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

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Date:	Thu, 26 Mar 2009 17:40:37 -0700
From:	Jesse Barnes <jbarnes@...tuousgeek.org>
To:	Theodore Tso <tytso@....edu>
Cc:	Ingo Molnar <mingo@...e.hu>, Jan Kara <jack@...e.cz>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Arjan van de Ven <arjan@...radead.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Nick Piggin <npiggin@...e.de>,
	Jens Axboe <jens.axboe@...cle.com>,
	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 19:59:36 -0400
Theodore Tso <tytso@....edu> wrote:

> On Fri, Mar 27, 2009 at 12:02:40AM +0100, Ingo Molnar wrote:
> > This isnt me streaming gigs of data in and out of the system 
> > dirtying 90% of all RAM. This is a trivial workload barely 
> > scratching the RAM and CPU capabilities of the system.
> 
> Have you tried with maxcpus set to say, 2?  My guess is you won't see
> the problems in that case.  So I'm not sure saying "barely scratching
> the CPU capabilities of the system" is completely fair.  I can
> probably get be able to get temporary access to a 16 CPU system, but
> that's not the kind of system that I normally get to use for my kernel
> devleopment.

Nope, I saw this with my dual CPU machine too (before I upgraded to
quad core)... Just doing kernel builds and/or icecream and/or VMware.
It didn't take much.  I have 8G of memory now but I used to have less
(3G iirc) and saw it then too.

> My normal development is not all that different from yours (make
> -j<numcpus*2>) and I do edit and save files while the compile is
> going.  I use emacs, but it calls fsync() when saving files, just like
> vim does.  The big difference is that for me, numcpus is normally 2.
> And my machine has 4 gigs of memory, not 12 gigs.  So I don't see
> these problems.  I agree that what you have isn't an "oddball
> workload"; as far as whether it is an "oddball system", it is
> certainly a system I would lust after.  And I acknowledge the world is
> a bit different from when Linus declared that 99% of the world was 1
> or 2 CPU's.  I suspect the percentage of machines with 16 CPU's is
> still somewhat small, though.

I'm surprised you haven't seen this then... Maybe your journal is
bigger?  Or some other config difference...

-- 
Jesse Barnes, Intel Open Source Technology Center
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