[ntp:questions] oscillations in ntp clock synchronization

David L. Mills mills at udel.edu
Tue Jan 15 04:51:02 UTC 2008


If chrony does discipline the clock frequency, it would use some kind of 
  phase-lock feedback loop (PLL). PLLs can show instability as you 
describe due to excessive loop gain. It gets pretty technical and a nerd 
treatment is in das Buch; however, the chrony designers should know 
about this and be able to correct it. The ntpd feedback loop stands on 
its head to avoid this problem, espcially as a scondary server with 
dependent clients; as this can be amplified and cause a whip effect.


Bill Unruh wrote:
> 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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