[time-nuts] Noob question, NTP stratum 1.
Paul Theodoropoulos
paul at anastrophe.com
Mon Jul 22 19:57:26 UTC 2019
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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