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

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Date:	Tue, 13 May 2008 15:05:09 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Matthew <jackdachef@...il.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, 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]
> >  >
> >  > Thisd would appear to be quite a considerable performance difference.
> >
> >  Indeed, that is of course a bug. The initial mail here mentions this as
> >  a regression - which kernel was the last that worked ok?
> >
> >  If someone would send me a blktrace of such a slow run, that would be
> >  nice. Basically just do a blktrace /dev/sda (or whatever device) while
> >  doing the hdparm, preferably storing output files on a difference
> >  device. Then send the raw sda.blktrace.* files to me. Thanks!
> >
> >  --
> >  Jens Axboe
> >
> >
> 
> Hi Jens,
> 
> I called this a "regression" since I wasn't sure if this is a real bug
> or just something introduced recently, I just started to use cfq as
> main io-scheduler so I can't tell ...
> 
> testing 2.6.17 unfortunately is somewhat impossible for me (reiser4;
> too new hardware - problems with jmicron)
> 
> google "says" that it seemingly already existed since at least 2.6.18
> (Ubuntu DapperDrake) [see:
> http://ubuntuforums.org/showpost.php?p=1484633&postcount=12]

Funky :/

> 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

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