[ntp:questions] Sub-millisecond NTP synchronization for local network

Richard B. Gilbert rgilbert88 at comcast.net
Thu Dec 4 22:21:51 UTC 2008


Unruh wrote:
> "Richard B. Gilbert" <rgilbert88 at comcast.net> writes:
> 
> 
>> There are other tools than NTPD.  One called "chrony" MAY meet your 
>> needs, or may not.  NTPD is very good at working over the Internet with 
>> its unpredictable queuing delays.  Chrony, as I understand it, is not so 
>> good at working over the internet.
> 
> No idea where you get this from. chrony works over the internet at least as
> well as does ntpd. Its philosophy of dealing with different delays is
> different than ntp's( although it can be set up to be virtually identical)
> 
> 
>> If you can't keep your machines up 24x7, chrony MAY be a better tool. 
>> It's possible that you will need something else entirely.
> 
> Possibly true. 
> 
> 
>> You may find that a hardware reference clock; e.g. a GPS timing 
>> receiver, will help.  With a GPS timing receiver, you will not be 
>> dependent on the internet for time sources.  NTPD will still need about 
>> thirty minutes to gain really tight synchronization.  Once gained, 
>> synchronization should be stable as long as the machine is up.
> 
> Actually it is much worse than that. On my system, on bootup the clock
> frequency can very by up to about 50PPM due to a Linux bug. In general it
> takes ntp about 10 hours to regain tight synchronisation. (In that case it
> is microsecond since it is synching to a GPS, but it is also on poll level
> 4 so it has lots of data and should converge faster than some other system
> on poll level 6-10). David Mills has always insisted that ntpd is designed
> for stable long time operation, and rapidity of response is a distant 49th
> or so in priority. 
> 

My Solaris 8 SPARC system seems to be able to synch with the GPS 
receiver in about 30 minutes.  I may reboot that system once a year 
unless we have a power outrage that lasts longer than the UPS battery!
Absent something like a power failure there's no real reason to reboot 
it at all.




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