On Fri, 17 Aug 2007, Mathieu Desnoyers wrote:
> > The cpu will definitely not be idle during these measurements and no
> > frequency scaling is active.
>
> The problem is that if the cpu is idle _before_ the measurements, the
> frequency will change differently from one cpu to another. Therefore,
> the cycle counters may have large offsets when you start your tests. So,
> if get_cycles() is executed on different CPUs (thread being migrated)
> between the beginning and the end of the test, the results would be
> skewed.
TSC measurements as done by this patch are associated with cpus. The
skew is irrelevant. Development of this patchset was done on a
system whose TSC are never synchronized.
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