NTP Performance

Richard Laager rlaager at wiktel.com
Wed Nov 20 22:02:32 UTC 2019

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:

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

server minpoll 3 maxpoll 3 prefer
fudge refid PPS

ntp2 uses the SHM refclock with a polling interval of 1 from a ublox 7
timing evaluation kit via gpsd:

server minpoll 1 maxpoll 1 prefer
fudge 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:


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