[ntp:questions] Extracting ntpq like information programmatically

unruh unruh at invalid.ca
Sun Mar 31 01:14:07 UTC 2013


On 2013-03-30, Mischanko, Edward T <Edward.Mischanko at arcelormittal.com> wrote:
> What can be done with ntpd's code to address the lack of speed in ramping
> down the time constant as needed to keep up with changes in the local clock?

Rewrite the code. 
Run chrony instead ( which responds much faster).


>
>> You are observing on a time scale much shorter than the loop time
>> constant.  ntpd has an adaptive time constant, and once it has gained
>> initial lock, that becomes quite long, as it assumes that offsets are
>> due to measurement errors, e.g. due to differential delays out and
>> return.  Also, in your case, you may actually have been using the same
>> best sample for the whole 15 minutes.
>> 
>> This strategy will generally give the best solution if the temperatures
>> of the oscillators are stable.  Where ntpd can have problems is in not
>> ramping down the time constant fast enough if the temperature changes
>> and the local clock frequency changes.
>> 
>> I'm not sure of the exact oversampling rate, but even after the
>> approximately 1 out of 8 sampling for the best of the last 8, I believe
>> it is still oversampling in relation to the loop time constant.
>> 
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