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"!
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