[ntp:questions] A Christmas puzzler - intermittent offset oscillations with a PPS source

Rick Jones rick.jones2 at hp.com
Fri Dec 21 19:51:36 UTC 2012

David Taylor <david-taylor at blueyonder.co.uk.invalid> wrote:
> Although the offset appears to have a 1.25 hour period from the MRTG
> graphs, examining the loopstats directly shows that the actual
> period is just over about 5/3 minutes - just over 100 seconds.  I
> don't know what's happening.  I would have put it down to poor GPS
> strength, except that the effect lasts almost a whole day, and GPS
> missing usually contributes bigger offset spikes.  I would not
> expect it to be the navigation mode of the GPS as the excursions are
> larger than I would expect between navigation and timing modes, but
> I don't have a lot of experience in that area.  One difference in
> the configuration is that #1 - the one with the offsets - runs and
> uses gpsd for the coarse seconds, whereas #2 relies on the rest of
> the network.  This seems to causes a higher CPU usage in #1, shown
> in there graphs:

>    http://www.satsignal.eu/mrtg/performance_raspi-1.php
>    http://www.satsignal.eu/mrtg/performance_raspi-2.php

Does the gpsd do anything every 5/3 minutes?  Or put another way, can
you find a similar periodicity in the CPU utilization?  If it does do
something interesting at that frequency and it involves a system call,
while the act of tracing would perturb things, you might find it in a
(timestamped) system call trace (strace) of the gpsd.

Perhaps the luck of process scheduling and the gpsd or some other
daemon holds-off the ntpd?  (Raspberry Pi's are single-core systems
right?)  Does the ntpd run at a higher (realtime?) priority than the

Might there be any other background dameons consuming more CPU on the
one system than the other?

rick jones
It is not a question of half full or empty - the glass has a leak.
The real question is "Can it be patched?"
these opinions are mine, all mine; HP might not want them anyway... :)
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