linux-kernel - Re: ext3 IO latency measurements (was: Linux 2.6.29)

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Date:	Thu, 26 Mar 2009 07:49:55 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Jan Kara <jack@...e.cz>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Theodore Tso <tytso@....edu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Nick Piggin <npiggin@...e.de>, David Rees <drees76@...il.com>,
	Jesper Krogh <jesper@...gh.cc>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Roland McGrath <roland@...hat.com>
Subject: Re: ext3 IO latency measurements (was: Linux 2.6.29)

On Thu, 26 Mar 2009 15:36:18 +0100
Jens Axboe <jens.axboe@...cle.com> wrote:

> On Thu, Mar 26 2009, Arjan van de Ven wrote:
> > On Thu, 26 Mar 2009 12:08:15 +0100
> > Jens Axboe <jens.axboe@...cle.com> wrote:
> > > 
> > > Did you capture the trace of the long delays in the read test
> > > case? It can be two things, at least. One is that each little
> > > read takes much longer than it should, the other is that we get
> > > stuck waiting on a dirty page and hence that slows down the reads
> > > a lot.
> > > 
> > 
> > would be interesting to run latencytop during such runs...
> 
> Things just drown in the noise in there sometimes, is my experience...
> And disappear. Perhaps I'm just not very good at reading the output,
> but it was never very useful for me. I know, not a very useful
> complaint, I'll try and use it again and come up with something more
> productive :-)
> 
> But in this case, I bet it's also the atime updates. If Ingo turned
> those off, the read results would likely be a lot better (and
> consistent).

latencytop does identify those for sure.
> 


-- 
Arjan van de Ven 	Intel Open Source Technology Centre
For development, discussion and tips for power savings, 
visit http://www.lesswatts.org
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