Re: [RFT] mmu optimizations branch

Ingo Molnar
2007-01-03T02:22:41Z

* Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org> wrote:

> >lat_ctx -s 0 [zero memory footprint]:
> >
> > -------------------------------------------------
> > #tasks native kvm-r4204 kvm-r4232(mmu)
> > -------------------------------------------------
> > 2: 2.02 180.91 9.19
> > 20: 4.04 183.21 10.01
> > 50: 4.30 185.95 11.27
> >
> >so here it's a /massive/, almost 20 times speedup!
> >
>
> Excellent. 10us is approximately the vmexit overhead on intel (we
> regularly see 100-120k exits/sec), so it means a context switch is
> exactly one exit. Hard to beat without nested page tables.

actually, the VM entry+exit cost on this CPU is around 3-4 microseconds,
so it's still 2 VM exits per context switch.

I debugged this a bit, and what happens is that when Linux does a
task-switch it does a cr3 load /and/ a write (look at __flush_tlb()) -
and both are causing a vm exit!

I have started paravirtualizing the Linux kernel for KVM. I have
eliminated the cr3 load from the Linux kernel via paravirtualization and
that way lat_ctx shows a ~5-6 usecs context-switch cost. That's pretty
good i think, compared to the 2-3 usecs of native. I'll send patches for
this tomorrow.

Ingo