linux-kernel - Re: [performance regression, bisected] scheduler: should_we

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Date:	Tue, 10 Sep 2013 16:15:20 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
Cc:	linux-kernel@...r.kernel.org, Paul Turner <pjt@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [performance regression, bisected] scheduler:
 should_we_balance() kills filesystem performance

On Tue, Sep 10, 2013 at 01:47:59PM +0900, Joonsoo Kim wrote:
> On Tue, Sep 10, 2013 at 02:02:54PM +1000, Dave Chinner wrote:
> > Hi folks,
> > 
> > I just updated my performance test VM to the current 3.12-git
> > tree after the XFS dev branch was merged. The first test I ran
> > which was a 16-way concurrent fsmark test to create lots of files
> > gave me a number about 30% lower than I expected - ~180k files/s
> > when I was expecting somewhere around 250k files/s.
> > 
> > I did a bisect, and the bisect landed on this commit:
> > 
> > commit 23f0d2093c789e612185180c468fa09063834e87
> > Author: Joonsoo Kim <iamjoonsoo.kim@....com>
> > Date:   Tue Aug 6 17:36:42 2013 +0900
> > 
> >     sched: Factor out code to should_we_balance()
.....
> > 
> > 			v4 filesystem		v5 filesystem
> > 3.11+xfsdev:		220k files/s		225k files/s
> > 3.12-git		180k files/s		185k files/s
> > 3.12-git-revert		245k files/s		247k files/s
> > 
> > The test vm is a 16p/16GB RAM VM, with a sparse 100TB filesystem
> > image sitting on a 4-way RAID0 SSD array formatted with XFS and the
> > image file is accessed by virtio+direct IO. The fsmark command line
> > is:
> > 
> > time ./fs_mark  -D  10000  -S0  -n  100000  -s  0  -L  32 \
> >         -d  /mnt/scratch/0  -d  /mnt/scratch/1 \
> >         -d  /mnt/scratch/2  -d  /mnt/scratch/3 \
> >         -d  /mnt/scratch/4  -d  /mnt/scratch/5 \
> >         -d  /mnt/scratch/6  -d  /mnt/scratch/7 \
> >         -d  /mnt/scratch/8  -d  /mnt/scratch/9 \
> >         -d  /mnt/scratch/10  -d  /mnt/scratch/11 \
> >         -d  /mnt/scratch/12  -d  /mnt/scratch/13 \
> >         -d  /mnt/scratch/14  -d  /mnt/scratch/15 \
> >         | tee >(stats --trim-outliers | tail -1 1>&2)
> > 
> > The workload on XFS runs to almost being CPU bound - the effect of
> > the above patch was that there was a lot of idle time left in the
> > system. The workload consumed the same amount of user and system
> > CPU, just instantaneous CPU usage was reduced by 20-30% and the
> > elaspsed time was increased by 20-30%.
> 
> Hello, Dave.
> 
> Now, I look again this patch and find one mistake.
> If we find that we are appropriate cpu for balancing, should_we_balance()
> should return 1. But current code doesn't do so. This correspond with
> your observation that a lot of idle time left.
> 
> Could you re-test your benchmark with below?

Sure. It looks like your patch fixes the problem:

			v4 filesystem		v5 filesystem
3.11+xfsdev:		220k files/s		225k files/s
3.12-git		180k files/s		185k files/s
3.12-git-revert		245k files/s		247k files/s
3.12-git-fix		249k files/s		248k files/s

Thanks for the quick turnaround :)

Tested-by: Dave Chinner <dchinner@...hat.com>

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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