[ntp:questions] Sudden change in precision and jitter

David Woolley david at ex.djwhome.demon.invalid
Sun Jun 2 09:24:17 UTC 2013


A C wrote:
> 
> That would be interesting since I have a cron job restarting it at an 
> odd hour away from any other cron jobs left.  I'll check and see if 

Why are you restarting it?  ntpd works  best if left to run continuously.

> there are any others that I missed and then move the restart somewhere 
> if there's an overlap.
> 
> I wish I could figure out how 4.2.6 and 4.2.7 differ in the calculations 
> because under 4.2.6 my jitter is always 0.061 while under 4.2.7 it is 
> always 0.122 no matter what I do.

There are two factors that affect precision:  the clock tick rate and 
the time needed to read the clock. The precision will tend to be based 
on the latter, rounded up to a multiple of the former.  This is then 
rounded to a power of two fraction of a second.

If the time to read is very close to an exact multiple of the hardware 
resolution, a small perturbation could change the number of ticks, which 
might then change the result into the next power of two range.
> 



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