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

unruh unruh at invalid.ca
Wed Mar 7 00:24:07 UTC 2012

On 2012-03-06, A C <agcarver+ntp at acarver.net> wrote:
> 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.

ntp limits all slewing to 500PPM. Thus that 2 sec would have taken over
an hour to slew. If it went faster than that then there is something
else wrong. Or what you called a slew was actually a step. 

> 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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