[ntp:questions] TSC, default precision, FreeBSD

David Mills mills at udel.edu
Sun Sep 6 03:53:23 UTC 2009


Richard,

No, what you suggest is properly called resolution, not precision. Do 
your homework; -21 is just fine for a  Sun faster than mine, where the 
precision is -20. Confirm that the jitter.c program, included in the NTP 
distribution for just that purpose, shows the gettimeofday() routine 
takes on average 500 ns per call.

Dave

Richard B. Gilbert wrote:

>Garrett Wollman wrote:
>  
>
>>In article <9fa237cd-eaf8-4303-8720-3e290547fa90 at g1g2000pra.googlegroups.com>,
>>Dave Hart  <davehart at gmail.com> wrote:
>>
>>    
>>
>>>I hope this help you understand NTP precision.  I also hope you will
>>>cease attempting to conflate it with any other definitions of
>>>"precision" or related terms.
>>>      
>>>
>>It's a pity that the authors of the NTP specification chose to
>>Humpty-Dumpty an existing word with a well-defined meaning.
>>
>>-GAWollman
>>    
>>
>
>At least it's stated what meaning it was "Humpty-Dumpty'd" to.
>Precision is the smallest possible difference between two consecutive 
>readings of the system clock.  It's normally expressed as a negative 
>power of two.  I once wrote a tiny program to read the clock twice in 
>succession and do the arithmetic and ran it on a Sun Ultra 10 
>workstation with a 440 MHz CPU.  IIRC the result was 2^-21.
>
>It has nothing whatever to do with accuracy, it simply specifies how 
>finely you can slice the time.
>
>_______________________________________________
>questions mailing list
>questions at lists.ntp.org
>https://lists.ntp.org/mailman/listinfo/questions
>  
>




More information about the questions mailing list