linux-kernel - Re: writeout stalls in current -git

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Date:	Wed, 7 Nov 2007 10:31:14 +1100
From:	David Chinner <dgc@....com>
To:	Torsten Kaiser <just.for.lkml@...glemail.com>
Cc:	Fengguang Wu <wfg@...l.ustc.edu.cn>,
	Peter Zijlstra <peterz@...radead.org>,
	Maxim Levitsky <maximlevitsky@...il.com>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Chinner <dgc@....com>, linux-fsdevel@...r.kernel.org
Subject: Re: writeout stalls in current -git

On Tue, Nov 06, 2007 at 10:53:25PM +0100, Torsten Kaiser wrote:
> On 11/6/07, David Chinner <dgc@....com> wrote:
> > Rather than vmstat, can you use something like iostat to show how busy your
> > disks are?  i.e. are we seeing RMW cycles in the raid5 or some such issue.
> 
> Both "vmstat 10" and "iostat -x 10" output from this test:
> procs -----------memory---------- ---swap-- -----io---- -system-- ----cpu----
>  r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa
>  2  0      0 3700592      0  85424    0    0    31    83  108  244  2  1 95  1
> -> emerge reads something, don't knwo for sure what...
>  1  0      0 3665352      0  87940    0    0   239     2  343  585  2  1 97  0
....
> 
> The last 20% of the btrace look more or less completely like this, no
> other programs do any IO...
> 
> 253,0    3   104626   526.293450729   974  C  WS 79344288 + 8 [0]
> 253,0    3   104627   526.293455078   974  C  WS 79344296 + 8 [0]
> 253,0    1    36469   444.513863133  1068  Q  WS 154998480 + 8 [xfssyncd]
> 253,0    1    36470   444.513863135  1068  Q  WS 154998488 + 8 [xfssyncd]
                                                ^^
Apparently we are doing synchronous writes. That would explain why
it is slow. We shouldn't be doing synchronous writes here. I'll see if
I can reproduce this.

<goes off and looks>

Yes, I can reproduce the sync writes coming out of xfssyncd. I'll
look into this further and send a patch when I have something concrete.

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group
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