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

Miroslav Lichvar mlichvar at redhat.com
Mon Jan 17 11:44:05 UTC 2011


On Sun, Jan 16, 2011 at 05:37:18PM -0800, Chris Albertson wrote:
> A longer poling interval is not a bad thing.  The polling interval is
> adjusted so as to reduce total noise.  There is a sweet spot where
> polling faster or slower is worse.

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
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!

> As an example, lets say you wanted to measure the thickness of a sheet
> of paper but your ruller only goes to 1/100 inch divisions.  You get
> soe gross errors if yu tried to measure one sheet.  But stack 1,000
> sheets and you will do well.   Longer polling interval works kind of
> the same  way.

Except the thickness is slowly changing during the production, so you
have to use a compromise to keep the noise down and to get the current
thinkness. 

> I think the longer poll time is telling you something good about the
> internal clock in the BSD system.

What exactly?

-- 
Miroslav Lichvar



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