[ntp:questions] NTP with GPS and RTC

Terje Mathisen "terje.mathisen at tmsw.no" at ntp.org
Mon Apr 29 05:51:21 UTC 2013

unruh wrote:
> On 2013-04-28, Harlan Stenn <stenn at ntp.org> wrote:
>> 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.
> I am not sure, but I thought that in the case of the Linux inability to
> consistantly calibrate the system clock, the frequency and tick values
> were always 0. It was the calibration behind the scenes which set what 0
> meant that was at fault.

That was my reaction as well, but then I realized that Harlan had to 
mean the actual tick value, i.e. nominal Hz rate and/or clock increment 
per tick.

With a reboot that results in a different base tick, this would lead to 
an obvious change in the validity of the ntp.drift numbers.

> Obviouly if the user changes those values (to bring the drift closer to
> zero for example so ntpd is not working so hard trying to compensate for
> a "way off" calibration) then ntpd should know about it.

This is effectively what Linux did for a number of years.


- <Terje.Mathisen at tmsw.no>
"almost all programming can be viewed as an exercise in caching"

More information about the questions mailing list