[time-nuts] Noob question, NTP stratum 1.

[Paul Theodoropoulos]

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