[ntp:questions] ntp & system without a rtc

Richard B. Gilbert rgilbert88 at comcast.net
Tue May 14 02:30:00 UTC 2013

On 5/10/2013 8:30 PM, unruh wrote:
> On 2013-05-10, Rick Jones <rick.jones2 at hp.com> wrote:
>> Richard B. Gilbert <rgilbert88 at comcast.net> wrote:
>>> I think you may be out of luck on this one.  If you can run NTPD 24x7
>>> you can have the correct time 24x7.  If you can't do this, NTPD is a
>>> poor choice.  The problem is that NTPD needs something like ten hours to
>>> select a time source and match the time!
>> NTPD is no speed daemon, and perhaps it is a subjective thing, or a
>> mis-interpreation on my part, but I notice NTPD declaring sync rather
>> sooner than 10 hours.  Rather sooner than one hour even (looking at
>> ntpq -p output).  Now, it may indeed take it a long time to get the
>> offset (term?) down to some acceptable level, but that depends on
>> one's definition of acceptable and the initial distance no?
> Yes. The 10 hours is for achieving the ultimate accuracy of a few
> microseconds offset. It has a halving time of a bit under an hour (lets
> say 45 min) . Ie,
> after 45 min, the size of the offsets is about 1/2 of what it
> was before. Because of stepping it has an error of about 100ms to start
> out.
> If you are happy with few millisecond precision, it will only take an hour
> or two.
> So if you start it out with the -g it will figure out is is way out
> after a few seconds, and step. But the rate in general will still be
> out, so it will rapidly drift away and ntpd will slowly bring the drift
> into order.
> Thus to get from hours out to hundreds of milliseconds out should occur
> very quickly.
>> rick jones

NTPD is NOT designed for fast convergence.  From start up to get the 
minutes correct, NTPD will need about thirty minutes!  To get the best 
time you are going to get, you will need to wait for about ten hours!

If fast convergence is your goal, you can use a a program called
CHRONY to achieve it.

If your goal is to know the time +/- 50 nanoseconds You are expected to 
operate your clock twenty-four hours a day.

With a GPS Timing Receiver you can keep time +/- 50 nano-seconds. 
Relatively few people NEED time with that sort of accuracy.  Many
who DO need that accuracy use NTP and a Global Positioning Satellite
to get the accuracy required.  A few of these people read this

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