[ntp:questions] TSC, default precision, FreeBSD

Richard B. Gilbert rgilbert88 at comcast.net
Sat Sep 5 19:15:55 UTC 2009

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.

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