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

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Message-ID: <20091002162240.GA12546@elte.hu>
Date:	Fri, 2 Oct 2009 18:22:40 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Jens Axboe <jens.axboe@...cle.com>, Mike Galbraith <efault@....de>,
	Vivek Goyal <vgoyal@...hat.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, riel@...hat.com
Subject: Re: IO scheduler based IO controller V10


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Fri, 2 Oct 2009, Jens Axboe wrote:
> > 
> > It's really not that simple, if we go and do easy latency bits, then 
> > throughput drops 30% or more.
> 
> Well, if we're talking 500-950% improvement vs 30% deprovement, I 
> think it's pretty clear, though. Even the server people do care about 
> latencies.
> 
> Often they care quite a bit, in fact.

The other thing is that latency is basically a given property in any 
system - as an app writer you have to live with it, there's not much you 
can do to improve it.

Bandwidth on the other hand is a lot more engineerable, as it tends to 
be about batching things and you can batch in user-space too. Batching 
is often easier to do than getting good latencies.

Then there's also the fact that the range of apps that care about 
bandwidth is a lot smaller than the range of apps which care about 
latencies. The default should help more apps - i.e. latencies.

	Ingo
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