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

David J Taylor david-taylor at blueyonder.co.not-this-bit.nor-this-part.uk.invalid
Fri Dec 16 08:07:46 UTC 2005

John Allen wrote:
> David,
> Yes, I have checked with and without FSB Spread Spectrum with both
> HALs.
> My observations for the 4 combinations are:
> Motherboard: A7N8X-X (nForce2)  Windows XP SP 2
> HAL: "ACPI Uniprocessor PC" (with APIC, halaacpi.dll):
> 1) FSB Spread Spectrum = 0.50% :  system clock unstable, NTP did not
> synchronise 2) FSB Spread Spectrum = disabled :  system clock
> unstable, NTP did not synchronise
> HAL: "Advanced Configuration and Power Interface (ACPI) PC" (no APIC,
> Halacpi.dll) 3) FSB Spread Spectrum = 0.50% :  system clock stable,
> NTP synchronised 4) FSB Spread Spectrum = disabled :  system clock
> stable, NTP synchronised
> It's a bit anecdotal, but it seems that FSB Spread Spectrum may not
> make a big difference. However, I'm still running (3), I'll have to
> check its behaviour over a longer period.
> It should be mentioned that according to the Microsoft KB article
> http://support.microsoft.com/?kbid=821893
> there is a quite fundamental difference between the two DLLs:
> - Halaapic.dll: uses the Real Time Clock (RTC) to generate clock
> interrupts - Halacpi.dll: uses the 8254 Programmable Interval Timer 
> (PIT) to
> generate clock interrupts
> John

That's most interesting, John.  I suppose with the spread-spectrum its 
effect on NTP would depend on what period the spread is over, i.e. if the 
spread is over microseconds but NTP measures over milliseconds, the 
frequency may appear stable.  You might want to enable the statistics 
collection and use a program like Meinberg's NTP Time Server Monitor:


to see what the stability actually is.  I'm also unclear whether the FSB 
speed will actually have any direct effect on the clocks seen by the 
processor, although it will obviously affect memory retrieval speed.

The information on the HALs was of interest - I have a book in which some 
of this stuff is written up, but not the timing stuff (IIRC).  "Windows 
Internals", fourth edition, is a good read (if you like that sort of 

I would also appreciate seeing this type of information in the Support 
Twiki (as well as searchable by Google).


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