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SubjectRe: tiobench read 50% regression with 2.6.30-rc1
On Thu, Apr 09 2009, Zhang, Yanmin wrote:
> Comparing with 2.6.29's result, tiobench (read) has about 50% regression
> with 2.6.30-rc1 on all my machines. Bisect down to below patch.
>
> b029195dda0129b427c6e579a3bb3ae752da3a93 is first bad commit
> commit b029195dda0129b427c6e579a3bb3ae752da3a93
> Author: Jens Axboe <jens.axboe@oracle.com>
> Date: Tue Apr 7 11:38:31 2009 +0200
>
> cfq-iosched: don't let idling interfere with plugging
>
> When CFQ is waiting for a new request from a process, currently it'll
> immediately restart queuing when it sees such a request. This doesn't
> work very well with streamed IO, since we then end up splitting IO
> that would otherwise have been merged nicely. For a simple dd test,
> this causes 10x as many requests to be issued as we should have.
> Normally this goes unnoticed due to the low overhead of requests
> at the device side, but some hardware is very sensitive to request
> sizes and there it can cause big slow downs.
>
>
>
> Command to start the testing:
> #tiotest -k0 -k1 -k3 -f 80 -t 32
>
> It's a multi-threaded program and starts 32 threads. Every thread does I/O
> on its own 80MB file.

It's not a huge surprise that we regressed there. I'll get this fixed up
next week. Can you I talk you into trying to change the 'quantum' sysfs
variable for the drive? It's in /sys/block/xxx/queue/iosched where xxx
is your drive(s). It's set to 4, if you could try progressively larger
settings and retest, that would help get things started.

Thanks!

--
Jens Axboe



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