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

Danny Mayer mayer at ntp.isc.org
Fri Dec 16 02:47:03 UTC 2005


David J Taylor wrote:
> 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.
> 

It's currently using the Performance Counter.

Danny




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