[time-nuts] weird Raspberry PPS+GPS NTP server behaviour

[Adam Kumiszcza]

On Mon, Mar 2, 2020 at 5:23 PM David J Taylor via time-nuts <
time-nuts at lists.febo.com> wrote:

> Hi!
>
> For several weeks I'm seeing strange behavior of my Raspberry Pi based NTP
> server with Uputronics GPS PPS expansion board (Ublox MAX-M8Q chip).
> Several weeks ago the mean daily jitter was below 300 ns, offset didn't
> exceed ±1 µs, provided no restart or configuration change was made. Now the
> offset often wanders even below 5 µs, steady change, and then comes back.
> You can see it at the screenshot from my Grafana server (attached). As you
> can see, the number of satellites visible and TDOP value should not be the
> cause of it.
>
> My current ntp.conf is as follows (I use ntpsec and gpsd):
>
> driftfile /var/lib/ntp/ntp.drift
> leapfile /var/lib/ntp/leap-seconds.list
>
> statsdir /var/log/ntpstats/
> statistics loopstats peerstats
> filegen loopstats file loopstats type day enable
> filegen peerstats file peerstats type day enable
>
> restrict default limited kod nomodify nopeer noquery
> restrict source nomodify noquery
> restrict 127.0.0.1
> restrict 192.168.0.0 mask 255.255.0.0
>
> tos mindist 0.002
>
> refclock shm unit 1 minpoll 4 maxpoll 4 flag4 1 prefer refid PPS
> refclock shm unit 0 minpoll 4 maxpoll 4 time1 0.057984 refid GPS
>
> #server nts.time.nl nts
> #server nts.ntp.se:4443 nts
>
> server tempus1.gum.gov.pl prefer
> server tempus2.gum.gov.pl
> server ntp.task.gda.pl
> server ntp.nask.pl
>
> server MY_GPSDO iburst
>
> Before that I thought it was because I used NTP pool, as some time ago
> similar behavior was mentioned here on the list, but, as you can see, I do
> not use it anymore and the same strange behavior remained.
>
> Any idea what could be the cause of this? Thanks in advance.
>
> Adam
> ====================================
>
> What are you doing to that poor RPi to drive its CPU up to 66 C!  Rather
> hot!
>
> That's the first thing I would investigate if the CPU load is really so
> low
> (9%).  Comparing, my oldest RPi server is a lowly RasPi 1B, and it runs at
> 2% CPU, ~43 C CPU (averaged over a day).  It lives in an unheated cupboard
> on an outside wall.
>
> Cheers,
> David
>

It's ~65°C on purpose. I'm using ntpheat (
https://blog.ntpsec.org/2017/02/01/heat-it-up.html) to keep stable
temperature no matter what it does. It turns my raspberry into kind of OCXO
:)
Anyway, ntpheat runs there for months, so I guess it's not the culprit.

Cheers,
Adam