Re: Question about min/max poll and an interesting plot showing maxpoll's effect
- From: "David L. Mills" <mills@xxxxxxxx>
- Date: Fri, 21 Apr 2006 20:43:44 +0000
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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