linux-kernel - Re: performance "regression" in cfq compared to anticipatory, deadline and noop

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Date:	Tue, 13 May 2008 21:23:43 +0200
From:	Matthew <jackdachef@...il.com>
To:	"Jens Axboe" <jens.axboe@...cle.com>
Cc:	"Kasper Sandberg" <lkml@...anurb.dk>,
	"Daniel J Blueman" <daniel.blueman@...il.com>,
	"Linux Kernel" <linux-kernel@...r.kernel.org>
Subject: Re: performance "regression" in cfq compared to anticipatory, deadline and noop

On Tue, May 13, 2008 at 8:40 PM, Jens Axboe <jens.axboe@...cle.com> wrote:
>
> On Tue, May 13 2008, Jens Axboe wrote:
>  > On Tue, May 13 2008, Matthew wrote:
>  > > On Tue, May 13, 2008 at 3:05 PM, Jens Axboe <jens.axboe@...cle.com> wrote:
>  > > >
>  > > > On Tue, May 13 2008, Matthew wrote:
>  > > >  > On Tue, May 13, 2008 at 2:20 PM, Jens Axboe <jens.axboe@...cle.com> wrote:
>  > > >  > >
>  > > >  > > On Sun, May 11 2008, Kasper Sandberg wrote:
>  > > >  > >  > On Sun, 2008-05-11 at 14:14 +0100, Daniel J Blueman wrote:
>  > > >  > >  > > I've been experiencing this for a while also; an almost 50% regression
>  > > >  > >  > > is seen for single-process reads (ie sync) if slice_idle is 1ms or
>  > > >  > >  > > more (eg default of 8) [1], which seems phenomenal.
>  > > >  > >  > >
>  > > >  > >  > > Jens, is this the expected price to pay for optimal busy-spindle
>  > > >  > >  > > scheduling, a design issue, bug or am I missing something totally?
>  > > >  > >  > >
>  > > >  > >  > > Thanks,
>  > > >  > >  > >   Daniel
>  > > >  > [snip]
>  > > >  > ...
>  > > >  > [snip]
>  > > >  > >  >
>  > > [snip]
>  > >
>  > > ...
>  > >
>  > > [snip]
>  > > >  > well - back to topic:
>  > > >  >
>  > > >  > for a blktrace one need to enable  CONFIG_BLK_DEV_IO_TRACE , right ?
>  > > >  > blktrace can be obtained from your git-repo ?
>  > > >
>  > > >  Yes on both accounts, or just grab a blktrace snapshot from:
>  > > >
>  > > >  http://brick.kernel.dk/snaps/blktrace-git-latest.tar.gz
>  > > >
>  > > >  if you don't use git.
>  > > >
>  > > >  --
>  > > >  Jens Axboe
>  > > >
>  > > >
>  > >
[snip]
...
[snip]
>  >
>  > They seem to start out the same, but then CFQ gets interrupted by a
>  > timer unplug (which is also odd) and after that the request size drops.
>  > On most devices you don't notice, but some are fairly picky about
>  > request sizes. The end result is that CFQ has an average dispatch
>  > request size of 142kb, where AS is more than double that at 306kb. I'll
>  > need to analyze the data and look at the code a bit more to see WHY this
>  > happens.
>
>  Here's a test patch, I think we get into this situation due to CFQ being
>  a bit too eager to start queuing again. Not tested, I'll need to spend
>  some testing time on this. But I'd appreciate some feedback on whether
>  this changes the situation! The final patch will be a little more
>  involved.
[snip]
...
[snip]
>
>  --
>  Jens Axboe
>
>

unfortunately that patch didn't help:

hdparm -t /dev/sde

/dev/sde:
 Timing buffered disk reads:  178 MB in  3.03 seconds =  58.67 MB/sec


hdparm -t /dev/sdd

/dev/sdd:
 Timing buffered disk reads:  164 MB in  3.00 seconds =  54.61 MB/sec

-> the first should be around 74 MB/sec, the second around 102 MB/sec

Thanks

Mat
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