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

Unruh unruh-spam at physics.ubc.ca
Fri Jan 25 18:03:43 UTC 2008


"David L. Mills" <mills at udel.edu> writes:

>Root,

>You could have saved a lot of time sweating the code if you looked at 
>the briefings. See especially the before and after time series and note 
>the 10 dB improvement in S/N.

I am sorry, but looking at the code is far simpler than trying to find
something in the documentation. Code I can use grep on, your documents I
cannot. 


>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.



>1. The basic principle is to select the samples that have not been 
>delayed in queues, leaving hopefully the ones delayed only by 
>propagation. It's better to discard the others that can only result in 
>less accurate measurements.

>2. The filter samples are correlated only if the total span of the 
>filter is less than the Allan intercept, generally assumed in the order 
>of 2000 s. For poll intervals above that, the samples become 
>uncorrelated, so fewer samples are used.

>3. Never use an old sample, only new onese. It often happens that the 
>last used minimum delay sample is older than the most recent sample. The 
>result is that the filter can toss out up to seven samples before 
>finding a new candidate. The clock discipline loop  is specifically 
>designed to deal with that, which is one reason the time constant is 
>larger than you might like.

>Dave

>root wrote:

>> OK, having looked at the code, I see what it is doing. It essentially 
>> takes the measurement with the shortest delay in the past 8 measurements.
>>  If that measurement happens to be the current one, it is actually used 
>> in the clock loop. (Yes, I know this characterisation of the clock filter is cruder
>> than reality).
>> 
>> 
>> This makes the smaller variance of chrony even more impressive, since in my
>>  tests, evey single measurement of offset was used to calculate the variance 
>> in the case of chrony, while
>> for ntp, only those "smallest" values as reported in peerstats were used. This also 
>> explains why the roundtrip variance in ntp was so much better than chrony's. 
>> 
>> snip

>> 
>> 
>> 
>> 




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