[ntp:questions] drift value very large and very unstable

Fran Horan fran.horan at jhuapl.edu
Fri Mar 7 17:01:57 UTC 2008


<snip lots of detail>

> So, the summary is that drift goes to 500ppm when stepping is disabled
> but runs normally when stepping is enabled and both situations never
> require a time step.  This makes no sense to me.  By the way, as
> mentioned previously, we require that time does not step backward due to
> a problem in some commercial software that cannot currently tolerate
> time moving backwards.
>
> Quite frankly, I don't think it's unreasonable that a system require
> time to monotonically increase.

Forgive me if this answer misses a point in the earlier details, or shows my
ignorance of NTP, but a few ideas/thoughts.

Oscillators and drift can go in either direction, fast or slow, its a
physics-based situation. You can't write code around that and provide a
software solution that is monotonic at all times. However, a single negative
step just at the start may be required before going monotic after that
event. (Not an expert, but that is my understanding).

With this ref clock and a GPS-drive IRIG source, you may only see a single
negative step when NTP first begins running on a new system with no drift
file, or a system that has been powered off a long time with a
battery-driven clock drifting over that long time. Once NTP is humming along
after the initial step and some updates, you shouldn't see a step again.
This makes me think that you should insert a delay in launching your
sensitive application, or block the application at some point, so it does
not see the (possible) first time step.

Fran Horan
JHU/APL





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