[ntp:questions] NTP vs chrony comparison (Was: oscillations in ntp clock synchronization)

Unruh unruh-spam at physics.ubc.ca
Tue Jan 22 18:35:33 UTC 2008


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

>Unruh,

>Please read the specification. The offset statistic is the 
>maximum-likelihood estimate of the remote clock offset relative to the 
>local clock and the sign really does matter. The best way to describe 
>this and keep the sign straight is to assume the signed offset is the 
>quantity in seconds to add to the local clock in order to maintain the 
>same time as the remote clock.

Of course the sign matters if you are trying to correct things. Sheesh. 
The sign does NOT matter to the standard deviation. That was ALL I was
saying. 



>The variance statistic, which is represented as an exponentially 
>weighted RMS average called jitter, is the expected error when computing 
>the offset statistic. Generally speaking, as long as the jitter is 
>unbiased, it does not materially affect the clock accuracy due to the 
>extreme lowpass characteristic of the discipline. You shoul be watching 
>the offset statistic, not the jitter statistic.

??? I am watching the offset. I am taking the offset as measured by the ntp
negotiation and calculating the rms deviation of that. 



>Dave

>Unruh wrote:

>> mayer at ntp.isc.org (Danny Mayer) writes:
>> 
>> 
>>>Unruh wrote:
>>>
>>>>>correction applied on that sample.
>>>>
>>>>No, this is offset as measured by the ntp procedure ( (t1+t4-t2-t3)/2 )
>>>>
>> 
>> 
>>>No, that's wrong. It is very carefully described in the NTPv4 draft 
>>>section 8 (p27):
>> 
>> 
>>>theta = T(B) - T(A) = 1/2 * [(T2-T1) + (T3-T4)]
>> 
>> 
>>>Not only do you have the wrong sign, the differences must be calculated 
>>>first, otherwise the errors in the calculation overwhelm the resulting 
>>>value. That's why it's written the way it is.
>> 
>> 
>> I was not writing code. I was telling you what time difference  I was refering to. And
>> sign is a convention as to whether you are saying positive is the computer
>> is fast or the external source is fast. Everything I talked about is sign
>> independent (standard deviation uses squares), and the difference is that 
>> as reported by ntp or chrony and
>> both are careful to to do the calculations with as high an accuracy as
>> possible. 
>> 
>> 




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