[ntp:questions] Re: Question about min/max poll and an interesting plot showing maxpoll's effect

David L. Mills mills at udel.edu
Fri Apr 21 20:43:44 UTC 2006


John,

Yes; that is the correct behavior. Note that the poll interval is 
directly tied to the clock discipline time constant. When you force to 
smaller poll intervals the time constant becomes smaller and the 
discipline can more closely follow the clock oscillator wander. However, 
the Nyquist rate has gone up and the clients have to poll faster in 
order to preserve overall system accuracy. To do otherwise, the clients 
would need some way to explicitly disregard the peer poll interval and 
expect to relax the timing accuracy. In principle, an option can be 
provided to do this, but it sure makes things even more complicated.

Dave

John Ackermann N8UR wrote:
> I am running a stratum 2 server that syncs to my pile o' stratum 1s.  In
> addition, just in case my lab explodes, I have a couple of external
> clocks.  I've set maxpoll to 6 on the server lines for the internal
> machines, to hold the poll interval at 64 seconds.
> 
> However, doing so seems to cause the polling interval for the other
> servers -- even though they don't specify a shorter maxpoll than the
> default, they continue to poll at 64 seconds.  Is that normal?
> 
> By the way -- if you want to see the effect of more tightly controlling
> ntpd through shorter polling, take a look at
> http://www.febo.com/time-freq/ntp/stats/clients.  These plots update
> every hour, so may look different by the time you see them, but at least
> for now they show a dramatic improvement in stability when switching
> from normal polling (which ran out to 1024 seconds) down to maxpoll 6.
> The level 2 server is holding within about +/- 4 microseconds to about a
> 4 microsecond offset from the server (a stratum 1) that is monitoring
> it.  Looking at the 7 day plot shows the dramatic change when I made the
> maxpoll adjustment.  (Of course "dramatic" is a relative word -- even
> polling at 1024 seconds, the server stayed within about a millisecond).
> 
> John
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