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

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

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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