[ntp:questions] Re: ntpd going wild

Johan Swenker no_spam_please at swenker.xs4all.nl
Sat Feb 18 00:43:31 UTC 2006

On Fri, 17 Feb 2006 10:45:54 +0100, Hermann Hastig wrote:

> Hi,
> Johan Swenker wrote:
>> Last summer something similar happened to me. but it strongly depended on
>> my configuration. So please tell us about your configuration (like
>> operating system etc.)
> It's a plain Linux 2.4 kernel on a Linux from scratch system, nothing
> extra that plays with the system's clock. The machine is located in a
> fairly large datacenter and does not have any other clock source than
> the three upstream servers from de.pool.ntp.org.
> So, I think I can say that the datacenter's network connection has
> definitly been stable as is the case with the temperature (as Richard
> guessed), and there's also no external clock that could mess things up
> somehow.
> I've watched the clock the last days. The drift file stays at 9.642

Sounds wrong to me. Please check (using ls -l) the last time the drift
file was written. My system, admittedly in a non-airconditioned room,
changes the frequency (grep freq /var/log/daemon.log) by 0.1 ppm or even

> while the offsets to the (new) servers move between 1 and 20 ms (except
> one that keeps shifting stratum between 3 and 5 and shows offsets up to
> 83 ms).
> Seems pretty fine to me (I don't care if the clock's a second wrong or
> not). Or am I wrong here?

Seems pretty bad to me. Even when you don't care if the clock is a second
worng or not, ntpd should be able to keep it synchronised with a few
milliseconds. Otherwise, you _do_ have a problem.

> About that maxpoll setting: Of course the offset may grow larger the
> longer the poll interval which in turn makes it more difficult to keep
> the local time right. But doesn't ntpd automatically keep the poll
> interval within limits that still allow to slew the clock, or does it
> simply go up until maxpoll, whatever may happen?

The poll interval is adjusted such that the jitter is low. I don't know
about the details, I can tell you the effects.
One of my systems had a GPS-receiver. It polls the configured internet
servers with minpoll. It has to happen this frequently, to be a good
match with the GPS-receiver. It also polls a system on my LAN. Even though
this polling happens with maxpoll, jitter is better than 10 times as low
as jitter from the internet servers. 
When I would disable the GPS-reciever, all polling will slowly grow to
maxpoll. Depending on the quality of the configured servers.

> Kind regards, Hermann

Regards, Johan

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