[ntp:questions] ntp & system without a rtc

unruh unruh at invalid.ca
Sat May 11 00:30:52 UTC 2013


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



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