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

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Date:	Fri, 10 Apr 2009 10:29:01 +0800
From:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: tiobench read 50% regression with 2.6.30-rc1

On Thu, 2009-04-09 at 11:57 +0200, Jens Axboe wrote:
> 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.
The files should be created before the testing and pls. drop page caches
by "echo 3 >/proc/sys/vm/drop_caches" before testing.

> 
> 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.
I tried 4,8,16,64,128 and didn't find result difference.


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