[ntp:questions] drift value very large and very unstable
David L. Mills
mills at udel.edu
Thu Mar 6 18:28:57 UTC 2008
1. As per the advice in the documentation, do not use iburst with
reference clock drivers. The only reason you might want to do this is to
reduce the time to set the clock on initial start. This is unnecessary
as the clock is now set on the first reply received. Using iburst anyway
will screw up the radio protocol in some drivers.
2. The temptation to reduce the poll interval below the default can be
counterproductive. The driver interface uses a median filter to clean up
nominal jitter due to serial port and interrupt latencies. Generally,
the jitter is reduced as the number of stages and the poll interval are
increased. There are cases, in particular with kernel PPS signals, where
a smaller poll interval can result in marginally better performance, but
in other cases it generally not a good idea.
3. The burst mode is designed for use when the poll interval of
necessity must be very long, like at least 1024 s. The current design
will rate-limit if burst is used with a poll interval of 512 s or less.
This is to protect busy servers with a minimum average default headway
of 16 s. Some operators might set the headway higher.
Kevin Oberman wrote:
>>From: Harlan Stenn <stenn at ntp.org>
>>Date: Thu, 06 Mar 2008 06:33:17 +0000
>>Sender: questions-bounces+oberman=es.net at lists.ntp.org
>>iburst is good. burst, very likely not.
> In general, I agree, but the context is for a connected reference clock.
> I would set maxpoll to 4 as there is no reason NOT to update from the
> reference clock on a frequent (16 second) basis, but I'm less certain if
> iburst is really appropriate. (Nor am I sure that it's inappropriate.)
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