linux-ext4 - Re: ext4 unlink performance

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Date:	Thu, 13 Nov 2008 22:11:21 -0600
From:	Bruce Guenter <lists-linux-ext4@...ce-guenter.dyndns.org>
To:	linux-ext4@...r.kernel.org
Subject: Re: ext4 unlink performance

On Thu, Nov 13, 2008 at 03:42:40PM -0500, Theodore Tso wrote:
> Hmm... how very strange.  Can you run the command:
> 
>        ps -eo pid,tid,class,rtprio,ni,pri,psr,pcpu,stat,wchan:16,comm
> 
> during one of the quiescent periods, and see what is in the WCHAN
> field for the unlink command?

Mostly I see this:

  PID   TID CLS RTPRIO  NI PRI PSR %CPU STAT WCHAN            COMMAND
 5932  5932 TS       -   0  19   0  2.4 D+   sync_buffer      rm

I had been running these benchmarks over dm-crypt (which is the target
environment for which I was testing).  So I re-ran the tests both bare
and with dm-crypt to compare.  The stalls reported by vmstat did not
show up when running the test bare.

I also sat and watched the drive LED while the unlink test was running.
The drive LED showed the occasional 1/2 second stall, but at the same
time vmstat was showing 5-20 second stalls, so there that would seem to
point to some kind of reporting problem.

For completeness, I re-ran the benchmark both with and without dm-crypt
to see if it was the cause of the problem, all times mounting with
barrier=0.  As expected, it did slow down the process, but the problem
remains:

ext4  128  plain  unlink:  665.523   elapsed  0.276  user  34.882  sys  5.28%
ext4  128  crypt  unlink:  907.934   elapsed  0.356  user  34.698  sys  3.86%
ext4  256  plain  unlink:  1435.964  elapsed  0.248  user  40.319  sys  2.82%
ext4  256  crypt  unlink:  1504.660  elapsed  0.304  user  35.186  sys  2.35%
ext3  128  plain  unlink:  133.863   elapsed  0.248  user  24.618  sys  18.57%
ext3  128  crypt  unlink:  133.092   elapsed  0.280  user  23.661  sys  17.98%
ext3  256  plain  unlink:  309.635   elapsed  0.296  user  27.362  sys  8.93%
ext3  256  crypt  unlink:  319.819   elapsed  0.268  user  23.713  sys  7.49%

Is there anything else I can try to see what's happening?

-- 
Bruce Guenter <bruce@...roubled.org>                http://untroubled.org/

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