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From: Nick Piggin <nickpiggin@yahoo.com.au>
To: "Alex Nixon (Intern)" <Alex.Nixon@eu.citrix.com>
Subject: Re: Large increase in context switch rate
Date: Thu, 24 Jul 2008 11:26:48 +1000
User-Agent: KMail/1.9.5
Cc: "Andi Kleen" <andi@firstfloor.org>,
       "Jeremy Fitzhardinge" <jeremy@goop.org>,
       "Peter Zijlstra" <peterz@infradead.org>, "Ingo Molnar" <mingo@elte.hu>,
       "Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
       "Ian Campbell" <Ian.Campbell@eu.citrix.com>,
       "Theodore Ts'o" <tytso@mit.edu>
References: <487E43D9.7080703@goop.org> <87mykgrxtv.fsf@basil.nowhere.org> <0E902970173AF84089673FA54B7FE78A329073@lonpexch01.citrite.net>
In-Reply-To: <0E902970173AF84089673FA54B7FE78A329073@lonpexch01.citrite.net>
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On Wednesday 23 July 2008 19:34, Alex Nixon (Intern) wrote:
> > -----Original Message-----
> > From: Andi Kleen [mailto:andi@firstfloor.org]
> > Sent: 17 July 2008 22:43
> > To: Jeremy Fitzhardinge
> > Cc: Alex Nixon (Intern); Peter Zijlstra; Ingo Molnar; Linux
> > Kernel Mailing List; Ian Campbell
> > Subject: Re: Large increase in context switch rate
> >
> > Jeremy Fitzhardinge <jeremy@goop.org> writes:
> > > OK, but that still doesn't account for the relatively large increase
> > > from 2.6.18 -> 2.6.26.  You're using HZ=100 in both cases,
> >
> > I presume.
> >
> > > The other variable is NOHZ and highres timers.  You could
> >
> > try turning
> >
> > > those off in 2.6.26.  Also, CONFIG_PREEMPT could well make a
> > > difference.  2.6.18-xen doesn't support CONFIG_PREEMPT at all, but
> > > pvops(-xen) does.
> >
> > If it's that easily reproducible you could just bisect it?
> >
> > -Andi
>
> I've bisected down to commit ba52de123d454b57369f291348266d86f4b35070 -
> [PATCH] inode-diet.  Before that kernbench consistently reports about
> 35k context switches (total), and after that commit about 53k.  The
> benchmarks are being run on a tmpfs.  I've verified the results on a
> different machine, albeit with an almost identical setup (the same
> kernels and debian distro, kernbench version, and benchmarking a build
> of the same source).
>
> Seems to be a mystery why that patch is (seemingly) the culprit...anyone
> have any ideas?  Maybe there's some other variable I'm not keeping
> constant?

Weird. It could possibly be triggering some different userspace behaviour
if blocksize reporting has changed anywhere. strace might help there.

Interesting if you could post the top results of profile=schedule for
a kernel with and without the patch.

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