RE: Large increase in context switch rate

From: Alex Nixon (Intern)
Date: Wed Jul 23 2008 - 05:34:56 EST

> -----Original Message-----
> From: Andi Kleen [mailto:andi@xxxxxxxxxxxxxx]
> 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@xxxxxxxx> 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

- Alex
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