[ntp:questions] Difference between PPS and PPS plus kernel discipline?

John Ackermann N8UR jra at febo.com
Sun Mar 26 02:59:41 UTC 2006

Today I changed the configuration file of two of my NTP servers to
enable the kernel timekeeping discipline, and have put plots of the
effect of the change at

The data is derived from the offset and dispersion fields of the
peerstats file.

When using kernel discipline, the offset and jitter shown by the ntp
stats are essentially 0, which makes sense since both the reference and
the unknown time come from the same source; they should track each other

When not using kernel discipline, there is a wander of several hundred
microseconds in one case, and several milliseconds in the other, and
significantly more jitter in both cases.

My question is about that wander -- is it showing the (in)stability of
the crystal on the motherboard, *or* is it simply showing that NTP by
itself doesn't steer the system time as tightly as the kernel
discipline?  Put another way, can I make any observations about the
quality of the motherboard crystal from this data, or am I looking at
NTP's PLL performance?

Bottom line, I'm trying to explain what these plots tell me, other than
the fact that I need an external reference to evaluate the performance
of the NTP/PPS system.



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