linux-kernel - Re: CFQ read performance regression

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Date:	Fri, 23 Apr 2010 12:48:17 +0200
From:	Miklos Szeredi <mszeredi@...e.cz>
To:	Jan Kara <jack@...e.cz>
Cc:	Corrado Zoccolo <czoccolo@...il.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Suresh Jayaraman <sjayaraman@...e.de>
Subject: Re: CFQ read performance regression

On Thu, 2010-04-22 at 17:53 +0200, Jan Kara wrote:
> On Thu 22-04-10 12:23:29, Miklos Szeredi wrote:
> > I have very little understanding of I/O scheduling but my idea of what's
> > really needed here is to realize that one queue is not able to saturate
> > the device and there's a large backlog of requests on other queues that
> > are waiting to be served.  Is something like that implementable?
>   I see a problem with defining "saturate the device" - but maybe we could
> measure something like "completed requests / sec" and try autotuning
> slice_idle to maximize this value (hopefully the utility function should
> be concave so we can just use "local optimization").

Yeah, detecting saturation may be difficult.

I guess that function depends on a lot of other things as well,
including seek times, etc.  Not easy to optimize.

I'm still wondering what makes such a difference between CFQ on 2.6.16
and CFQ on 2.6.27-34, why is the one in older kernels performing so much
better in this situation?

What should we tell our customers?  The default settings should at least
handle these systems a bit better.

Thanks,
Miklos

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