Re: [RFC -v3 PATCH 0/3] directed yield for Pause Loop Exiting

Avi Kivity
2011-01-04T10:35:15Z

On 01/04/2011 12:32 PM, Mike Galbraith wrote:
> On Tue, 2011-01-04 at 11:09 +0200, Avi Kivity wrote:
> > On 01/04/2011 08:42 AM, Mike Galbraith wrote:
>
> > > If I were to, say, run a 256 CPU VM on my quad, would this help me get
> > > more hackbench or whatever oomph from my (256X80386/20:) box?
> >
> > First of all, you can't run 256 guests on x86 kvm.
>
> I didn't want 256 guests. Just one great big dog slow virtual playpen :)

I meant a 256 vcpu guest. Certainly you can run 256 guests, you're
limited only by virtual memory. Current kvm limit is 64 vcpus.

> > > > Both Intel and AMD CPUs have a feature that detects when a virtual
> > > > CPU is spinning on a lock and will trap to the host.
> > >
> > > Does an Intel Q6600 have this trap gizmo (iow will this do anything at
> > > all for my little box if I were to try it out).
> >
> > Likely not. Run
> > http://git.kernel.org/?p=virt/kvm/qemu-kvm.git;a=blob_plain;f=kvm/scripts/vmxcap;hb=HEAD,
> > look for 'PAUSE-loop exiting'. I think the first processors to include
> > them were the Nehalem-EXs, and Westmeres have them as well.
>
> Oh darn, need new box to get all the toys. Thanks.
>

I have a patchset somewhere that emulates pause-loop exiting by looking
at rip during timer interrupts. Unfortunately timer interrupts are very
rare compared to pause-loop exits, so it's not very helpful.

--
error compiling committee.c: too many arguments to function