Periodic spike in offset for pps

[Martin Boissonneault]

Hi Doug,

I'm researching a similar issue on my own Pi 3B+. It's also running 
ADS-B receivers, and there is a "reset if heard nothing for a while" 
"feature" in the dump<freq> decoders it uses. The drop in aviation 
traffic causes them to not hear a plane in that period and reset, 
causing a CPU spike. But it's not the complete answer. IIRC, turning the 
decoders off left a periodic spike visible.

I'm setting up a clean, dedicated Pi 4 just for NTP use, and I'm 
recording ntpviz's output for each modification I make and pick the 
better modification. I haven't put the GNSS on it yet, but I see no 
spike so far. Of course, with a StdDev of ~50uS and offset of ~-2uS, it 
might just be too small to see.

Martin

On 2020-05-05 14:08, Doug Curtis via users wrote:
> I apologize if this has been answered previously.
>
> I've setup a few ntp servers on raspberry Pis using the ntpsec howto 
> with clockmaker.  Currently, I'm using a pi2, pi3 and pi4 with 2 of 
> them using the Adafruit gps hat and one of them using a "Gowoops GPS 
> Module 
> <https://www.amazon.com/Gowoops-Module-Antenna-Arduino-Microcomputer/dp/B01MRNN3YZ/ref=cm_cr_arp_d_bdcrb_top?ie=UTF8>".  
> I setup ntpviz to run on a separate machine and I'm noticing a regular 
> offset spike for shm(1) in my graphs on all of them.
>
> After changing the ntpviz period to 2 hours, it appears they are all 
> spiking after 34 minutes. https://world-mail.org/ntpgraphs/gps3/
> Here is a 24 hour period of one of the others: 
> https://world-mail.org/ntpgraphs/gps2/
>
> Since I've been seeing this issue on all three of them, I designated 
> one as the test pi so I've gone through and stopped all non-essential 
> services that I had enabled (ntplogtmp, ntploggps).  I disabled the 
> /var/log/ntpstats nfs mount.  I changed it from dhcp to static ip 
> assignments as well.
>
> Since they were all setup on different days, the version of gpsd is 
> only slightly different but here is the version of the test pi:
> 3.20.1~dev (revision release-3.20-695-g2de5ac5de)
>
> Here is the ntpd version:
> ntpd ntpsec-1.1.8+ 2020-05-04T14:44:13Z (git rev 574d871ed)
>
> Here is what the ntp.conf looks like:
>
> driftfile /var/lib/ntp/ntp.drift
> leapfile /etc/ntp/leap-seconds.list
>
> logfile /var/log/ntp.log
>
> statsdir /var/log/ntpstats/
> statistics loopstats clockstats
> #filegen peerstats  file peerstats  type day enable
> filegen loopstats  file loopstats  type day enable
> filegen clockstats file clockstats type day enable
> #filegen sysstats file sysstats type day enable
>
> restrict default kod limited nomodify nopeer noquery
> restrict -6 default kod limited nomodify nopeer noquery
> restrict 127.0.0.1
> restrict -6 ::1
>
> refclock shm unit 1 maxpoll 4 refid PPS flag4 1 prefer
>
> server rolex.usg.edu <http://rolex.usg.edu> iburst
> server ntp3.mattnordhoff.com <http://ntp3.mattnordhoff.com> iburst
>
> refclock shm unit 0 maxpoll 4 time1 0.138 refid GPS flag4 1
>
> I can't think of anything else to look for or try.
>
> Thanks,
>
> Doug
>
>
>
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