linux-kernel - Re: tiobench read 50% regression with 2.6.30-rc1

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Date:	Thu, 9 Apr 2009 11:57:06 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: 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@...cle.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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