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Re: [Xen-devel] Xen, Windows 2003 and APIC

  • To: "Keir Fraser" <keir.fraser@xxxxxxxxxxxxx>
  • From: "George Dunlap" <George.Dunlap@xxxxxxxxxxxxx>
  • Date: Fri, 5 Dec 2008 16:42:57 +0000
  • Cc: xen-devel@xxxxxxxxxxxxxxxxxxx, Gary Pennington <gary.pennington@xxxxxxx>, et-mgmt-tools@xxxxxxxxxx
  • Delivery-date: Fri, 05 Dec 2008 08:43:21 -0800
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IIRC, only w2k3sp2+, vista, and w2k8 have lazy TPR.  Not positive
about w2k3sp1, but I think it doesn't.  WinXP does not have lazy TPR
in any service pack, AFAIK.

What service pack of w2k3 did you do those tests with?

Hopefully, sometime in the next few weeks, I'll be able to release my
'xenalyze' tool, which will help a lot with analyzing what's really
going on with these kinds of workloads.


On Fri, Dec 5, 2008 at 2:45 PM, Keir Fraser <keir.fraser@xxxxxxxxxxxxx> wrote:
> On 05/12/2008 14:03, "Gary Pennington" <Gary.Pennington@xxxxxxx> wrote:
>> My next question is: What is really happening when APIC is specified for
>> a windows guest and why does performance vary so much according to whether
>> it's specified or not?
> Older Windows kernels update the APIC TPR a lot, and unless you have a very
> modern Intel processor every one of those TPR updates causes a vmexit.
> Modern Windows (including possibly latest w2k3 service pack, but I'm not
> totally certain) includes lazy TPR, which gets rid of the vast majority of
> TPR updates, and hence will go much faster.
>  -- Keir
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