Just a shot in the dark, but it looks to me like an errant overly-greedy
cron job or systemd timer - perhaps something else is polling the serial
port on a schedule. If you set up hourly graphs you might be able to
correlate it with something else firing on the server at those intervals.
Unfortunately the spikes don't line up with uniformity within the two
hour intervals of the graph, so my theory could just be a rabbit-hole.
On 11/20/19 14:02, Richard Laager via devel wrote:
I now have a second stratum 1 server, with an independent setup. This
allows me to compare the two. Why does ntp1 have this very specific
repeating pattern of local clock offset? It's roughly +7 us, -5 us, +2
us, -4 us and then repeats again, over and over. We can also see that in
the histogram, which has three local peaks, as opposed to ntp2's which
looks like a normal distribution.
Here are the two NTPviz URLs:
https://ntp1.wiktel.com/day/
https://ntp2.wiktel.com/day/
The server hardware is the same... same chassis, same motherboard, CPUs,
etc. Both have their respective PPS connected via a motherboard serial
port. Both are running Ubuntu 18.04 with NTPsec 1.1.7.
They are in different buildings. The hand-picked network servers they
reference are slightly different, but I don't think that's relevant here.
ntp1 uses the PPS refclock with a polling interval of 3 from a telecom
GPSDO (OCXO):
server 127.127.22.0 minpoll 3 maxpoll 3 prefer
fudge 127.127.22.0 refid PPS
ntp2 uses the SHM refclock with a polling interval of 1 from a ublox 7
timing evaluation kit via gpsd:
server 127.127.28.2 minpoll 1 maxpoll 1 prefer
fudge 127.127.28.2 refid GPS
I could try fiddling around with the polling interval. Next steps might
be to try raising the polling the interval to 4 and/or lowering it to 1.
The driver_pps documentation states, "This driver incorporates a good
deal of signal processing to reduce jitter using the median filter
algorithm in the driver. As the result, performance with minpoll
configured at 4 (16 s) is generally better than the kernel PPS discipline."
Another idea might be to switch to the PPS refclock on ntp2 to see if it
behaves similarly with the ublox PPS.
Before I start blindly flailing about, does anyone have recommendations?
As a second question, if you look at the ntp2 weekly graphs, there is a
single huge transient. Any idea what that might be? I've seen these
about once or twice a week since I put this in a couple weeks ago:
https://ntp2.wiktel.com/week/
--
Paul Theodoropoulos
www.anastrophe.com
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