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

On Fri, 2005-07-29 at 19:29 +0100, Keir Fraser wrote:
> 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.

Not being a hardware expert, terminology and details might be a bit off,
but my understanding is that yes, this level of instability from the 
hardware time source (not necessarily a crystal) is not uncommon.  HPET
time source tends to be more reliable (stable) than the TSC.  There
tend to be many factors that affect the accuracy of the TSC, most of
which I do not grok.

Much of the complexity in the Linux time subsystem is there to deal with
the inadequacy of hardware time sources.

>   -- Keir
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Michael Hohnbaum                             503 578 5486
hohnbaum@xxxxxxxxxx                          t/l 775 5486

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