[ntp:questions] Why does ntp keep changing my conf file?

David Woolley david at ex.djwhome.demon.invalid
Fri Sep 17 21:59:01 UTC 2010


Daniel Havey wrote:

> So, you think that a PC clock will drift 20-50ms in 5 seconds?  Seems like a lot, but whatever.  Let me see if I've got this right, you tell me I might get say synchronization of ~10ms with ntpd running on a lan with everybody on the same switch or perhaps one switch away?
> 

On its own no.  With the help of ntpdate, yes.  I wouldn't actually 
expect to reach this level very often, but I'd certainly expect several 
milliseconds.  You will get these large steps because you are using 
individual measurements, which are very vulnerable to scheduling delays 
and network propagation delays, which ntpd, itself, will average out (it 
will effectively low pass filter the jitter out of the measurements).

If you seriously need very tight synchronisation, you should use ntpd or 
possibly chrony, with a pulse per second source parallel wired to all 
the machines.  In extreme cases, you may need to make sure the 
transmission lines from the PPS source are or equal length.




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