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Re: [Xen-devel] System time monotonicity

On 26/3/07 21:14, "John Levon" <levon@xxxxxxxxxxxxxxxxx> wrote:

>> 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).
> And what about cross-VCPU monotonicity? It's called very frequently and
> having to fake monotonicity via a single variable across all CPUs would
> not be pleasant, unless I can think of something smarter...

Yet it is what you need to do if you want to absolutely guarantee monotonic
time across all VCPUS. I don't think there's any way around that, but as Ian
says it's still going to be cheaper than reading a chipset timer. I'd be
inclined to settle for per-vcpu and per-OS-task monotonicity.

 -- Keir

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