On 2011-12-23, Richard B. Gilbert <rgilber...@comcast.net> wrote: > On 12/22/2011 11:35 PM, unruh wrote: >> On 2011-12-23, Richard B. Gilbert<rgilber...@comcast.net> wrote: >>> On 12/22/2011 2:11 PM, Paul Sobey wrote: >>>> Dear All, >>>> >>>> I work for a firm which requires clocks to be synchronised to quite a >>>> high degree of accuracy. >>>> >>>> We have an existing ntp-based infrastructure but want to improve on it >>>> to the point where the bulk of our hosts are synchronised to single >>>> digit microseconds of each other if possible. We have about 400 hosts in >>>> production, spread across about 15 sites. >>>> >>>> I hear from many vendors and industry colleagues that 'ntp just isn't >>>> suitable for high precision work and anything less than 1-2ms precision >>>> requires ptp or direct connection to gps clock'. I find these numbers >>>> somewhat suspect, and wanted to ask the advice of you experts. In >>>> particular I've read several threads on this list and other sites which >>>> suggest that highly accurate synchronisations are possible, assuming OS >>>> and network jitter can be minimised. >>>> >>>> Our internal testing to this point is that a stock ntpd pointed against >>>> a stratum 1 clock on a low contention gigabit ethernet (stratum 1 source >>>> and client less than 1ms apart) reports its own accuracy at approx 200 >>>> microseconds. Further tuning the ntp config by adding the minpoll 4, >>>> maxpoll 6 and burst keywords result in ntpd reported accuracy dropping >>>> to within 10-20 microseconds (as reported by ntpq -p and borne out by >>>> loopstats). Further improvements can be made running ntpd in the RT >>>> priority class. >>>> >>>> My questions to you all, if you've read through the above waffle are: >>>> >>>> - what is a sensible expected accuracy of ntpd if pointed at several >>>> stratum 1 time sources across a low jitter gigabit network (we'd >>>> probably spread them over several UK and US sites for resiliency but all >>>> paths are low jitter and highly deterministic latency) >>>> >>>> - are there any obvious tunables to improve accuracy other than >>>> minpoll/burst and process scheduling class, and how agressive can the >>>> polling cycles be sensible made? >>>> >>>> - can ntpd's own reported offset (ntpq -p or loopstats) be trusted >>>> (assuming high priority means it gets scheduled as desired)? I've quoted >>>> our apparent numbers at several people and the response is always 'pfft >>>> you can't trust ntpd to know its own offset' - but nobody can ever tell >>>> me why >>>> >>>> I appreciate these may appear to be silly questions with obvious answers >>>> - I am grateful in advance for your patience, and any research sources >>>> you may direct me to. >>>> >>>> Many thanks, >>>> Paul >>> >>> If you can possibly site a GPS antenna and receiver at your location, >>> you can get microsecond accuracy or better. The receiver will output a >>> "tick" each second. One edge of the tick signal will be within about >>> 50 nanoseconds of the "start" of a second. >>> >>> The receivers cost anywhere from $100 and up. Some people need, or just >>> want this level of accuracy. You do need to be able to site an antenna >>> with a clear view of the sky. >>> >>> The last time I heard, there were twenty-seven GPS satellites in >>> service. There are usually anywhere from three to five or six above the >>> horizon at any given time. Given at least three satellites in >>> line-of-sight your GPS receiver can figure out the latitude, longitude, >>> and elevation of your antenna. Once it has done this it only needs to >>> see a single GPS satellite to get the time. >>> >>> This kind of accuracy is far more than most people really need. It's >>> there if you need it even if you only need it for "bragging rights"! >> >> Or timing how long it takes neutrinos to get from Cern to Grand Sasso. >> >>> > > How do you "tag" a neutrino so that you can say with assurance that the > the neutrino that left Cern is the same neutrino that arrives at Sasso?
The neutrinos are created by collisions of protons which produce pions which decal amonst other things to neutrinos. The protons come in bunches, so knowing when the protons raced by (at 99.9%c) the neutrinos are created in the same line and thus you know what the timing is between when the proton went by and the neutrino was absorbed. So you tag it by time. > > _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
