Bill, 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.
Dave 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. > _______________________________________________ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
