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

Brian Utterback brian.utterback at sun.com
Thu Jan 24 20:54:59 UTC 2008

Unruh wrote:
> situation, but have no reasons for that worry. The very worst case is if
> the system runs for a while on very short poll intervals, and then suddenly
> has very log poll intervals. The short period estimation of the drift is
> not a good estimator of the long period drift. But I suspect that NTP would
> have problemsi in that situation as well. 

Perhaps. Perhaps not. The NTP reference code chooses its own poll
interval based on the clock stability and the sample jitter. For
a frequency correction to be valid, the clock offset must be greater
than the sample jitter. As the frequency gets closer to the correct
value the poll interval must get longer. See, NTP has a different
design goal than chrony. The goal of NTP is not merely to keep the
clock in sync, but to also discipline the frequency, while also
providing a stable time synchronization network. If your goal is
to keep the offset as low as possible, just keep the poll interval
as short as possible. That doesn't take much work.

Brian Utterback

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