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

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Message-ID: <4D22F7E3.7000508@redhat.com>
Date:	Tue, 04 Jan 2011 12:35:15 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Mike Galbraith <efault@....de>
CC:	Rik van Riel <riel@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Chris Wright <chrisw@...s-sol.org>
Subject: Re: [RFC -v3 PATCH 0/3] directed yield for Pause Loop Exiting

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

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