linux-kernel - Re: IO scheduler based IO controller V10

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Date:	Mon, 28 Sep 2009 20:24:57 +0200
From:	Mike Galbraith <efault@....de>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Jens Axboe <jens.axboe@...cle.com>,
	Ulrich Lukas <stellplatz-nr.13a@...enparkplatz.de>,
	linux-kernel@...r.kernel.org,
	containers@...ts.linux-foundation.org, dm-devel@...hat.com,
	nauman@...gle.com, dpshah@...gle.com, lizf@...fujitsu.com,
	mikew@...gle.com, fchecconi@...il.com, paolo.valente@...more.it,
	ryov@...inux.co.jp, fernando@....ntt.co.jp, jmoyer@...hat.com,
	dhaval@...ux.vnet.ibm.com, balbir@...ux.vnet.ibm.com,
	righi.andrea@...il.com, m-ikeda@...jp.nec.com, agk@...hat.com,
	akpm@...ux-foundation.org, peterz@...radead.org,
	jmarchan@...hat.com, torvalds@...ux-foundation.org, mingo@...e.hu,
	riel@...hat.com
Subject: Re: IO scheduler based IO controller V10

On Mon, 2009-09-28 at 13:48 -0400, Vivek Goyal wrote:

> Hmm.., so close to 25% reduction on average in completion time of konsole.
> But this is in presece of writer. Does this help even in presence of 1 or
> more sequential readers going?

Dunno, I've only tested sequential writer.

> So here latency seems to be coming from three sources.
> 
> - Wait in CFQ before request is dispatched (only in case of competing seq readers).
> - seek latencies
> - latencies because of bigger requests are already dispatched to disk.
> 
> So limiting the size of request will help with third factor but not with first  
> two factors and here seek latencies seem to be the biggest contributor.

Yeah, seek latency seems to dominate.

	-Mike

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