Re: cfq performance gap

["AVANTIKA R. MATHUR" <mathur@linux.vnet.ibm.com>]

Jens Axboe wrote:
> On Fri, Dec 08 2006, Avantika Mathur wrote:
>   
> > 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@suse.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.
> > > >
> > > > 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.
> >     
>
> It's certainly the slice and idling hurting here. But at the same time,
> I don't really think your test case is very interesting. The test area
> is very small and you have 16 threads trying to read the same thing,
> optimizing for that would be silly as I don't think it has much real
> world relevance.
>
>   
Could a database have similar workload to this test?
> That said, I might add some logic to detect when we can cheaply switch
> queues instead of waiting for a new request from the same queue.
> Averaging slice times over a period of time instead of 1:1 with that
> logic, should help cases like this while still being fair.
>   
Thank you for looking at this issue.
I've found an IBM/SUSE bugzilla bug for the same performance gap on 
rawio. There was a fix for this bug included in SLES10-RC1, do you know 
why it was not added in mainline?

Thanks again,
Avantika Mathur
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