[ntp:questions] Popcorn on prefer takes out system clock

A C agcarver+ntp at acarver.net
Tue Mar 6 19:01:47 UTC 2012

Last night I had a large popcorn event coming from a server marked 
"prefer" (for the benefit of the ATOM refclock) upset my system clock 
(ntpd ver 4.2.7p259).  It knocked the clock out by about two seconds by 
slewing the clock correction very quickly (almost a step response, 
actually) from which it never recovered.  I had to restart ntpd to allow 
a clock step to bring the clock back in line.

I switched away from that server to another but happened to spot this in 
the log today from the remote server that used to be the prefer: 944d 8d popcorn 2147483647.997598 s

The system was quite stable up until the moment this popcorn appeared 
and then it fell apart quickly.  It would appear that the control loop 
does not gracefully handle a very gross error from the system peer.  I 
would have expected something like this to be ignored and the server 
rejected in favor of the other configured servers but it doesn't seem to 
be the case.

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