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Re: [Xen-devel] write_tsc in a PV domain?

To: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Subject: Re: [Xen-devel] write_tsc in a PV domain?
From: Alan Cox <alan@xxxxxxxxxxxxxxxxxxx>
Date: Fri, 28 Aug 2009 10:49:15 +0100
Cc: Jeremy Fitzhardinge <jeremy@xxxxxxxx>, "Xen-Devel \(E-mail\)" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
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> No, I don't think this is true.  An enterprise app that binds processes
> to fixed physical processors on a physical machine can make
> assumptions about the results of rdtsc that aren't valid when
> the vcpus can skip between pcpus.  Further, like Linux itself,

They rarely make the right assumptions

> applications may test assumptions about tsc at startup that are
> assumed to remain valid for the life of the app, which is
> perfectly reasonable on a physical machine

No it isn't because of things like suspend/resume.

> True, but any app that tries to run on a NUMA machine without
> being aware of the idiosyncracies of a NUMA machine probably
> has worse problems to deal with than tsc sync.  Further, there

Disagree - this is true if your NUMA factor is large but quite a few
machines today are "vaguely NUMA" - the NUMA factor is low enough the app
doesn't need to care. Anyway you don't need NUMA to see TSC skew between

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