linux-kernel - Re: Performance regression in IO scheduler still there

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Date:	Tue, 10 Nov 2009 11:47:30 -0500
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Corrado Zoccolo <czoccolo@...il.com>
Cc:	Jan Kara <jack@...e.cz>, jens.axboe@...cle.com,
	LKML <linux-kernel@...r.kernel.org>,
	Chris Mason <chris.mason@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mike Galbraith <efault@....de>
Subject: Re: Performance regression in IO scheduler still there

Corrado Zoccolo <czoccolo@...il.com> writes:

> Jeff, Jens,
> do you think we should try to do more auto-tuning of cfq parameters?
> Looking at those numbers for SANs, I think we are being suboptimal in
> some cases.
> E.g. sequential read throughput is lower than random read.

I investigated this further, and this was due to a problem in the
benchmark.  It was being run with only 500 samples for random I/O and
65536 samples for sequential.  After fixing this, we see random I/O is
slower than sequential, as expected.

> I also think that current slice_idle and slice_sync values are good
> for devices with 8ms seek time, but they are too high for non-NCQ
> flash devices, where "seek" penalty is under 1ms, and we still prefer
> idling.

Do you have numbers to back that up?  If not, throw a fio job file over
the fence and I'll test it on one such device.

> If we agree on this, should the measurement part (I'm thinking to
> measure things like seek time, throughput, etc...) be added to the
> common elevator code, or done inside cfq?

Well, if it's something that is of interest to others, than pushing it
up a layer makes sense.  If only CFQ is going to use it, keep it there.

Cheers,
Jeff
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