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Re: [Xen-devel] RE: Saving/Restoring IA32_TSC_AUX MSR

To: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Subject: Re: [Xen-devel] RE: Saving/Restoring IA32_TSC_AUX MSR
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Fri, 11 Dec 2009 10:50:29 -0800
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx, "Dugger, Donald D" <donald.d.dugger@xxxxxxxxx>, "Xu, Dongxiao" <dongxiao.xu@xxxxxxxxx>, Keir Fraser <keir.fraser@xxxxxxxxxxxxx>, "Nakajima, Jun" <jun.nakajima@xxxxxxxxx>, "Zhang, Xiantao" <xiantao.zhang@xxxxxxxxx>
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On 12/11/09 10:35, Dan Magenheimer wrote:
However, the vcpu number is definitely useful to usermode
apps, so they
can get some idea how they're moved between (v)cpus.  I don't
think it
will matter to them that it isn't pcpu.
My point is that an app running on native Linux can
safely assume that, if TSC_AUX==3 at time T1 and
TSC_AUX is still 3 at time T2,it is running
on the same processor and the same node at both T1
and T2.  In a virtual environment it cannot even
assume it is running on the same machine.
Further if the app sees that TSC_AUX==2 at time T3
and TSC_AUX==3 at time T4, on native Linux it
can safely assume that it is running on a different
processor.  While rarer, in a virtual environment,
this may also be a false assumption.

That's why I say the information is misleading.

Sure, but that info is, at best, of heuristic value, and won't cause any correctness problems if it is wrong. The performance may suck, but that's part of the larger problem of running NUMA-aware code in a virtual environment.


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