[ntp:questions] Why does ntp keep changing my conf file?

David Woolley david at ex.djwhome.demon.invalid
Sat Sep 18 21:38:21 UTC 2010

David J Taylor wrote:

> So, if I understand you correctly, you are saying that because ntp 
> measures and reports an offset, it will try and correct the clock in the 
> opposite direction, so that the actual clock error is likely to be less?
So will ntpdate, but ntpdate only uses the most recent offset, whereas 
ntpd uses (to a first approximation) an exponential rolling average of 
the recent offsets, such that it makes effective use of quite a lot of 
them.  That has a similar effect to using longer and longer averaging 
periods in MRTG.  The time of the clock is much more accurate than any 
one offset would suggest.

Complicating factors are that there is a non-linear finite impulse 
response filter (best of 8) ahead of the main, infinite impulse response 
one, the actual filter is  not just solving for an offset, but also for 
the frequency, and the filter parameters are varied depending on an 
estimate of the quality of the time data. 

More information about the questions mailing list