[ntp:questions] Meinberg NTP monitor, silly question

unruh unruh at wormhole.physics.ubc.ca
Wed Dec 23 05:45:53 UTC 2009


On 2009-12-22, David Woolley <david at ex.djwhome.demon.invalid> wrote:
> David J Taylor wrote:
>
>> 
>> I do wish there were some way of speeding this up - a variable loop 
>> bandwidth or something like that.
>> 
>
> It already does have one, and has had one since at least version 3.

One problem ntp has its clock filter algorithm. It throws away 80% of
the incoming data. (It accepts a new reading only if that new reading
has a smaller delay than any of the last 8 readings. This effectively
means that the time between measurements is 8 times the poll interval.
Ie, if you have a poll interval of 6 (64 sec) the effective poll
interval for ntpd is 8 min. And at poll interval 10, the effective
interval is about 2 hr. the system certainly cannot respond to anything
more rapidly than that, and must do so more slowly in order to remain
stable. This is true even if the delay has a very small variance. 
Secondly because the only memory is contained in the rate, the system
has no way of actually estimating the rate of the clock with respect to
the remote clock. Ie, if the offset is positive, the rate is increased a
bit, if negative, it is decreased a bit, and that's it. But there is far
more information than that in the collection of offsets. With three
offsets, one can both estimate the drift, and the uncertainlty in that
drift. It is like walking down the street, staring only at i one square
inch of the the sidewalk
under your feet, and correcting when you see the curb in that 1 square
inch, and the grass, as opposed to looking ahead and behind you. 

Note that the time scales ( 1hr for halving the error) I mentioned is
for a poll interval of 4 (16 sec) .






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