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

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Date:	Fri, 2 Oct 2009 07:24:41 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Jens Axboe <jens.axboe@...cle.com>
cc:	Ingo Molnar <mingo@...e.hu>, 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

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.

And Mike's patch didn't look big or complicated. 

> You can't say it's black and white latency vs throughput issue,

Umm. Almost 1000% vs 30%. Forget latency vs throughput. That's pretty damn 
black-and-white _regardless_ of what you're measuring. Plus you probably 
made up the 30% - have you tested the patch?

And quite frankly, we get a _lot_ of complaints about latency. A LOT. It's 
just harder to measure, so people seldom attach numbers to it. But that 
again means that when people _are_ able to attach numbers to it, we should 
take those numbers _more_ seriously rather than less.

So the 30% you threw out as a number is pretty much worthless. 

		Linus
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