[ntp:questions] ntpd goes into oscillation

unruh unruh at wormhole.physics.ubc.ca
Wed Dec 30 18:40:09 UTC 2009

On 2009-12-30, Hal Murray <hal-usenet at ip-64-139-1-69.sjc.megapath.net> wrote:
> 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?)

Linux-- Mandriva 2008.1 ( kernel 
This  oscillation has just started. 

> What is the temperature like?

The temp should be relatively stable ( certainly no air conditioner, although
possibly cooler than usual because the heating has been turned down for
the Christmas break-- After all nothing happens in the University during
Christmas. Research-- what's that?.
HOwever it so happens that I have the internal temp recorded via
lmsensors for the past 5 days, and there is some evidence of a one
degree flucutation (the resolution of the onboard thermometers) 
 with about the same period. No idea what that is,
since there is no airconditioning/heating. But I suppose it could be
causing that fluctuation in the rate and thus the offset. 
Which somehow started on Dec 24.
Lets see what happens when classes begin again next week.

> 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.

Actually, no, the offset is the integral, not the derivative, of the
drift, so the offset will be a bunch of peaks.
> 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.)
Yes, but then this "fix" whatever it is,  has been the same for months now. 

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