[ntp:questions] once 'ntpd' stops, does the drift file value continue to get used for clock adjustments?

unruh unruh at wormhole.physics.ubc.ca
Fri Mar 18 00:45:27 UTC 2011

On 2011-03-17, Gautam Thaker <gautam.h.thaker at lmco.com> wrote:
> I have a stable drift value on my fedora 12 node and everything is
> running well. (offsets are well below 1msec). If I now stop 'ntpd'
> will my time start to drift relative to the 'correct time' by the PPM
> value in the drift file? Or is it that ntpd has already set things up
> so that corrections as per last value of drift file will continue
> indefinitely? If it is the latter and if indeed my drift value is

It depends on the clock correction model. ntp has two modes. In one it
sends a request to the operating system (eg adjtimex) to alter the
kernel clock rate. In the other, once a second it resets the clock to
the "right time" as per the frequency slewing algorithm. In the later
case, the clock is left alone do run at the wrong speed, and if you stop
ntp, the clock will wander off. In the other case (eg Linux, BSD I
elieve) the the clock rate will have been adjusted in the kernel,and it
should continue at the right rate. Exactly which operating system use
which mode, I am not sure. 

> quite good (as ntpd had run for many days already), it seems I can go
> for many more days w/o ntpd and not get very much out of sync at all.

It you run linux, yes. 

> Any comments welcome. Thanks.
> Gautam

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