Heiko, Be careful when making generalizations about refclocks. Some, including the Spectracom, Austron, Traconex, TrueTime and others using the median filter will not workproperly at other than default poll. Same for the modem and audio drivers.
I don't knkow what version you are using, but the current (development) version sets the time on the first poll received from a reference clock. But, !! note carefully !! On one of our servers the first poll recieved sometimes has a large error (400 ms) that torques the time until brought right 15 minutes later. Frankly, our servers run for months without disturbance, so an initial delay to synchronize to a reference clock is not a big deal. As a general rule of thumb and confirmed by theory and practice, the nominal errors increase by a factor of two for each fourfold increase in poll interval; thus, reducing the poll from 64 to 16 s reduces the errors by half. This is only true when the time constant is above the Allan intercept, which is usually in the vicinity of 2000 s. By design, the time constant is 32 times the poll interval, so 16-s poll is on the hairy edge. Dave Heiko Gerstung wrote: > Richard B. Gilbert schrieb: > >> Heiko Gerstung wrote: >> >>> Hi! >>> >>> I created a NTP cheat sheet as a short reference for the NTP >>> configuration file statements and other NTP related stuff. I am >>> pretty sure that I missed something important and I would appreciate >>> any feedback from you if you can spare a few minutes. >>> >>> Here is the link to our NTP download page: >>> http://www.meinberg.de/english/sw/ntp.htm >>> >>> You'll find the cheat sheet near the bottom of this page. >>> >>> Thanks, >>> Heiko >> >> >> The first thing I noticed was text describing how to change the >> MINPOLL and MAXPOLL options. I would add that tinkering with MINPOLL >> and MAXPOLL is usually not a good idea. If you need to ask how, you >> probably should not be changing them. > > > Thanks a lot for your feedback! > > In my experience it definitely helps a lot to set the minpoll/maxpoll > values to a lower value when talking to reference clock drivers and your > local NTP servers. On our time server appliances we use minpoll 4 > maxpoll 4 for talking to the integrated refclock (GPS, MSF, DCF77 or > IRIG) and this dramatically improves the reaction times of ntpd when it > comes to things like initial synchronization, reception outtages and so > on. Although the parse driver (which we use for all our refclocks) > already has a "fast sync" feature which allows ntpd to accept the > refclock after a few seconds (something like iburst for refclocks), > other refclock drivers can benefit from being able to pass on sync data > to ntpd four times faster than the default. > > To me there is no sense in only getting one sample in 64 seconds when > your expensive and highly accurate hardware reference clock offers one > very good sample every second. > > But well, this cheat sheet was intended to be used by people with some > knowledge of NTP since there are a lot more advanced configuration > options mentioned that are probably not very interesting for absolute > NTP beginners. > > Best regards, > Heiko _______________________________________________ questions mailing list [email protected] https://lists.ntp.org/mailman/listinfo/questions
