linux-kernel - Re: CFQ read performance regression

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Date:	Thu, 22 Apr 2010 17:53:59 +0200
From:	Jan Kara <jack@...e.cz>
To:	Miklos Szeredi <mszeredi@...e.cz>
Cc:	Corrado Zoccolo <czoccolo@...il.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Jan Kara <jack@...e.cz>, Suresh Jayaraman <sjayaraman@...e.de>
Subject: Re: CFQ read performance regression

On Thu 22-04-10 12:23:29, Miklos Szeredi wrote:
> > >
> > > This is on a vanilla 2.6.34-rc4 kernel with two tunables modified:
> > >
> > > read_ahead_kb=512
> > > low_latency=0 (for CFQ)
> > You should get much better throughput by setting
> > /sys/block/_your_disk_/queue/iosched/slice_idle to 0, or
> > /sys/block/_your_disk_/queue/rotational to 0.
> 
> slice_idle=0 definitely helps.  rotational=0 seems to help on 2.6.34-rc
> but not on 2.6.32.
> 
> As far as I understand setting slice_idle to zero is just a workaround
> to make cfq look at all the other queues instead of serving one
> exclusively for a long time.
  Yes, basically it disables idling (i.e., waiting whether a thread sends
more IO so that we can get better IO locality).

> 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").

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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