Unruh wrote: > "Richard B. Gilbert" <[EMAIL PROTECTED]> writes: > >> Unruh wrote: >>> "Richard B. Gilbert" <[EMAIL PROTECTED]> writes: >>> >>>> Hal Murray wrote: >>>>>> It's the poll interval of ntpd. Ntpq does not poll! The poll interval >>>>>> varies between 2^MINPOLL and 2^MAXPOLL. You have set MINPOLL=MAXPOLL=4 >>>>>> giving a poll interval of 2^4 or 16 seconds. This is usually the >>>>>> correct choice for a GPS receiver. >>>>> Why do you say that? >>>> Because the GPS time signal is extremely accurate! >>>>> Or let me ask it another way, how would you >>>>> decide what the right polling interval is? >>>>> >>>> NTPD uses much longer poll intervals over the internet or even over a >>>> local network because of the variable delays introduced by the network. >>>> No two packets are guaranteed the same path between two points unless >>>> there IS only one path. In addition to the variations in travel time >>>> due to differing paths, there are also variable queuing delays! >>> No. The longer poll intervals are mainly about keeping packets off the >>> servers. In >>> principle it is always better to poll more. (in practice with the ntp >>> model, this is only partially true-- you want the 8 times the poll interval >>> to be close tothe Allan minimum if the noise model really is exponential >>> phase and 1/f drift noise. ( on most modern networks not the greatest >>> assumption-- day to night temp variations are probably more important for >>> the frequency noise). > >> ntp presents a very light load on the servers unless you have some >> idiots polling at two second intervals (other than an initial burst at >> startup). > >> The longer poll intervals allow ntpd to measure small errors very >> accurately. > > If you want good time, use short polls. If you want good frequency, use > long polls. ntp tries to strike a balance by having the poll interval be
And if you want both good time AND good frequency? Ideally the system should provide both. > roughly the minimum of the Allan variance. But since it assumes, rathr than > measures the location of that minimum, it is pretty useless for any real > life situation. Thus, If you have continuous connectivity to the server, > short poll intervals will give you the best time ( but not the best > frequency) If you occasionally loose connectivity for some period, then the > lack of good freq will hurt you as the time will drift off too fast. > Longer poll intervals thus allow you to measure frequency drifts more > accurately, but if you have continuous connectivity, why do you care if you > know the drift rate accurately? > > The primary reason for long poll intervals is not to swamp the public > servers. -- while one packet is not too bad, 10^6 packets pers second > because of for example a very badly designed router which has your server's > address hardwired in totally swamps your connction. > > So, if it is your own server which only you use, poll as often as you wish. > If it is someone elses, don't. > > _______________________________________________ questions mailing list [email protected] https://lists.ntp.org/mailman/listinfo/questions
