce the swap deadlocks you - regularly had when using apt with my page cache patch - <braunr> it happens when lots of dirty pages are write back to their pagers - <braunr> so apt, or a big file copy or anything that writes several MiB - very quickly is a good candidate - <braunr> written* - <antrik> braunr: nice... - <braunr> antrik: well in a way, yes, as it will allow us to track it more - easily - - -## IRC, freenode, #hurd, 2012-07-15 - - <braunr> oh btw, i think i can say with confidence that the hurd *doesn't* - deadlock - <braunr> (at least, concerning swapping) - <braunr> lol, one of my hurd systems has been hitting the "swap deadlock" - for more than an hour, and suddenly got out of it - <braunr> something is really wrong in the pageout daemon, but it's not a - deadlock - <youpi> a livelock then - <braunr> do you get out of livelocks ? - <braunr> i mean, it's not even a "lock" - <braunr> just a big damn tricky slowdown - <youpi> yes, you can, by giving a few more resources for instance - <youpi> depends on the kind of livelock of course - <braunr> i think it's that - <braunr> the pageout daemon clearly throttles itself, waiting for pagers to - complete - <braunr> and another dangerous thing is the line in vm_resident, which only - wakes on thread to avoid starvation - <braunr> hum, during the livelock, the kernel spends much time waiting in - db_read_address - <braunr> could be a bad stack - <braunr> so, the pageout daemon seems to slow itself as much as waiting - several seconds between each iteration when under load - <braunr> but each iteration possibly removes clean pages - <braunr> so at some point, there is enough memory to unblock waiting pagers - <braunr> for now i'll try a simple solution, like limiting the pausing - delay - <braunr> but we'll need more page lists in the future (inactive-clean, - inactive-dirty, etc..) - <braunr> limiting the amount of dirty pages is the only way to really make - it safe actually - <braunr> wow, the pageout loop is still running even after many pages were - freed, and it unable to free more pages - <braunr> i think i have an idea about the livelock - <braunr> i think it comes from the periodic syncing - <bddebian> Too often? - <braunr> that's not the problem - <braunr> the problem is that it can happen at the same time with paging - <bddebian> Oh - <braunr> if paging gets slow, it won't stop the periodic syncing - <braunr> which will grab any page it can as soon as some are free - <braunr> but then, before it even finishes, another sync may occur - <braunr> i have yet to check that it is possible - <braunr> and i don't understand why syncing isn't done by the kernel - <braunr> the kernel is supposed to handle the paging policy - <braunr> and it would make paging really scale - <bddebian> It's done on the Hurd side? - <braunr> (instead of having external pagers make one request for each - object, even if they're clean) - <braunr> yes - <bddebian> Hmm, interesting - <braunr> ofc, with ext2fs --debug, i can't reproduce anything - <bddebian> Ugh - <braunr> sync are serialized - <braunr> grmbl - <braunr> there is a big lock taken at sync time though - <braunr> uhg - - -## IRC, freenode, #hurd, 2012-07-16 - - <braunr> all right so, there *is* a deadlock, and it may be due to the - default pager actually - <braunr> the vm_page_laundry_count doesn't decrease at some point, even - when there are more than enough free pages - <braunr> antrik: the thing is, i think the deadlock concerns the default - pager - <antrik> the deadlock? - <braunr> yes - <braunr> when swapping - - -## IRC, freenode, #hurd, 2012-07-17 - - <braunr> i can't even reproduce the swap deadlock when using upstrea ext2fs - :( - <braunr> upstream* - - -## IRC, freenode, #hurd, 2012-07-19 - - <braunr> the libpager deadlock patch looks wrong to me - <braunr> hm no, the libpager patch is ok acually - - -## [[synchronous_ipc]] - -### IRC, freenode, #hurd, 2012-07-20 - - <braunr> but actually after reviewing more, the debian patch for this - particular issue seems correct - <antrik> well, it's most probably done by youpi, so I would be shocked if - it wasn't correct... ;-) - <braunr> he wasn't sure at all about it - <antrik> still ;-) - <braunr> :) - <antrik> well, if you also think it's correct, I guess it's time to push it - upstream... - - -## IRC, freenode, #hurd, 2012-07-23 - - <braunr> i still can't conclude if we have any pageout deadlock, or if it's - simply a side effect of the active and inactive lists getting very very - large - <braunr> but almost every time this issue happens, it somehow recovers, - sometimes hours later - - -# See Also - - * [[ext2fs_deadlock]] |