Messages in this thread | | | From | Nick Piggin <> | Subject | Re: Large increase in context switch rate | Date | Thu, 24 Jul 2008 11:26:48 +1000 |
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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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