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

unruh unruh at wormhole.physics.ubc.ca
Mon Jan 17 17:59:59 UTC 2011


On 2011-01-17, Miroslav Lichvar <mlichvar at redhat.com> wrote:
> 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.

This is a bit of a myth. It depends entirely on what you want to
optimize. To optimize the offset error, more polling is better. If you
want to optimize the drift rate, then there is a "sweet spot" but it
depends on all sorts of stuff, including the daily temp fluctuations due
to the computer's being used more during the say. It is not at all clear
what ntp is set up to optimize but as stated below, reducing network use
is part of it. It also chooses an "Allan intercept" which is not one
measured for your system but some "average value ( from 1995?). 

>
> 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

??? 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?
>




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