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

unruh unruh at wormhole.physics.ubc.ca
Mon Apr 18 06:33:31 UTC 2011

On 2011-04-18, Chris Albertson <albertson.chris at gmail.com> wrote:
> On Sun, Apr 17, 2011 at 7:58 PM, C BlacK <rblak at non.net> wrote:
>> Why would it take ntpd ten hours to achieve its accuracy? ?Can this be explained in laymans terms and
>> mathematically
> In very simple terms....
> Ignore NTP and computers and think about an old mechanical clock.
> Lets say you wanted to adjust the rate of the clock to one part in
> 10,000.  But the hands can only be read to the nearest one second but
> you are lucky to have an atomic clock nearby.  In theory it would take
> you 10,000 seconds to know if you adjusted the clock correctly.  In
> other words it would take 10,000 seconds for the clock to gain or
> loose one second relative to your standard.
> NTP works kind of like that.  It uses a set of reference clocks and
> watches the rate of your local clock relative to the reference
> clock(s) and depending on details it make time some time

Nice analogy. If only chrony did not demonstrate that using exactly the
same time interchange with an accurate clock, the computer can achieve
usec accuracy within less than an hour. Ie, your analogy simply does not
hold as a general statement. 


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