Re: [Xen-devel] RE: [PATCH] record max stime skew (was RE: [PATCH] strictly increasing hvm guest time)

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 10/7/08 01:24, "Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx> wrote:

> 7) CONJECTURE: Result of natural skews between platform
> timer and tsc, plus jitter.  Unfixable.
> 
> Possible, untested, not sure how.

I ended up suspecting this on one of the test platforms I originally did the
Xen-system-time implementation on. It was an old AMD white box iirc. On that
system, TSC and platform time seemed to have a significant and inexplicable
jitter at around 1Hz. The jitter was 100s of ppm, which was totally
unexpected for what should be crystal-based oscillators. And the test code
was simple enough that it was hard to suspect that either (I think I was
just dumping the counters every second or two after reading them as close
together as I could).

 -- Keir



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