[ntp:questions] Advice for a (LAN-interconnected) WLAN Testbed w/ ARM nodes

Miroslav Lichvar mlichvar at redhat.com
Tue Aug 3 21:42:03 UTC 2010

On Tue, Aug 03, 2010 at 09:14:34PM +0100, David Woolley wrote:
> Miroslav Lichvar wrote:
> >The most important thing is to set minpoll and maxpoll for the server
> >specified in ntp.conf. Generally, lower is better unless the network
> >load to the NTP server is a concern. With ntp-4.2.4 the allowed
> >minimum is 4, with 4.2.6 and it's 3.
> This is not the official policy, which is that the defaults are about right.

The defaults are ok if you don't want to use too much network
bandwidth, which is necessary for public servers. They could have been
sufficient to get the best achievable accuracy many years ago when a
typical network jitter was in tens of milliseconds, but with today's
network conditions even 6 might be too high.

> Setting low values will result in excess jitter, because you will
> tend to track the effects of varying network load, rather than
> smoothing them out.

But there will be more clock updates. Noise in frequency may go up,
but the offset will be the same or better, unless there are network
congestions that last longer than the clock filter can handle (8 *
poll interval).

Miroslav Lichvar

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