[ntp:questions] Mixed maxpoll values for mixed LAN/Internet servers - sensible?

Unruh unruh-spam at physics.ubc.ca
Thu Feb 26 18:53:50 UTC 2009

"David J Taylor" <david-taylor at blueyonder.neither-this-bit.nor-this.co.uk> writes:


>It's been suggested that if I have a mixture of a "known-good" (i.e. 
>GPS/PPS-based) LAN server, and some Internet-based backup servers, I could 
>use an ntp configuration file with different maxpolls, with the idea that 
>syncing more often to a good source will produce even lower offsets. 
>Something like:

>  server lan-gps-pps  prefer iburst  maxpoll 6
>  server 0.uk.pool.ntp.org iburst
>  server 1.uk.pool.ntp.org iburst
>  server 2.uk.pool.ntp.org iburst

>If I do this, is the maxpoll for the Internet servers also clamped at 6, 
>or will it gradually up to 1024s (10).

No, not clamped. Maxpoll refers to the specific server it is referenced to . In your
case lan-gps-pps.

Because of the way ntp disciplines the clock, the disadvantage of low poll
intervals is that the rate discipline of the clock is poorer. Now, if you
remain attached to the source, that is not critical, since the more
frequency polling makes up for it. But if you ever get disconnected from
that source, your clock will drift off correct time faster. Actually your
poll interval should be tuned to your particular computer's clock. If your
computer receives a lot of variable use ( big high cpu programs for 15 min
and then nothing for the next 15 min) poll 10 is too long. If it sits there
chugging along doing exactly the same stuff all the time, It could even be
too short. ntp has no way of measuring the real "Allan intercept" for your
machine. chrony does that automatically, and as a result actually controlls
the clock better than does ntp. But that is another topic. 

>Is this a sensible thing to do for a client without a refclock?

It depends. Note that it also depends on who owns that refclock. They may
not want people querying it so often. 
Certainly Mills would get annoyed if you clamped the poll interval to the
udel servers at maxpoll 6
Of course if you own it, you can do as you wish.


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