[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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