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

David Taylor david-taylor at blueyonder.co.uk.invalid
Sat Dec 22 08:06:55 UTC 2012

On 21/12/2012 19:51, Rick Jones wrote:
> 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
> gpsd?
> Might there be any other background dameons consuming more CPU on the
> one system than the other?
> rick jones

Thanks for your input, Rick.  There's nothing obvious on the CPU plot 
which correlates with the NTP offset plot, so I'm unsure whether they 
are related.  As an experiment, I've now removed NTP's dependency on 
gpsd (although it didn't appear to be selected as a source, and was 
/not/ the "prefer" source), and I will see whether the oscillation 

If I still see the problem, I will restart the system without gpsd enabled.

The systems are nominally identical, except that the oscillating system 
has the serial data fed via GPIO pins, and the stable system is using 
USB for the serial I/O, but that serial data isn't being used anywhere 
(NTP relies on the LAN servers for its coarse time, and experimentally 
it's now doing that on /both/ systems).
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