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Re: [xen-devel] System time monotonicity

To: "dan.magenheimer@xxxxxxxxxx" <dan.magenheimer@xxxxxxxxxx>, Ian Pratt <Ian.Pratt@xxxxxxxxxxxxx>, "Tian, Kevin" <kevin.tian@xxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: Re: [xen-devel] System time monotonicity
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Thu, 10 Apr 2008 08:08:50 +0100
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On 9/4/08 19:36, "Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx> wrote:

> OK, as long as the maximum uncorrected drift between physical TSCs
> does not exceed the guest-expected granularity of its virtual
> platform timer, I agree its good enough.

Ignoring power-saving events, TSCs are crystal-driven and hence we can
expect specified tolerance of a few ppm across temperature extremes, and in
practice over few-second periods I would expect tolerance of better than
1ppm. *However* I have seen platform timers (which also should be
crystal-driven) which inexplicably exhibit much worse behaviour.

> It appears that TSC drift for each pcpu is corrected by Xen
> once per second.  Any idea for real systems out there what the
> maximum drift (per second) is?  Will this be affected by
> existing or future power-savings designs (e.g. is it possible
> for the TSCs in one socket to be slowed down while the TSCs
> in another socket are not)?  If so, as Kevin points out,
> some kind of affinity enforcement might be necessary for
> time-sensitive VMs.

P-state changes are informed to Xen so we can re-sync the local TSC
immediately. The tricky ones are unannounced thermal events because software
does not get informed about those. On some systems we can turn them off, on
others (new Intel platforms) TSC is constant-rate regardless. In a normal
running system thermal events are rare.

 -- Keir

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