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Date: Wed, 1 Nov 2000 16:48:04 -0200 (BRDT)
From: Rik van Riel <riel@conectiva.com.br>
To: David Mansfield <lkml@dm.ultramaster.com>
cc: 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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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)

regards,

Rik
--
"What you're running that piece of shit Gnome?!?!"
       -- Miguel de Icaza, UKUUG 2000

http://www.conectiva.com/		http://www.surriel.com/

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