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

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Date:	Thu, 26 Mar 2009 15:36:18 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Arjan van de Ven <arjan@...radead.org>
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, 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).

-- 
Jens Axboe

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