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. > _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
