[ntp:questions] Accuracy of NTP - Advice Needed

Dave Hart davehart at gmail.com
Fri Dec 23 03:25:18 UTC 2011


On Thu, Dec 22, 2011 at 19:11, Paul Sobey <buddha at the-annexe.net> wrote:
> - can ntpd's own reported offset (ntpq -p or loopstats) be trusted
>  (assuming high priority means it gets scheduled as desired)? I've quoted
>  our apparent numbers at several people and the response is always 'pfft
>  you can't trust ntpd to know its own offset' - but nobody can ever tell
>  me why

This is an interesting issue.  Consider the case where ntpd has just
started and is reporting 50 msec offset from its source(s).  Using
ntpd 4.2.7, within the first five (with drift file) or ten (without)
minutes that offset will be reduced to less than a half millisecond,
thanks to the new for 4.2.7 initial offset slew, which also avoids
perturbing the frequency correction during that initial slew.

During such startup, it is reasonable to assume the best estimate of
UTC available on that system is the system clock plus the reported
offset from ntpq.  However, once ntpd has settled down, the best
estimate of UTC will be the system clock alone, ignoring any offset
reported by ntpq.  To help understand why, recognize why ntpd has a
damped response rather than working furiously to eliminate any
apparent offset as fast as possible:  doing so would be respecting
noise that gets drowned out by signal through the slower feedback
loop.

ntpq -c "rv 0 rootdisp" gives you the estimate of maximum error
between the local clock and the ultimate source of time (GPS or other
refclock).  You won't find small-microseconds numbers there.  While
the average and instant error is likely to be much less, if you're
careful in talking about ntpd accuracy, you'll want to minimize root
dispersion.

A good way to measure ntpd performance is to use a local PPS marked
noselect as well as a nearby NTP server with PPS.  With peerstats
enabled, you can consider the PPS offsets logged by the noselect
refclock a good measurement of the offset of the local clock.

Cheers,
Dave Hart


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