[ntp:questions] Timekeeping broken on Windows XP with multimedia timer enabled (-M option)

Martin Burnicki martin.burnicki at meinberg.de
Mon Jan 25 14:15:27 UTC 2010

Evandro Menezes wrote:
> On Jan 21, 10:42 am, Martin Burnicki <martin.burni... at meinberg.de>
> wrote:
>> Ah, that's interesting. I did know that new CPU cores may not suffer from
>> switched CPU clocks anymore, but I didn't know this is because they are
>> driven by the FSB clock. So I assume the QPC clock frequency reported by
>> Windows can also correspond to the FSB clock.
> Only indirectly.  You certainly know that the CPU clock is a multiple
> of the FSB clock.  The unit of the result returned by RDTSC is still
> CPU clock ticks, always.  So, when the CPU clock multiplier is changed
> from, say, 3.5 to 1.0, due to a power management decision, the TSC
> circuitry is changed accordingly, so that its unit is the same as the
> CPU's.  The result is that when measuring the CPU clock, one will
> still get it right.
> However, the precision of the TSC is not 1 CPU clock tick anymore, but
> the FSB multiplier.  So it cannot be used so easily to measure how
> many CPU clock cycles a sequence of instructions takes anymore.
> AFAIK, on AMD processors it's still possible to chose between variant
> and invariant TSC.  But I think that the tendency is for BIOS makers
> to not offer this option and just enable the invariant TSC, since only
> developers care about its precision (and they can use a performance
> counter for the same purpose).

Yes, it helps indeed ;-)

Interesting details I didn't know, yet.

Martin Burnicki

Meinberg Funkuhren
Bad Pyrmont

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