[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 8 20:59:40 UTC 2005

Martin Burnicki wrote:
> I think the 10/15 ms switching is just used to maintain a kind of
> synchronization between the standard tick rate and the MM timer tick
> rate.
> Martin


Thanks for that detailed explanation, which I've saved for future 

What is still unexplained on this system, though, is why on Friday at 
14:30 the system change from (I suspect) a single stable clock timing (one 
unaffected by running MM applications) to a 10/15ms timing which affects 
NTP like crazy?

Looking now, the system is as stable with the MM timer running at 1KHz as 
it ever was before.  I'm going to see what your NTP Time Server Monitor 
says, as I've got the statistics from before the problem.  (Thanks for the 
excellent tool, by the way).

Nov 29:  frequency -7.6 .. -7.5, offset -0.006 .. +0.006

During the fault:
Dec 04:  frequency -68..-63,  offset -0.6 .. +0.6

Dec 08: frequency -16.2 .. -7.56,  offset -0.06..+0.06 (but still 

So I am coming back towards the drift value I had before the fault, and 
the stability is similar (the 0.06 looks like a single transient when I 
tried stopping and restarting QuickTime to check my own measurement 

OK, I'm now revising my view of what happened.  My best guess now is that 
before the fault occurred, I was running with the MM timer permanently 
enabled, and that's what made the system stable.  Something caused that to 
stop.  What?  Does XP SP2 permanently run the MM timer at 1KHz?  News to 
me if it does!

Too much thinking out loud!


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