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

David L. Mills mills at udel.edu
Tue Jan 22 15:36:33 UTC 2008


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.

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.

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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