[ntp:questions] NTP with GPS and RTC

Harlan Stenn stenn at ntp.org
Sun Apr 28 10:16:36 UTC 2013


unruh writes:

> I advise to subtract 20 from the drift file to give an estimate of how
> fast ntpd recovers from a wrong drift, not that that is the perferred
> way of operating. Note that Linux for a while would vary in the rate
> from bootup to bootup by up to 50PPM, and ntpd-- operating as
> designed-- would have to try to recover from that.  Ie, a wrong drift
> value is NOT a complely out of this world situation. Similarly, a
> machine drifting out by 100ms is also not out of the oridinary for a
> lengthy shutdown (esp since the rtc is uncorrected.) If it is out by
> more, then the stepping of ntpd will come into play.

For years I've wanted the drift file to include the frequency/tick
values in-use when the drift file was written.  Doing this would allow
ntpd to know if the tick value or frequency that it reads from the drift
file matches the current operating values or not, and take appropriate
steps.

H


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