[ntp:questions] NTP vs chrony comparison (Was: oscillations in ntp clock synchronization)

Unruh unruh-spam at physics.ubc.ca
Wed Jan 23 09:27:36 UTC 2008


Just an update: I started chrony with a 60ms offset. It had the right drift
file. It took about 1 min ( having collected about 4 samples from the
servers at minpoll 4) to drive the offset down to about 100 usec (Yes, a
1000 fold improvement in about 50 sec.) Ie, the time constant for
correction of offset errors is enough time to collect enough samples to
determine that the offset really is statistically way off. 

I also started chrony without a drift file. In this case it took about 5
min to get a frequency within 10% of the long term stable frequency and
that "error" disappeared within 1/2 hour.

I have also patched chrony so that it can put itself at max priority. It
seems clear to me that the reason that NTP was so much better at the round
trip scatter was that it was running at max priority. Ie, the large spikes
in the round trip times was because chrony was not being woken up, or was
swapped out, rather than any problem with the network.

However I will have to run chrony again for a while to collect statistics.
However, even without the high priority, chrony did better than NTP at
keeping the clock disciplined, and this taming of the round trip
fluctuations should help. 

To compare the transient response of chrony and NTP, look at the graphs for
flory (bottom graph on the right at
www.theory.physics.ubc.ca/chrony/chrony.html)
and fluxon (fourth down on the right).Both suffered a sudden change in the
drift rate of the clock it appears. 
On the NTP controlled clock there seems to have been a sudden .2PPM change
in the drift rate of the clock on Jan22.8. This caused a 500usec error in the offset
errors in the clock, which took a few hours to settle down. 
Contrast this with fluxon at Jan 21.27 where it seems to have suffered a
2PPM sudden change in the drift (ten times the change that flory suffered).
This caused only a 200 usec offset, which chrony corrected within 5 min.
The similar jump at 21.4 behaved in the same way. Ie, a jump 10 times as
big had an effect less than 1/2 as large, and fixed on the timescale of
over 20 times faster. 






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