Re: Large increase in context switch rate

[Peter Zijlstra <peterz@infradead.org>]

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.