[ntp:questions] Stick to PPS, even if the prefer server fails

Unruh unruh-spam at physics.ubc.ca
Thu Mar 26 04:46:09 UTC 2009

Thank you. Your quote at the top is correct
"Listen carefully to what I say; it is very complicated."
But I have no idea if I have listened carefully.

So, I have PPS running the shm refclock, with the seconds supplied by the
local clock (ie the reading from the system clock) and the usec from the
PPS signal.  Since I do not expect the local clock to suddenly shift by a
second, its usec discipline should be fine. I mark it as the preferred
server. (The PPS shm is run only once the local clock is disciplined to
within .1 sec by the other sources). 

Does this make sense? Or have I totally midunderstood.

mills at udel.edu (David Mills) writes:

>Bill,If you had taken the trouble to look at the documentation , you 
>would have found the "Mitigation Rules and the Prefer Peer"  
>http://www.eecis.udel.edu/~mills/ntp/html/prefer.html page which clearly 
>describes how the contraptoin works.

>Unruh wrote:

>>Dave Hart <davehart at gmail.com> writes:
>>>On Mar 25, 9:19=A0pm, Unruh <unruh-s... at physics.ubc.ca> wrote:
>>>>I have no idea why and whether kernel PPS code is any better ( or worse)
>>>>than say PPS discipline using the shm PPS refclock using parallel port
>>>>interrupt. Ie, both can discipline
>>>>to about 1-2usec level.
>>>Re-read the thread, then.  A kernel clock disciplined by PPS allows
>>>PPS to continue to discipline the clock when ntpd's PPS implementation
>>>stops doing so because of a prefer peer problem.
>>I must admit that I am completely confused by this prefer peer problem. I
>>would think that ntp would use the PPS/refclock preferentially no matter
>>what external servers are doing. But clearly I am not understanding
>>something. If it does not that would seem to me to be a bug in ntp, not
>>something one should be rewriting kernel code to overcome. 

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