[ntp:questions] Slow convergence of NTP with GPS/PPS

Kevin Oberman oberman at es.net
Tue Sep 30 21:11:17 UTC 2008


> From: "David McConnell" <davidm at pipstechnology.co.uk>
> Date: Tue, 30 Sep 2008 14:04:18 +0100
> Sender: questions-bounces+oberman=es.net at lists.ntp.org
> 
> 
> Hi
> 
> We are using Linux ntpd with GPS/PPS reference clock to discipline the time
> on our systems.
> 
> Our application requires good time accuracy (better than 5ms) but it also
> needs to get there quickly (as quickly as possible, but ideally taking no
> more than about 15 minutes).
> (The Linux/ntpd is running on a remote embedded device that is frequently
> restarted - possibly once a day or so - so we cant wait hours for
> convergence).
> 
> Currently ntpd can take hours to achieve the desired acuracy.
> 
> So, the question is simple - is there any way to significantly speedup the
> convergence of ntpd (using GPS/PPS reference clock)?
> 
> We would be prepared to compromise somewhat on accuracy and jitter.
> (Currently accuracy and jitter values are excellent with jitter as low as 1
> microsecond and accuracy better than 10 uS but it can take a day or two to
> get there).
> 
> It does not seem unreasonable to expect that the ntpd could achieve the
> required accuracy within 15 minutes or so - but nothing we have tried seems
> to work.
> Have tried modifying some of the tinker values, but we dont really
> understand what they all do - and have not really had any success.
> 
> So to summarise:
> 
> 1) Is it possible to speedup ntpd convergence (using GPS/PPS reference
> clock)?
> 2) If so, how - and what are the tradeoffs?

Sorry for the dup for everyone on the mailing list, but I need to send
it unsigned to make it to the news group.

Most important is to start ntpd at boot time with the -g option so that
it will immediately set the time. Then adjust your ntp.conf to set the
maxpoll and minpoll to 4 for your reference clock. 
"minpoll 4 maxpoll 4"

This will get the time synced to close to correct, hopefully a few
microseconds, within a couple of minutes, depending on your hardware.
-- 
R. Kevin Oberman, Network Engineer
Energy Sciences Network (ESnet)
Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab)
E-mail: oberman at es.net			Phone: +1 510 486-8634
Key fingerprint:059B 2DDF 031C 9BA3 14A4  EADA 927D EBB3 987B 3751

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