[ntp:questions] how long does it take ntpd to sync up

David Woolley david at ex.djwhome.demon.invalid
Sun Aug 28 10:53:00 UTC 2011

Harlan Stenn wrote:

> ntp *does* a fit/figure of the expected needed adjustment - your
> sentence implies that it does not (at least that's how it reads to me).

No it doesn't.  It is basically a feedback controller.

If you take a modern central heating controller, which varies the output 
by varying how much of a ten minute cycle the pump is running, you have 
something similar to version 3 ntpd, except that it used 4 seconds.

The central heating controller will use the measured difference from the 
target temperature to adjust the on to off proportion, and also include 
some of the integral of that, to, eventually, remove any remaining offset.

A fitting process would be more like the controller measuring the rate 
of temperature change when the heat was off, and when it was on, and 
then calculating exactly how much on to off time to apply in one go. 
(In practice, it isn't as simple as that for the central heating system, 
as there is a lag involved for the heat to get from the boiler flame to 
the the thermostat.

> Do you want to re-do the algorithmic analysis of everything NTP does to
> make sure that it behaves OK in all circumstances if you bump the limit
> to 10000ppm?  And what about kernels that do not currently allow changes
> to be applied at that rate?

Nothing is going to work perfectly in every situation.  Unfortunately, 
it is also rare that you don't lose something for what you gain, so you 
actually have to weight the importance of the ability to cope in 
different circumstances.

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