[ntp:questions] NTP slow to start correction after a drift

Mike K Smith mks-usenet at dsl.pipex.com
Tue May 13 09:38:31 UTC 2008


On 12 May, 15:16, "Richard B. Gilbert" <rgilber... at comcast.net> wrote:
> Mike K Smith wrote:

> > Looks like I should be reducing maxpoll. I guess the design of NTP is
> > optimised for clocks with predictable drift rates, and a sudden
> > variation in drift rate takes longer to correct.
>
> You DO know that NTPD adjusts the poll interval to fit the current
> conditions???  It will increase the poll interval to MAXPOLL only when
> the clock is stable and very close to being correct.  The default values
> of MINPOLL and MAXPOLL are correct for all but the weirdest cases.

I know that ntpd adjusts the poll interval to fit the current
conditions, but I am describing a case where the current conditions
changed. The clock had been stable for around a week, and the polling
interval had increased to 1024 seconds, then something changed. It
looks like the clock started drifting by about 2ppm, the poll interval
didn't change for three hours causing a 15ms offset before beginning
to correct the drift.
I initiated this thread to help me understand why ntpd took so long to
respond. I had expected to see the poll interval decrease and the
offset swing back towards zero after the first couple of polls showed
the increased offset.

> Are you operating your machines in a controlled (temperature)
> environment?  If the temperature bounces around, so will your clock.
> NTPD will correct it but if the temperature drops five degrees in five
> minutes when the air conditioning kicks in, NTPD may have a little
> difficulty keeping up.

The systems are in air-conditioned equipment rooms, I wasn't expecting
to frequency changes due to temperature.




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