[ntp:questions] dispersion has high peak when reference clock first appears
nowhere at mail.ru
Tue Apr 9 16:14:12 UTC 2013
After some thinking i came to a conclusion that i was totally wrong.
A dispersion is not a measure of quality of local time. It's a measure of
quality of currently selected sys peer.
But my task is to answer the question "are we synchronized very good?".
To answer it i will rely on system leap value and ( if sys peer exists )
on system offset. And i will exclude system dispersion from this list.
Also I will create some kind of a timer. The timer will count from last
state with sys peer selected and will drop synchronization marker
if no sys peer was selected, for example, for 4 hours.
2013/4/9 Brian Utterback <brian.utterback at oracle.com>
> On 04/08/13 08:03, Nickolay Orekhov wrote:
>> I've got external util that estimates quality of synchronization. One of
>> the clues that it uses is current sys peer dispersion.
>> When clock goes down for a long period of time it's dispersion filter gets
>> filled with MAXDISPERSE ( 16.0 )
>> And than there's a peak dispersion when clock goes up again and get's
>> In general I don't think that's logical. Because I have very good
>> synchronization with low self dispersion and than there will be a peak
>> because some clock appeared from nowhere.
>> I'm thinking about some additional code. For example, one can delay clock
>> selection until all filter will be filled with good dispersion, which is
>> not equal to MAXDISPERSE. It will smooth the moment of clock appearance.
> That's pretty much how it works now. The dispersion is calculated using
> all 8 pieces of polling data in the billboard. If a clock has been off the
> air too long, all of the data regarding the clock times out or is cleared
> and the billboard doesn't have any polled data for the clock and the
> dispersion is calculated as the max for all eight pieces of data. As each
> new piece of polling data is entered the dispersion is re-calculated and
> because the data is weighted by age each piece of data has the effect of
> approximately reducing the dispersion by half. But ntpd will not select a
> clock with dispersion over one second. This is the main reason that iburst
> was created, to allow the billboard to be filled quickly with valid data,
> reducing the dispersion below one second and allowing the clock to be
> selected. Before iburst you had to wait for 5 polling periods.
> Always code as if the guy who ends up maintaining your code will be a
> violent psychopath who knows where you live. - Martin Golding
> Brian Utterback - Solaris RPE, Oracle Corporation.
> Ph:603-262-3916, Em:brian.utterback at oracle.com
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