[ntp:questions] oscillations in ntp clock synchronization

Bill Unruh unruh at physics.ubc.ca
Mon Jan 14 23:53:00 UTC 2008


I have been keeping track of the clocks on my various systems. One is
synced using ntp from a GPS 18LVM pps source. The others are synced from
that machine using chrony. Almost all of the systems have a very strange
oscillation in them, with a time scale of about 1.5 hours. On the ntp
system, this oscillation in the offset has an amplitude of about 2-3 usec.,
while on the others, which use chrony, the oscillation is about 20 usec. 
The periods are roughly the same ( within about 10%) on all the systems.
These are especially obvious in the clock rates which are set by chrony,
such that the rates have oscillation of about .5 PPM . 

To add to the bizareness, the real time clocks, which chrony also monitors,
also has oscillations in the rate chrony derives for the rtc. On some of
the machines the rtc oscillations are in phase with the rate oscillations
of the system, and on some they are out of phase. Now, if the rtc were
itself a perfect clock, and if the rate oscillations in the system clock
were due to some "instability" in the chrony or ntp algorithm, then you
would expect to see the rtc oscillation be exactly the same size as the
system oscillation, out of phase and the same amplitude. But most are in
phase and all have amplitudes 2-10 times larger than the oscillation in the
system. 

(To see the data, see www.theory.physics.ubc.ca/chrony/chrony.html)
Does anyone have the ghost of a clue as to what could be going on here?

Note that exactly the same kind of oscillations can be seen in the ntp
offset on the string computer, but with a period about 10-20% smaller than
on the others, and an amplitude about 10 times smaller. 

What it looks like is that both the chrony and the ntp algorithms are
acting like amplifiers, with a huge peak  Q peak at around a frequency of 1.5hr.




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