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

John Ackermann N8UR jra at febo.com
Fri Apr 21 21:14:21 UTC 2006


Thanks for the clarification, Dave.

If that's the expected behaviour, wouldn't it make more sense for the 
minpoll and maxpoll parameters to be specified on a global basis, rather 
than as a server option?  Setting it as if it's a per-server option 
would lead one to believe that only that server's polling is being affected.

John
----
David L. Mills wrote:
> 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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> 
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