[ntp:hackers] Fast startup with refclocks

David L. Mills mills at udel.edu
Wed May 11 18:58:12 PDT 2005


There was some discussion awhile back about fast startup for refclocks. 
One suggestion was to use the iburst flag. This is a lose in some 
refclocks and can interfere with the polling mechanism.

However, there is a better scheme that synchronizes the refclock within 
four seconds. I have done this for the WWVB, PST and Arbiter drivers. It 
is already used in the ACTS driver, but for another reason during a 
call. The scheme is very simple and amounts to reading the time every 
second until the peer dispersion falls below MAXDISTANCE. See lines 
375/6 in refclock_wwvb.c in ntp-dev. This is likely to work in any 
driver that can poll the clock once per second, which avoids the median 
filter, until the distance falls below threshold, in which case switch 
back to the regular poll interval. This works because the root delay and 
root distance for refclocks is zero.

I have modified the ATOM driver to produce a clockstats line for every 
second if fudge flag4 is lit. This is intended for collecting Allan 
deviation data. If you use this, use the disable ntp command to open the 
discipline loop.


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