[ntp:questions] Polling interval in FreeBSD vs. Windows

unruh unruh at wormhole.physics.ubc.ca
Mon Jan 17 23:54:40 UTC 2011


On 2011-01-17, David Woolley <david at ex.djwhome.demon.invalid> wrote:
> Miroslav Lichvar wrote:
>> On Sun, Jan 16, 2011 at 05:37:18PM -0800, Chris Albertson wrote:
>
>> Yes, there is a sweet spot, but ntpd isn't looking for it. It strongly
>> prefers longer polling interval to save network bandwidth. If you want
>
> I believe it is intended to select the sweet spot, although it is 
> possible that it has been left behind by technology changes.
>
>> the best accuracy, you will need to set maxpoll according to the
>> network jitter and clock stability you have.
>> 
>> For a typical clock oscillator and the standard kernel PLL, poll 3
>> will give you better accuracy than poll 4 when the network jitter is
>> about 100 microseconds or less. Such jitter is not uncommon on LAN,
>> sometimes I observe 100us jitter to close pool.ntp.org servers!
>
> I suspect you are using offset as your figure of merit.  You cannot do 
> that in an unqualified way.

?? Surely what you want is that your computer's clock shows the correct
time. If your computer disciplines the clock to have the true rate (1
sec of computer time equals one second of UTC time) but was 10 hours
off, I think most people would count that as a failure of ntp. 
Not sure what you mean with "You cannot do that in an unqualified way".




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