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Date: Thu, 2 Nov 2000 08:19:06 +0100 (CET)
From: Mike Galbraith <mikeg@wen-online.de>
To: Rik van Riel <riel@conectiva.com.br>
cc: David Mansfield <lkml@dm.ultramaster.com>,
        lkml <linux-kernel@vger.kernel.org>
Subject: Re: [BUG] /proc/<pid>/stat access stalls badly for swapping process,
  2.4.0-test10
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Message-ID: <Pine.Linu.4.10.10011020800010.1299-100000@mikeg.weiden.de>
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On Wed, 1 Nov 2000, Rik van Riel wrote:

> On Wed, 1 Nov 2000, David Mansfield wrote:
> 
> > I'd like to report what seems like a performance problem in the latest
> > kernels.  Actually, all recent kernels have exhibited this problem, but
> > I was waiting for the new VM stuff to stabilize before reporting it. 
> > 
> > My test is: run 7 processes that each allocate and randomly
> > access 32mb of ram (on a 256mb machine).  Even though 7*32MB =
> > 224MB, this still sends the machine lightly into swap.  The
> > machine continues to function fairly smoothly for the most part.  
> > I can do filesystem operations, run new programs, move desktops
> > in X etc.
> > 
> > Except: programs which access /proc/<pid>/stat stall for an
> > inderminate amount of time.  For example, 'ps' and 'vmstat'
> > stall BADLY in these scenarios.  I have had the stalls last over
> > a minute in higher VM pressure situations.
> 
> I have one possible reason for this ....
> 
> 1) the procfs process does (in fs/proc/array.c::proc_pid_stat)
> 	down(&mm->mmap_sem);
> 
> 2) but, in order to do that, it has to wait until the process
>    it is trying to stat has /finished/ its page fault, and is
>    not into its next one ...
> 
> 3) combine this with the elevator starvation stuff (ask Jens
>    Axboe for blk-7 to alleviate this issue) and you have a
>    scenario where processes using /proc/<pid>/stat have the
>    possibility to block on multiple processes that are in the
>    process of handling a page fault (but are being starved)

I'm experimenting with blk.[67] in test10 right now.  The stalls
are not helped at all.  It doesn't seem to become request bound
(haven't instrumented that yet to be sure) but the stalls persist.

	-Mike

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