On Friday 23 December 2011 03:25:18 Dave Hart wrote: > On Thu, Dec 22, 2011 at 19:11, Paul Sobey <[email protected]> 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.
This is great - appreciate you taking the time to write it. Paul _______________________________________________ questions mailing list [email protected] http://lists.ntp.org/listinfo/questions
