Quoting Dr. Nikolaus Klepp (dr.klepp@???):

> Just for your amusement: systemd breaks linuxcnc on RPi4 - who's not
> into machining will most likely not see the fun part of it, but anyway
> :)
>
> https://www.youtube.com/watch?v=RDKaFJmB254

Nik --

Thank you for the link. I think it should be of interest for people
wanting to observe an inquisitive mind doing good diagnosis work,
whether they are interested in linuxcnc or not, e.g., right around the
11:35 minute mark (out of 14:43) he arrives at the important conclusion,
through logical testing, that systemd's timesyncd implements an
un-gentle jerk-forward-or-back of time all at once -- as opposed to the
gradual adjustment any of the real NTP daemons would do -- and that such
rough time discontinuities are problematic for real-time operations.

(It's distinctive that the entire RPi series lacks by default any RTC
circuitry, so it's particularly vulnerable to such software problems.)

Starting around 13:00, he tests his hypothesis by switching to ntpd, and
indeed the observed symptom immediately goes away.

-- 
Cheers,              "Given where we started from, which was crawling up some 
Rick Moen            beach, somewhere, and no one even knows where that was, we 
rick@???  haven't really done that badly. And I would much rather be
McQ! (4x80)          a rising ape than a falling angel."   -- Terry Pratchett