Large increase in context switch rate

From: Jeremy Fitzhardinge
Date: Wed Jul 16 2008 - 14:54:41 EST

Hi Ingo,

We have Alex Nixon doing some profiling of Xen kernels, comparing current pvops Xen and native with the last "official" Xen kernel

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 (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.

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