[ntp:questions] NTPD can take 10 hours to achieve stability

Chris Albertson albertson.chris at gmail.com
Mon Apr 18 23:57:17 UTC 2011


On Mon, Apr 18, 2011 at 3:49 PM, unruh <unruh at wormhole.physics.ubc.ca> wrote:

>> Thanks for all the great answers.  Now for a harder question, how does the accuracy of the local clock source
>> affect the accuracy of ntpd.
>
> What local clock source?

That is a good question:  "Which clock?"  One clock might be the
reference clock for a stratum 1 NTP server.  This is typically a GPS
receiver and these vary a LOT.  Some can supply a pulse per second
with 1 sigma error of only a hand full of nanoseconds and other GPSes
have hundred of microseconds of error.  That is a factor of over
10,000 difference in accuracy.

The other clock is the crystal inside the computer that NTP tries to
"discipline" by using the above reference clock.  I think what matters
most is "stability" rather than "accuracy".   In other words it is
fine if this oscillator runs at the wrong speed but not good if the
speed is not constant with respect to time.  The best oscillators are
housed in temperature controlled ovens but these are not common inside
PCs.






-- 
=====
Chris Albertson
Redondo Beach, California



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