[ntp:questions] The libntp resumee...

David Woolley david at ex.djwhome.demon.co.uk.invalid
Mon Sep 8 21:19:52 UTC 2008


Kay Hayen wrote:
> 
> 
> How would it be difference from using the restrict command manually? 

Because manual use would normally be based on significant thought and 
measurements over an extended period.
> 
> And why would it not be NTP?

Because a key part of NTP is the algorithm used to identify and reject 
unreliable sources of time.  These actually work better if you have many 
sources.

> > 
> Well, dispersion is going down only with more samples to base estimation on, 

The calculation initially assumes that the source jitter might be very 
large until it has evidence to the contrary.

> isn't it? And we need that quick, if we want the server to influence the 
> hosts behind it quickly, say after a "NTP LAN" failure ended (some people 
> have dedicated LANs for NTP).

iburst covers that.

> 
> I was not concerned with how the kernel makes the adjustments, but rather that 
> the a fixed time change over the period is known. The slew rate is known, 
> isn't it?

The actual change in time in any period should be zero, within 
statistical error.  The real excess slew rate should also be zero within 
statistical error.  The assumed length of a tick, which is probably the 
reciprocal of what you mean by the slew rate, is continuously varying. 
You would need to integrate this to get the excess number of ticks over 
a period, which is, I think your concept of error.

(The big argument between chrony and ntpd is about whether ntpd really 
gives the best estimate of true time for real inputs.)

> 
> Let me use a car analogy, these things work. :-)
> 
> Lets assume a three lane high way with 3 cars that try to drive at the same 
> speed. The car to the left is driving at (near) constant speed. The driver in 
> the middle accelerates and braces according to his motor behaviour as well as 
> the observed difference in speed between him and the other one. Now what 
> should the driver to the right do?
> 
> In my view, he could take the acceleration of his neighbour into account when 
> making estimates of his own error.

Analogies are always unsafe in fora, but the second car doesn't actually 
know its acceleration (remember, if they could actually see the road, 
they would use that as reference).  All it knows is how hard its driver 
is pushing on the accelerator.

Moreover, the drivers are looking at each other through mirrors that are 
vibrating violently and unpredictably, such that the apparent position 
of the neighbours is varying much more than their true relevant 
position.  To a significant extent the mirrors are moving independently 
of each other (this probably requires that the third driver actually be 
the middle one, to make the physical model sensible).




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