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

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Message-ID: <20100726143300.GG12449@redhat.com>
Date:	Mon, 26 Jul 2010 10:33:00 -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).

Not sure if CFQ will be worse than deadline with slice_idle=0. CFQ has
some inbuilt things which should help.

- Readers preempt Writers
- All writers go in one single queue (at one prio level), readers get
  their individual queues and can outnumber writers.

So I guess CFQ with slice_idle=0 should not be worse than deadline in terms
of read latencies.

Vivek
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