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

Chuck Swiger cswiger at mac.com
Fri Sep 17 22:29:20 UTC 2010

On Sep 17, 2010, at 2:21 PM, Daniel Havey wrote:
> So, you think that a PC clock will drift 20-50ms in 5 seconds?

Goodness, no, a typical PC quartz crystal has a frequency stability typically measured in tens of PPM (ie, ~ 10E-5 to 10E-6); even a bad one ought to not drift by as much as a millisecond or so in 5 seconds.  However, running "ntpdate -b" every second or so will tend to bounce the clock forwards and backwards by an amount more closely related to the latency (and the *variability* of the latency) of network traffic, which tends to be on the order of milliseconds to tens of milliseconds.

> 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?

You should easily obtain better than ~10ms sync with ntpd over a LAN.  With a recommended setup of three or four local NTP servers all referencing each other as peers, you should be getting more like ~1ms sync.

> I would actually kind of like to test it myself ;^)  But measuring time skew between machines may not be as easy as it seems at first glance ;^)  Surely there must be a paper somewhere that already does this?

Something like:


...or PHK's stuff:




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