linux-kernel - Re: [RFC PATCH] cfq-iosced: Implement IOPS mode and group

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Message-ID: <20100726143023.GF12449@redhat.com>
Date:	Mon, 26 Jul 2010 10:30:23 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Corrado Zoccolo <czoccolo@...il.com>
Cc:	Christoph Hellwig <hch@...radead.org>,
	linux-kernel@...r.kernel.org, axboe@...nel.dk, nauman@...gle.com,
	dpshah@...gle.com, guijianfeng@...fujitsu.com, jmoyer@...hat.com
Subject: Re: [RFC PATCH] cfq-iosced: Implement IOPS mode and group_idle
 tunable V3

On Sat, Jul 24, 2010 at 11:07:07AM +0200, Corrado Zoccolo wrote:
> On Sat, Jul 24, 2010 at 10:51 AM, Christoph Hellwig <hch@...radead.org> wrote:
> > To me this sounds like slice_idle=0 is the right default then, as it
> > gives useful behaviour for all systems linux runs on.
> No, it will give bad performance on single disks, possibly worse than
> deadline (deadline at least sorts the requests between different
> queues, while CFQ with slice_idle=0 doesn't even do this for readers).

> Setting slice_idle to 0 should be considered only when a single
> sequential reader cannot saturate the disk bandwidth, and this happens
> only on smart enough hardware with large number of spindles.

I was thinking of writting a user space utility which can launch
increasing number of parallel direct/buffered reads from device and if
device can sustain more than 1 parallel reads with increasing throughput,
then it probably is good indicator that one might be better off with
slice_idle=0.

Will try that today...

Vivek
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