Re: paravirtualization & cr3-cache feature

[Daniel Veillard <veillard-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

On Mon, Feb 12, 2007 at 05:21:26AM -0800, Dor Laor wrote:
> >>
> >> >hi,
> >> >    Can someone briefly explain what the cr3-cache feature on intel
> VT
> >> >processors is and how it is used to improve performance in
> >> >paravirtualization.
> >>
> >> In short, this is a unique Intel VT feature that supposed to save
> vmexit
> >> on cr3 switch. The vmexit operation is quite expensive at the moment
> >> (something like 2-5 ms!). PV kernels can use this feature by sharing
> >  ^^^^^^^^^^^^^^^^^^^^^^^^
> >
> >Where do you measured this unbelievable large number? On AMD-V I
> >measured around 2400 processor cycles for a VMEXIT/VMENTRY roundtrip. I
> >would really really wonder if Intel-VT is that slow...
> 
> Intel is indeed slower than AMD with this one.
> I didn't measure such roundtrip specifically. We usually see that the
> VMETNRY/VMEXIT cycle limits our performance (for io/mmio,..)
> On Intel core duo we cannot get more than 150k-200k VMEXITS per second
> for regular guest doing extensive IO. 
> 
> Anyway 2ms for a 2.4G processor equals 4800 cycles so it's not that far.

  Hum, ms means millisecond in the SI system, so on a 2.4 billion cycle/s
2ms would be 4.8 million cycles, hence IMHO the unbelievably large perception.
I assume you mean VMEXITS would be around  2-5 microsecond, which would fit
quite better in the overall picture :-)

Daniel

-- 
Red Hat Virtualization group http://redhat.com/virtualization/
Daniel Veillard      | virtualization library  http://libvirt.org/
veillard-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org  | libxml GNOME XML XSLT toolkit  http://xmlsoft.org/
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