[ntp:questions] Loopstats update - how often?

unruh unruh at invalid.ca
Thu Mar 15 15:51:01 UTC 2012

On 2012-03-15, David J Taylor <david-taylor at blueyonder.co.uk.invalid> wrote:
>> On Thu, Mar 15, 2012 at 07:56, David J Taylor
>> <> wrote:
>>> If I have a configuration including:
>>> _________________________________
>>> server  minpoll 5 maxpoll 5        iburst
>>> server 0.uk.pool.ntp.org iburst  minpoll 10
>>> _________________________________
>>> how often should the loopstats be updated?  I was expecting something 
>>> near
>>> to every 32 seconds, but it seems that sometimes it can be more than 64
>>> seconds.  Is that correct?
>> Ignoring the pool server, it could be as much as 32 * 8 seconds
>> between loopstats updates.  Some polls of will not change
>> the offset estimate for that peer, as an older entry in the clock
>> filter register is preferred over the latest.
>> Cheers,
>> Dave Hart
> That explains what I am seeing, Dave.  Makes sense not to repeat 
> unchanging data!

The clock filter selects the data with the smallest round trip time, to
fix the problem with asymmetric round trips (on the theory tht the
shortest time is more likely to be symmetric round trip). 
The filter is 8 items deep, and thus on average it selects the same item
the whole time it is in the clock filter. Ie, on average a bit less than
7 out of 8 inputs are thrown away (it is actually avout 80% that are
thrown away).
This is for network polled items where round trip time makes sense. For
refclocks about 40% are thrown away to get rid of "popcorn" deviations.
(non-gaussian large offsets) as I understand it. 

> Cheers,
> David 

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