✘Local RMS Time Jitter: Ooops

Gary E. Miller gem at rellim.com
Thu Oct 13 01:26:13 UTC 2016

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

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:


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

You can see the full data here:


Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
	gem at rellim.com  Tel:+1 541 382 8588
-------------- next part --------------
A non-text attachment was scrubbed...
Name: local-jitter.png
Type: image/png
Size: 15694 bytes
Desc: not available
URL: <http://lists.ntpsec.org/pipermail/devel/attachments/20161012/4836c265/attachment.png>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 455 bytes
Desc: OpenPGP digital signature
URL: <http://lists.ntpsec.org/pipermail/devel/attachments/20161012/4836c265/attachment.bin>

More information about the devel mailing list