[ntp:questions] systems won't synchronize no matter what

Chuck Swiger cswiger at mac.com
Sat Oct 30 16:09:17 UTC 2010

On Oct 29, 2010, at 11:30 PM, David J Taylor wrote:
> - why add  maxpoll 9 to all the servers?  That seems wrong.  I see the note: "The option `maxpoll 9' is used to prevent PLL/FLL flipping on FreeBSD.".  So is that a FreeBSD specific issue, and if so, why not fix it in the OS?

Because it's not a bug.  There are different ways of controlling clock adjustments (Phase Locked Loop vs Frequency Locked Loop), and the FreeBSD kernel will switch between them depending on how often adjtime() is being invoked.  The only real issue is that logging about it can be annoying, but I think newer versions of ntpd have changed the log level to only display the message if debugging is enabled.  See:



"In operation, PLL mode is preferred at small update intervals and time constants and FLL mode at large intervals and time constants. The optimum crossover point between the PLL and FLL modes, as determined by simulation and analysis, is the Allan intercept. As a compromise, the PLL/FLL algorithm operates in PLL mode for update intervals of 256 s and smaller and in FLL mode for intervals of 1024 s and larger. Between 256 s and 1024 s the mode is specified by the API. This behavior parallels the NTP daemon behavior, except that in the latter the weight given the FLL prediction is linearly interpolated from zero at 256 s to unity at 1024 s."


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