Updated ntpd misbehaves

[roman55]

Two days ago there was a security update for ntp,CESA-2017:0252, pointing to ntp-4.2.6p5-10.el6.centos.2.x86_64.rpm. After installing it the jitter values of my ntpd jumped up by a factor of 100.
Only after reverting to ntp-4.2.6p5-10.el6.centos.1.x86_64 did it go back to normal. You can clearly see that in the attached plot of offsets and jitter.

ntp1.png (25.98 KiB) Viewed 730 times

The machine has a local GPS driven by gpsd and some local and remote servers:

Code: Select all

     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
+SHM(0)          .GPS.            0 l   48   64  377    0.000    3.052   1.060
*SHM(1)          .PPS.            0 l   47   64  377    0.000    0.037   0.003
-dnsx.mathconsul 162.23.41.10     2 u   39   64  377   14.183   -1.909   0.406
-metasntp13.admi .MRS.            1 u   50   64  377   14.326   -0.614   0.387
-vega.mathconsul 192.168.100.32   2 u   40   64  367    0.024   -0.540   0.385
+sensor2.mathcon .PPS0.           1 u   64   64  376    0.244   -0.035   0.010
 LOCAL(0)        .LOCL.           8 l    -   64    0    0.000    0.000   0.000
 ...

Startup messages in syslog do not show any difference. Besides the security fixes, are there any other changes that could explain this strange observation? Other servers without precise local clocks don't show the problem, but there the normal jitter is much higher to begin with.