[ntp:questions] How do I prevent sudden system time jumps.

Harlan Stenn stenn at ntp.org
Fri Jul 15 03:38:25 UTC 2011


Bill wrote:
> On 2011-07-12, Lars Ericsson <laeatwork at gmail.com> wrote:
> > Hi all,
> >
> > I have been running the ntp client on a Linux platform for some time and
> > have not seen any problems.
> >
> > I recently run into a strange behavior where our communication software
> > failed on some critical timeouts.
> > After some investigation we found out the the system time suddenly made a
> > jump with 200ms.
> 
> Yup. ntpd makes huge statements about the importance of keeping things
> monotonic, and then makes jumps if the time is more than 125ms out.

Yes, for many things it is Important that time stays monotonic.  When it
cannot, what should NTP do?  I have some ideas about this, but they are
for another go-round with the Standards folks.

> Now, usually that indicates that there is something wrong with the
> system clock, so you should check it out. This is especdially true if
> the system has been running for a while and then jumps. It could also
> be that the servers you have are inconsistant, and out from each otehr
> by some large amount and you have too few servers for ntp to figure
> out which are giving good time.

Yup.

> > Normally NTP will smoothly adjust the time without any noticeable
> > jumps, and then this sudden step :o)
> >
> > My question is: How do I prevent NTP clock adjustments from
> > jumping. OK, it will take some time to adjust but that's OK.

Be careful what you wish for.  "steps" are an aberrant case.  There are
several things you can do about this, each with costs/benefits.

H



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