[ntp:questions] ntpd goes into oscillation

Hal Murray hal-usenet at ip-64-139-1-69.sjc.megapath.net
Wed Dec 30 10:25:21 UTC 2009

In article <slrnhjkta6.4e4.unruh at wormhole.physics.ubc.ca>,
 unruh <unruh at wormhole.physics.ubc.ca> writes:
>ntpd has suddenly broken out into oscillations. it is fed by a Garmin
>18 LVC PPS via shm. The oscillation has a period of just under an hour (
>about 50 min) and an amplitude of about 10usec. in the offsets (
>amplitude of about .005PPM in the rate). Since this is acting as the
>clock for a number of other machines, they are also showing the
>oscillation especially in the rate.
>While I suppose this could be something in the GPS itself, it looks more
>like an oscillation in ntpd.
>Nothing changed when the oscillations started. Ntpd had been started
>on Dec 14, and this change began on Dec24.
>You can see the graph on www.theory.physics.ubc.ca/chrony/chrony.html ,
>the graph for the machine called string.
>Has anyone else ever seen this kind of thing?

What OS?  (version?)

What is the temperature like?

If you feed a sawtooth into a PLL, the offset will be the derivative,
a square wave.  The amplitude of the square wave is smaller with
higher gain.  A sawtooth with a 1 hour period is possible from
air conditioners.

I've seen oscillations on boxes using the pool, or at least stuff
that looks like oscillations to my eyeball.  That's on Linux.
(They have been "fixing" the timekeeping code.  I wouldn't be
surprised by anything.)

These are my opinions, not necessarily my employer's.  I hate spam.

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