[ntp:questions] strange behaviour of ntp peerstats entries.

David L. Mills mills at udel.edu
Mon Jan 28 03:24:25 UTC 2008


Danny,

True; there is an old RFC or IEN that reports the results with varying 
numbers of clock filter stages, from which the number eight was the 
best. Keep in mind these experiments were long ago and with, as I 
remember, ARPAnet sources. The choice might be different today, but 
probably would not result in great improvment in the general cases. Note 
however that the popcorn spike supressor is a very real Internet add-on.

The number of stages  may have unforseen consequences. The filter can 
(and often does) introduce additional delay in the feedback loop. The 
loop time constant takes this into account so the impulse response is 
only marginaly affected. So, the loop is really engineered for good 
response with one accepted sample in eight. Audio buffs will recognize 
any additional aamples only improve the response, since they amount to 
oversampling the signal. Audio buffs will also recognize the need for 
zeal in avoiding undersampling, which is why the poll-adjust algorithm 
is so squirrely.

Dave

Danny Mayer wrote:

> Unruh wrote:
> 
>>mayer at ntp.isc.org (Danny Mayer) writes:
>>
>>
>>>Unruh wrote:
>>>
>>>>Brian Utterback <brian.utterback at sun.com> writes:
>>>>
>>>>
>>>>>Unruh wrote:
>>>>>
>>>>>>"David L. Mills" <mills at udel.edu> writes:
>>>>>>
>>>>>>>You might not have noticed a couple of crucial issues in the clock 
>>>>>>>filter code.
>>>>>>
>>>>>>I did notice them all. Thus my caveate. However throwing away 80% of the
>>>>>>precious data you have seems excessive.
>>>>
>>>>Note that the situation can arise that the one can wait many more than 8
>>>>samples for another one. Say sample i is a good one. and remains the best
>>>>for the next 7 tries. Sample i+7 is slightly worse than sample i and thus
>>>>it is not picked as it comes in. But the next i samples are all worse than
>>>>it. Thus it remains the filtered one, but is never used because it was not
>>>>the best when it came in. This situation could keep going for a long time,
>>>>meaning that ntp suddenly has no data to do anything with for many many
>>>>poll intervals. Surely using sample i+7 is far better than  not using any
>>>>data for that length of time.
>>
>>>On the contrary, it's better not to use the data at all if its suspect. 
>>>ntpd is designed to continue to work well even in the event of loosing 
>>>all access to external sources for extended periods.
>>
>>>>And this could happen again. Now, since the
>>>>delays are presumably random variables, the chances of this happening are
>>>>not great ( although under a condition of gradually worsening network the
>>>>chances are not that small), but since one is running ntp for millions or
>>>>billions of samples, the chances of this happening sometime becomes large. 
>>>>
>>
>>>There are quite a few ntpd servers which are isolated and once an hour 
>>>use ACTS to fetch good time samples. This is not rare at all.
>>
>>And then promplty throw them away because they do not satify the minimum
>>condition? No, it is not "best" to throw away data no matter how suspect.
>>Data is a preecious comodity and should be thrown away only if you are damn
>>sure it cannot help you. For example lets say that the change in delay is
>>.1 of the variance of the clock. The max extra noise that delay can cause
>>is about .01 Yet NTP will chuck it. Now if the delay is 100 times the
>>variance, sure chuck it. It probably cannot help you. The delay is a random
>>process, non-gaussian admitedly, and its effect on the time is also a
>>random process-- usually much closer to gaussian. And why was the figure of
>>8 chosen ( the best of the last 8 tries) why not 10000? or 3? I suspect it
>>came off the top of someone's head-- lets not throuw away too much stuff,
>>since it would make ntp unseable, but lets throw away some to feel
>>virtuous. Sorry for being sarcastic, but I would really like to know what
>>the justification was for throwing so much data away.
> 
> 
> No, 8 was chosen after a lot of experimentation to ensure the best 
> results over a wide range of configurations. Dave has adjusted these 
> numbers over the years and he's the person to ask.
> 
> Danny




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