linux-kernel - Re: cfq performance gap

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Message-Id: <1165615793.9200.11.camel@dyn9047017105.beaverton.ibm.com>
Date:	Fri, 08 Dec 2006 14:09:53 -0800
From:	Avantika Mathur <mathur@...ibm.com>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: cfq performance gap

On Fri, 2006-12-08 at 13:05 +0100, Jens Axboe wrote:
> On Thu, Dec 07 2006, Avantika Mathur wrote:
> > Hi Jens,
> 
> (you probably noticed now, but the axboe@...e.de email is no longer
> valid)

I saw that, thanks!
> > I've noticed a performance gap between the cfq scheduler and other io
> > schedulers when running the rawio benchmark.
> > Results from rawio on 2.6.19, cfq and noop schedulers:
> >
> > CFQ:
> >
> > procs           device    num read   KB/sec     I/O Ops/sec
> > -----  ---------------  ----------  -------  --------------
> >   16         /dev/sda       16412     8338            2084
> > -----  ---------------  ----------  -------  --------------
> >   16                        16412     8338            2084
> >
> > Total run time 0.492072 seconds
> >
> >
> > NOOP:
> >
> > procs           device    num read   KB/sec     I/O Ops/sec
> > -----  ---------------  ----------  -------  --------------
> >   16         /dev/sda       16399    29224            7306
> > -----  ---------------  ----------  -------  --------------
> >   16                        16399    29224            7306
> >
> > Total run time 0.140284 seconds
> >
> > The benchmark workload is 16 processes running 4k random reads.
> >
> > Is this performance gap a known issue?
> 
> CFQ could be a little slower at this benchmark, but your results are
> much worse than I would expect. What is the queueing depth of sda? How
> are you invoking rawio?

I am running rawio with the following options:
rawread -p 16 -m 1 -d 1 -x -z -t 0 -s 4096
 
The queue depth on sda is 4.

> 
> Your runtime is very low, how does it look if you allow the test to run
> for much longer? 30MiB/sec random read bandwidth seems very high, I'm
> wondering what exactly is being tested here.
> 

rawio is actually performing sequential reads, but I don't believe it is
purely sequential with the multiple processes.
I am currently running the test with longer runtimes and will post
results once it is complete. 
I've also attached the rawio source.

Thanks,
Avantika


Download attachment "rawio-2.4.2.tar.gz" of type "application/x-compressed-tar" (13100 bytes)