[ntp:questions] Re: Windows timekeeping - sudden degradation - why?

David J Taylor david-taylor at blueyonder.co.not-this-bit.nor-this-part.uk.invalid
Thu Dec 15 08:45:01 UTC 2005

Hal Murray wrote:
>> I would have though that "spread spectrum" (implying a continually
>> changing random frequency) was definitely something to avoid for
>> accurate timekeeping!  As you say, changing the HAL is not something
>> to be undertaken lightly.
> Is "spread spectrym" as used for CPU clocks really going to
> do anything evil to timekeeping?  Is it really random?  I've been
> assuming it was some simple modulation pattern - sine or sawtooth.
> It's all on one chip in a cutthroat business so they aren't going
> to pay much for it.
> What does Windows use for timekeeping?
> Linux (on most boxes) uses the interrupts from the TOY/battery-backed
> clock, usually running off a 32 KHz watch crystal.  I got surprised
> by this a few years ago.  The main complication with using the main
> CPU clock is SMP systems.
> It's fairly easy to compare the two crystals.  Both track temperature
> very well.

For NTP, Windows uses one of the CPU clocks (I forget now if it's RDTSC or 
the Performance Counter) to interpolate between the Windows ticks (which 
are at 10 - 15ms intervals).  That's why I asked the OP to try the simple 
test of disabling the "spread-spectrum" in the BIOS - to see if it did 
make a difference.

I do agree with you that's it's like to be a very simple spread-spectrum 


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