[ntp:questions] Poul-Henning Kamp and re-write of NTP

Martin Burnicki martin.burnicki at meinberg.de
Mon Dec 8 11:32:47 UTC 2014

Paul wrote:
> I asked before but I'll try again since "tsc" has come up.
> Mills says (Mitigation Rules and the prefer Keyword):
> "The clock combine algorithm uses the survivor list to produce a weighted
> average of both offset and jitter. Absent other considerations discussed
> later, the *combined offset* is used to discipline the system clock"

Unfortunately, the above seems true.

> phk says (20141107 - Probably Noon):
> "In classical NTPD we always pick one server at a time, and steer our clock
> only with the measurements from that server."
> So which of these is correct?  Or does "classical NTPD" refer to an 18th
> century version?  I know other people make the same assertion but I fail to
> understand what they mean.

Some time ago I've run a test where firstly ntpd was configured to use 
only a single GPS PCI card as time source, which resulted in very nice 

However, after I had added some NTP servers on the LAN to the 
configuration and these servers were accepted as "candidates" marked 
with a '+' in the output from "ntpq -p" the loopstats became much worse, 
which was obviously since the NTP servers provided much more jitter than 
the GPS refclock.

I'll see if I can find the loopstats graph, and post a link to it.


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