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

Dave Hart davehart at gmail.com
Wed Jan 19 03:35:48 UTC 2011


On Wed, Jan 19, 2011 at 02:47 UTC, Mike S <mikes at flatsurface.com> wrote:
> At 02:18 PM 1/18/2011, unruh wrote...
>> Especially as ntpd uses only one of every 8 polls,
>
> Is this relatively recent behavior?

No.  See the "Clock filter algorithm" slide 13 in:

http://www.eecis.udel.edu/~mills/database/brief/arch/arch.pdf

> I can't provide NTP version #s
> immediately, but it used to be I could "watch ntpq -p", and see the offset
> and jitter change with each poll. More recently, I don't see this. I haven't
> counted, but 1 of 8 sounds about right.
>
> Is it ignoring 7 of 8 (if so, why, instead of 8x polling interval?), or is
> it averaging the 8, then using the average?

Use:

ntpq -p -c "rv &1"

to see the detailed peer variables for the first association, or
change the 1 to 2 for the second association, etc.  Among those are
the 8 clock filter entries (or 24 since each entry has a dispersion,
delay, and apparent offset).  Whichever entry has the lowest delay is
presumed (wisely IMO) to have the least error in the apparent offset.

Cheers,
Dave Hart



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