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Re: [Xen-devel] Windows SMP

  • To: "Keir Fraser" <keir.fraser@xxxxxxxxxxxxx>
  • From: "Andrew Lyon" <andrew.lyon@xxxxxxxxx>
  • Date: Mon, 29 Dec 2008 21:40:37 +0000
  • Cc: James Harper <james.harper@xxxxxxxxxxxxxxxx>, Dirk Utterback <dirk.utterback@xxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx, Venefax <venefax@xxxxxxxxx>
  • Delivery-date: Mon, 29 Dec 2008 13:41:10 -0800
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On Mon, Dec 29, 2008 at 8:14 AM, Keir Fraser <keir.fraser@xxxxxxxxxxxxx> wrote:
> On 29/12/2008 02:59, "Venefax" <venefax@xxxxxxxxx> wrote:
>> I had to disable both, and PAE. Only APIC=0 would not make any difference. I
>> will some further testing with Citrix Xenserver 5, using the same virtual
>> machine and another copy with their vmpd drivers. I bet that there is no
>> difference in performance. It seems to be a Xen architectural issue. Any
>> ideas?
> The problem is almost certainly APIC related. APIC=0 actually has no effect
> for a multi-processor HVM guest, since APICs are architecturally absolutely
> required in x86 multi-processor systems.
> The problem is most likely lots of emulated APIC TPR writes slowing things
> down. Possible fixes:
>  1. Run a Windows guest with the 'lazy TPR' optimisation -- w2k3sp2+, w2k8,
> vista. Or run 64-bit Windows which will write TPR in a different way which
> most Intel/AMD CPUs can virtualise efficiently.
>  2. Run a new enough Intel processor which has automatic TPR handling even
> for 32-bit Windows guests.

Does "PTR handling" have a cpu feature flag I can grep for in
/proc/cpuinfo, or perhaps there is a list of cpus that support that

I run Xen on two systems, a dual Xeon 5420 and a Core 2 Quad, neither
seem to suffer from bad performance with SMP Windows hvm's but most of
the windows hvm's I run are not heavily loaded so I would like to know
if my system is likely to suffer from this problem as it would
probably become noticible once load increases.


>  3. Run Citrix drivers which patch Windows to avoid TPR writes.
>  -- Keir
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