[ntp:questions] Extracting ntpq like information programmatically

unruh unruh at invalid.ca
Tue Apr 2 01:59:24 UTC 2013


On 2013-04-01, David Woolley <david at ex.djwhome.demon.invalid> wrote:
> unruh wrote:
>
>> 
>> Well, modulo Mills' prejudices as to how ntpd should work. In short, if
>> the user has different requirements from those that Mills had, there may
>> be a way of using the data differently to get better results. But you
>> really really have to know what you are doing. 
>> 
> As the OP mentioned PIDs (proportional, integral, differential 
> controllers), I think he is assuming the Millsian process control 
> algorithm type of structure, rather than the linear regression structure 
> used by chrony.  He's just assuming a much smaller loop time constant 
> than is actually used.

I think he really does not know what he wants or what ntp, or chrony,
does. It may be that for his purposes a different control structure
would work better. Eg different weights in the PID or different time
constants. Or perhaps a linear regression structure would work better or
even some non-linear structure. But he needs to know what he wants, and
needs to know how to get it from the data, keeping in mind all of the
things that can go wrong. Eg, how will his control structure handle an
hour of high one way delays in the network? Or sporadic delays. Of the
remote server going down and the clock having the freewheel. Or one of
the many other things that can go wrong. Thus a system which 90 % of the
time works better than ntpd, but 10 % goes into totally unstable
oscillation would probably not satisfy him. (or even 1%). And any
looking at the system over some fairly short period of time ( eg the 15
min he used as his base) might show that his system works much better. 



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