[ntp:questions] 500ppm - is it too small?

David J Taylor david-taylor at blueyonder.not-this-part.nor-this.co.uk.invalid
Thu May 7 16:12:46 UTC 2009

I've recent been suggesting the Windows port of NTP as a program suitable 
for an application where the timekeeping needed to be within a second or 
two.  Yes, NTP is overkill, but it has the advantages of multiple servers, 
best server selection, adaptive poll rate, and memory of the clock drift 
etc.  However, on quite a few installations - at a guess between 1% and 
5% - NTP has failed because the click frequency error appears to be too 
great for NTP to correct.

Is there any feeling for changing the 500ppm limit, perhaps to 1000ppm or 
even as much as 5000ppm (to pull a figure out of the hat)?  Or is 500ppm 
generally believed to be the worst error which should be compensated?

One possibility is that some of the problem PCs are portables, with some 
sort of power-saving or even hibernation scheme.  I don't have direct 
visibility of the type of PC.


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