Re: [RFC] VMX CR3 cache

Marcelo Tosatti
2008-01-28T17:35:30Z

On Mon, Jan 28, 2008 at 06:17:34PM +0100, Ingo Molnar wrote:
>
> * Marcelo Tosatti <marcelo-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org> wrote:
>
> > lat_ctx numbers (output is "nr-procs overhead-in-us"):
> >
> > cr3-cache:
> > "size=0k ovr=1.30
> > 2 6.63
> > "size=0k ovr=1.31
> > 4 7.43
> > "size=0k ovr=1.32
> > 8 11.02
>
> when i did the testing then i was able to get zero VM exits in the
> lat_ctx hotpath and get the same performance as on native. The above
> numbers you got suggest that this is not the case. (your lat_ctx numbers
> on native are probably around 1 usec, right?)

Actually, native is around 5usec:

# bin/x86_64-linux-gnu/lat_ctx -N 10 2

"size=0k ovr=1.09
2 5.21

So it does get pretty close to native, but the problem is that its
difficult to see improvements on macrobenchmarks (even hackbench which
remotely resembles a real scenario is not improving).

> If that is the case, could
> you check via profile=kvm (or whatever other method you use to profile
> KVM) where the VM exits come from?
>
> Ingo
>
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