On 7/22/19 11:52, Achim Gratz wrote:
Well, I have jitter (PPS timestamps) within ~2µs and the occasional
"hair" that goes out to about 30µs. The distribution is a little bit
wider on the three systems that are loaded with extra server tasks, but
not much. The thing to note is that you need to boot with "nohz=off",
set a fixed frequency as high as possible and use an interval timer for
controlling the CPU load (I use all four cores as workers and adjust
them at 40Hz).
Yes, nohz off is necessary for sure. I use a bunch of assorted tweaks
and am able to keep RMS time jitter under 300ns at 95th percentile in a
given 24 hour interval. However, it takes very, very, *very* little user
(me) activity on the device itself to cause larger spikes into the
microsecond realm. I upgraded my devices from Raspbian Stretch to Buster
over the weekend, causing a spike to 3 microseconds offset. However,
offsets have settled in even lower since the upgrade.
I use,among other things, partrt to set up 'realtime' cpu partitioning,
and schedtool to nail ntpd to core 3 and gpsd to core 2 - but I suspect
any improvement I see probably falls into the psychosomatic realm. ;)
Disabling as many cronjobs and running services as possible provides by
far the most benefit to stability. Oh, and staying off the command line!
--
Paul Theodoropoulos
www.anastrophe.com
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