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

unruh unruh at wormhole.physics.ubc.ca
Wed Jul 13 18:43:48 UTC 2011


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. 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. 


>
> 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.

a)Run chrony. ntpd will only use a max of 500PPM rate adjustment. Chrony
can go up to 100000PPM to adjust away a large time discrepancy. 

b) Find out why your clock is suddenly finding itself out by that huge
amount. 


>
> Regards
> Lars




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