✘Local RMS Time Jitter: Ooops

[Gary E. Miller]

Yo All!


Well, this one was a bit odd.  I updated a RasPi from Linux kernel 2.4.22
to 2.4.24, at the same time I updated to NTPsec git head.  What could go
wrong?

Well, at 4.4.22, repeated calls to clock_gettime() would return about
every 1 micro sec.  From 4.4.24, that does not return more than about
every 2 micro sec.  So sys_precision degrades from -20 to -19.

This is the RasPi kernel bug, don't hold your breath:

    https://github.com/raspberrypi/linux/issues/1680

The bad effect was shown in yesterday's image.  The good news is that
KPPS accuracy is much better than sys_precision, so changing the code
to not ignore data better than sys_precision makes the RasPi ntpd even
better than before.

This change is in commit 67c2abdabfea27bd84bdf934ba68bb40560dbcbe

See the attached for preliminary results.  Before 23:25 Oct 11 you can
see the data loss at about 1 micro sec.  At 23:25 Oct 11 you see that
degraded to about 2 micro sec.  After the patch at 19:30 Oct 12 you can
see the RMS jitter data looks much better than before the knerle bump.
The PPS refclock jitter is still a smidgen worse than before the kernel
change.

You can see the full data here:

    https://pi4.rellim.com/tmp/#local_rms_time_jitter


RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
	gem at rellim.com  Tel:+1 541 382 8588
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