[ntp:questions] very slow convergence of ntp to correct time.

David L. Mills mills at udel.edu
Tue Jan 29 00:42:48 UTC 2008


Eric,

There are actually two burst modes IBURST when the server is unreachable 
and BURST when it is. They are independent of each other and both can be 
used at the same time. Currently, IBURST uses 6 packets, as that is a 
couple more than to pass the distance threshold and synchronize the 
clock. This actually is recommended; the following packets whether burst 
or not are delayed so that the average headway does not exceed the 
specified threshold, by default 16 s. The BURST mode also obeys the 
headway restrictions, but is intended to de-jitter in the cases I mentioned.

What set off my bell in response to your remark was an interesting 
observation when watching the clock filter operate. Start the daemon 
with a -d flag and watch the clock_filter and local_clock traces. Notice 
that there are often several samples discarded as not younger than the 
last used sample. This is a normal situation; however, it reveals that 
the probability of using another sample just after usin one is 
relatively low. In other words, when you find a sample you might as well 
give up and wait for the next burst. This needs to be confirmed.

Dave

Eric wrote:

> On Mon, 28 Jan 2008 22:09:11 +0000, "David L. Mills" <mills at udel.edu> wrote
> for the entire planet to see:
> <snip>
> 
>>However, you give me an idea. Why not shut down the burst when the clock 
>>filter delivers the first sample? Gotta think about that.
>>
>>Dave
> 
> 
> Hi Dave - 
> 
> I'm pleased to know I've provoked some new thoughts.  If I understand your
> post, burst mode was intended to get enough (lousy) samples into and
> through the clock filters to allow for initial sync.  Once the pipeline is
> loaded no more "extra" polls are needed.  
> 
> But the rest of this sub-thread was about poll intervals that get so large
> that the intervening equipment forgets about the flow and always, from then
> on, gives lousy performance on the one and only poll in that interval.  
> 
> I guess we could kill two birds with one stone and shut down burst as you
> suggest, until the interval gets longer, when it could make a reappearance,
> perhaps as only a pair of packets.
> 
> - Eric
> 




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