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

Martin Burnicki martin.burnicki at meinberg.de
Thu Jan 21 16:42:21 UTC 2010

Evandro Menezes wrote:
> On Jan 21, 4:43 am, Martin Burnicki <martin.burni... at meinberg.de>
> wrote:
>> AFAIK this should only affect the TSCs, so the setting should not matter
>> at all if the PM timer is used.
> Another detail on RDTSC: newer cores, such as Intel's Core2 and AMD's
> Phenom, return a constant-rate readout by RDTSC.  In actuality, the
> counter that RDTSC is driven by the FSB clock instead of by the CPU
> clock and, even if the later changes due to power management, it's
> adjusted accordingly.

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.

> But it all boils down to the HAL, which will use whatever's necessary
> to make sure that what QueryPerformanceCounter returns is independent
> of the CPU clock.

There may still be limitations depending on the age of specific items, e.g.
the HAL can only decide which timer to use if the pros and cons of the
timers have been known to the programmers, and appropriate code has been
integrated into the HAL, when the specific HAL version was released.

Anyway, thanks for the hints.

Martin Burnicki

Meinberg Funkuhren
Bad Pyrmont

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