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RE: [Xen-devel] Re: Live migration fails due to c/s 20627

To: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Subject: RE: [Xen-devel] Re: Live migration fails due to c/s 20627
From: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Date: Wed, 16 Dec 2009 07:20:00 -0800 (PST)
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx, kurt.hackel@xxxxxxxxxx, "Dugger, Donald D" <donald.d.dugger@xxxxxxxxx>, "Xu, Dongxiao" <dongxiao.xu@xxxxxxxxx>, Fraser <keir.fraser@xxxxxxxxxxxxx>, "Nakajima, Jun" <jun.nakajima@xxxxxxxxx>, Keir, "Zhang, Xiantao" <xiantao.zhang@xxxxxxxxx>
Delivery-date: Wed, 16 Dec 2009 07:21:19 -0800
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> From: Jeremy Fitzhardinge [mailto:jeremy@xxxxxxxx]
> On 12/15/2009 08:14 PM, Dan Magenheimer wrote:
> > My argument is simply that if TSC_AUX cannot ALWAYS
> > be trusted by an application, apps will NEVER trust it.
> > And if apps NEVER trust it, why expose it at all?
> The cpu/node info is only of heuristic value anyway; it is never 
> trustworthy in an absolute sense.  Apps just use it to try to 
> optimise 
> their own memory allocation and use patterns, but they can't rely on 
> that info for actual correctness.

Well, "heuristic" implies a reasonably high probability of
getting the right answer.  Would you agree that the probability
that TSC_AUX gets the "right" answer is much higher
in a physical environment than in a (non-pinned) virtual
environment?  And if the heuristic is wrong more often
than right, that using that heuristic is a bad idea?

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