Re: [RFT] mmu optimizations branch

[Ingo Molnar <mingo-X9Un+BFzKDI@public.gmane.org>]

* 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

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