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<p>Hi Doug,</p>
<p>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.<br>
</p>
<p>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.</p>
<p>Martin<br>
</p>
<div class="moz-cite-prefix">On 2020-05-05 14:08, Doug Curtis via
users wrote:<br>
</div>
<blockquote type="cite"
cite="mid:CACSTJtS9HPbTSYKKx8dNEv4UQSirKxog6wePS-9yraK_pe8wBw@mail.gmail.com">
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
<div dir="ltr">I apologize if this has been answered previously.
<div><br>
</div>
<div>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 "<a class="gmail-a-link-normal"
href="https://www.amazon.com/Gowoops-Module-Antenna-Arduino-Microcomputer/dp/B01MRNN3YZ/ref=cm_cr_arp_d_bdcrb_top?ie=UTF8"
style="font-size:13px;box-sizing:border-box;text-decoration-line:none;color:rgb(85,85,85);font-family:"Amazon
Ember",Arial,sans-serif" moz-do-not-send="true">Gowoops
GPS Module</a>". 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.</div>
<div><br>
</div>
<div>After changing the ntpviz period to 2 hours, it appears
they are all spiking after 34 minutes. <a
href="https://world-mail.org/ntpgraphs/gps3/"
moz-do-not-send="true">https://world-mail.org/ntpgraphs/gps3/</a></div>
<div>Here is a 24 hour period of one of the others: <a
href="https://world-mail.org/ntpgraphs/gps2/"
moz-do-not-send="true">https://world-mail.org/ntpgraphs/gps2/</a></div>
<div><br>
</div>
<div>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.</div>
<div><br>
</div>
<div>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:</div>
<div>3.20.1~dev (revision release-3.20-695-g2de5ac5de)<br>
</div>
<div><br>
</div>
<div>Here is the ntpd version:</div>
<div>ntpd ntpsec-1.1.8+ 2020-05-04T14:44:13Z (git rev 574d871ed)<br>
</div>
<div><br>
</div>
<div>Here is what the ntp.conf looks like:</div>
<div><br>
</div>
<div><font face="monospace">driftfile /var/lib/ntp/ntp.drift<br>
leapfile /etc/ntp/leap-seconds.list<br>
<br>
logfile /var/log/ntp.log<br>
<br>
statsdir /var/log/ntpstats/<br>
statistics loopstats clockstats<br>
#filegen peerstats file peerstats type day enable<br>
filegen loopstats file loopstats type day enable<br>
filegen clockstats file clockstats type day enable<br>
#filegen sysstats file sysstats type day enable<br>
<br>
restrict default kod limited nomodify nopeer noquery<br>
restrict -6 default kod limited nomodify nopeer noquery<br>
restrict 127.0.0.1<br>
restrict -6 ::1<br>
<br>
refclock shm unit 1 maxpoll 4 refid PPS flag4 1 prefer<br>
<br>
server <a href="http://rolex.usg.edu"
moz-do-not-send="true">rolex.usg.edu</a> iburst<br>
server <a href="http://ntp3.mattnordhoff.com"
moz-do-not-send="true">ntp3.mattnordhoff.com</a> iburst<br>
<br>
refclock shm unit 0 maxpoll 4 time1 0.138 refid GPS flag4 1</font><br>
</div>
<div><br>
</div>
<div>I can't think of anything else to look for or try.</div>
<div><br>
</div>
<div>Thanks,</div>
<div><br>
</div>
<div>Doug</div>
<div>
<div><br>
</div>
<div><br>
</div>
</div>
</div>
<br>
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