From: Mike Galbraith <mikeg@wen-online.de>
To: Linus Torvalds <torvalds@transmeta.com>
Cc: Jens Axboe <axboe@suse.de>, MOLNAR Ingo <mingo@chiara.elte.hu>,
Rik van Riel <H.H.vanRiel@phys.uu.nl>,
Kernel Mailing List <linux-kernel@vger.kernel.org>,
Alan Cox <alan@lxorguk.ukuu.org.uk>
Subject: Re: [BUG] /proc/<pid>/stat access stalls badly for swapping process, 2.4.0-test10
Date: Fri, 10 Nov 2000 08:34:14 +0100 (CET) [thread overview]
Message-ID: <Pine.Linu.4.10.10011100732250.601-100000@mikeg.weiden.de> (raw)
In-Reply-To: <Pine.LNX.4.10.10011091005390.1909-100000@penguin.transmeta.com>
On Thu, 9 Nov 2000, Linus Torvalds wrote:
>
>
> As to the real reason for stalls on /proc/<pid>/stat, I bet it has nothing
> to do with IO except indirectly (the IO is necessary to trigger the
> problem, but the _reason_ for the problem lies elsewhere).
>
> And it has everything to do with the fact that the way Linux semaphores
> are implemented, a non-blocking process has a HUGE advantage over a
> blocking one. Linux kernel semaphores are extreme unfair in that way.
>
> What happens is that some process is getting a lot of VM faults and gets
> its VM semaphore. No contention yet. it holds the semaphore over the
> IO, and now another process does a "ps".
>
> The "ps" process goes to sleep on the semaphore. So far so good.
>
> The original process releases the semaphore, which increments the count,
> and wakes up the process waiting for it. Note that it _wakes_ it, it does
> not give the semaphore to it. Big difference.
>
> The process that got woken up will run eventually. Probably not all that
> immediately, because the process that woke it (and held the semaphore)
> just slept on a page fault too, so it's not likely to immediately
> relinquish the CPU.
>
> The original running process comes back faulting again, finds the
> semaphore still unlocked (the "ps" process is awake but has not gotten to
> run yet), gets the semaphore, and falls asleep on the IO for the next
> page.
>
> The "ps" process actually gets to run now, but it's a bit late. The
> semaphore is locked again.
>
> Repeat until luck breaks the bad circle.
>
> (This schenario, btw, is much harder to trigger on SMP than on UP. And
> it's completely separate from the issue of simple disk bandwidth issues
> which can obviously cause no end of stalls on anything that needs the
> disk, and which can also happen on SMP).
Unfortunately, it didn't help in the scenario I'm running.
time make -j30 bzImage:
real 14m19.987s (within stock variance)
user 6m24.480s
sys 1m12.970s
procs memory swap io system cpu
r b w swpd free buff cache si so bi bo in cs us sy id
31 2 1 12 1432 4440 12660 0 12 27 151 202 848 89 11 0
34 4 1 1908 2584 536 5376 248 1904 602 763 785 4094 63 32 5
13 19 1 64140 67728 604 33784 106500 84612 43625 21683 19080 52168 28 22 50
I understood the above well enough to be very interested in seeing what
happens with flush IO restricted.
-Mike
[try_to_free_pages()->swap_out()/shm_swap().. can fight over who gets
to shrink the best candidate's footprint?]
Thanks!
-
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next prev parent reply other threads:[~2000-11-10 7:34 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <Pine.Linu.4.10.10011091452270.747-100000@mikeg.weiden.de>
2000-11-09 18:31 ` [BUG] /proc/<pid>/stat access stalls badly for swapping process, 2.4.0-test10 Linus Torvalds
2000-11-10 7:34 ` Mike Galbraith [this message]
2000-11-10 10:47 ` Mike Galbraith
2000-11-10 17:07 ` Linus Torvalds
2000-11-10 21:42 ` [BUG] /proc/<pid>/stat access stalls badly for swapping process,2.4.0-test10 David Mansfield
2000-11-11 6:20 ` Linus Torvalds
2000-11-01 18:38 [BUG] /proc/<pid>/stat access stalls badly for swapping process, 2.4.0-test10 David Mansfield
2000-11-01 18:48 ` Rik van Riel
2000-11-02 7:19 ` Mike Galbraith
2000-11-02 21:59 ` Val Henson
2000-11-03 1:37 ` Jens Axboe
2000-11-03 5:56 ` Mike Galbraith
2000-11-03 15:45 ` Mike Galbraith
2000-11-03 19:38 ` Jens Axboe
2000-11-04 5:43 ` Mike Galbraith
2000-11-02 8:40 ` Christoph Rohland
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