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Re: [Xen-devel] HPET/PIT timer accuracy


On 29 Jul 2005, at 19:17, Michael Hohnbaum wrote:

The TSC is not a reliable time source.  There is no guarantee of
synchronization between multiple CPUs.  Also note that the TSC will
stop in ACPI C3 mode.  Probably not an issue for Xen today, but could
be in the future, especially as servers start doing more power
management.  The HPET is preferable for a time source on systems
that this is available on.

We're not relying on TSC synchronisation between CPUs. They can run at different frequencies, stop in deep sleep, and so on. But we *do* want the frequency of each one to be as stable as possible. I'd expect to get <1ppm stability from a crystal source at constant temperature, no problem.

 -- Keir


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