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

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Message-ID: <AANLkTinwcGdvp-Q+-dqNHAjL1O4oDbi-8GkyyV=k6-Pc@mail.gmail.com>
Date:	Thu, 29 Jul 2010 21:57:43 +0200
From:	Corrado Zoccolo <czoccolo@...il.com>
To:	Vivek Goyal <vgoyal@...hat.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 Mon, Jul 26, 2010 at 4:33 PM, Vivek Goyal <vgoyal@...hat.com> wrote:
> 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.

I was thinking more to the fact that read requests are not sorted:
they will basically be serviced in FIFO order, while deadline will
sort them and possibly increase locality. In the reader vs writer
case, cfq may have a small edge. Basically, they will severely
underperform vs. cfq with slice != 0, though.

>
> Vivek
>



-- 
__________________________________________________________________________

dott. Corrado Zoccolo                          mailto:czoccolo@...il.com
PhD - Department of Computer Science - University of Pisa, Italy
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