Messages in this thread | | | Subject | Re: Large increase in context switch rate | From | Peter Zijlstra <> | Date | Thu, 17 Jul 2008 11:17:30 +0200 |
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On Wed, 2008-07-16 at 11:54 -0700, Jeremy Fitzhardinge wrote: > Hi Ingo, > > We have Alex Nixon doing some profiling of Xen kernels, comparing > current pvops Xen and native with the last "official" Xen kernel > 2.6.18.8-xen. > > One obvious difference is that the kernbench context switch rate is way > up, from about 30k to 110k. Also, the user time went up from about 375s > to 390s - and that's comparing pvops native to 2.6.18.8-xen (pvops Xen > was more or less identical). > > I wonder if the user time increase is related to the context switch > rate, because the actual context switch time itself is accounted to the > process, or because of secondary things like cache and tlb misses. Or > perhaps the new scheduler accounts for things differently? > > Anyway, I'm wondering: > > * is the increased context switch rate expected? > * what tunables are there so we can try and make them have > comparable context switch rates? > > This is an issue because the Xen/pvops kernel is showing a fairly large > overall performance regression, and the context switches a specifically > slow compared to the old Xen kernel, and the high switch rate is > presumably compounding the problem. It would be nice to have some knobs > to turn to see what the underlying performance characteristics are.
Is this specific to Xen?, as a native kernel doesn't do more than ~3k cs/s with make -j3 on my dual core.
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