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Re: [xen-devel] System time monotonicity

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Subject: Re: [xen-devel] System time monotonicity
From: "Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx>
Date: Tue, 8 Apr 2008 10:34:41 -0600
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>On 26/3/07 19:50, "Ian Pratt" <Ian.Pratt@xxxxxxxxxxxx> wrote:
>> On your system it appears to be a couple of microseconds out, which is
>> on the high side of what we've observed. Normally you only see that kind
>> of mismatch on systems with TSCs running off different crystals.
> More likely a jittery chipset timer -- we've observed less-than-ideal
> stability from some chipset timers, which can throw us off a bit when
> independently sync'ing the TSCs (which each CPU does for its TSC
> independently every couple of seconds).
> -- Keir

Sorry, a little slow on responding here, only took a year ;-)

Where is the code that does this independent TSC sync'ing?  I see
code in smpboot.c that seems to do this at startup (though exactly
how I admit I haven't yet figured out... looks like some kind of
rendezvous loop triggered by the BP?).  But I don't see where/how
this gets called "every couple of seconds", nor do I see any writing
to the TSC (except setting BP and each AP to zero at startup).


If Xen could save time in a bottle / then clocks wouldn't virtually skew /
It would save every tick / for VMs that aren't quick /
and Xen then would send them anew
(with apologies to the late great Jim Croce)
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