[ntp:questions] Test internal clock

Rob Neal hundoj at comcast.net
Fri Mar 20 13:24:58 UTC 2009

On Fri, 20 Mar 2009, Unruh wrote:

> hundoj at comcast.net (Rob Neal) writes:
>>> It almost seems like a religious group.  Most people try to convert
>>> the world to Mills' NTPD, and then there is the dissident who tries
>>> to push Chrony in every thread.
>> 	Chrony has an impulse response that is ill-advised in
>> 	a network of NTP servers. It's really rather crude, IMHO.
> An interesting claim. What is it about the chrony impulse response that you
> do not like and why is it "ill-advised" in a network of ntp servers? I
> really am interested in the answer. I would think it is far less crude than
> is ntpd, but would love to understand your reasons.
 	The slew rate. NTP limits the slew rate to 500 PPM,
 	for purposes of error bound analysis and conformance
 	to common software constraints. It is, in the end,
 	a PLL.

 	Chrony does not appear to have either such analysis
 	of errors, nor a bound on the slew rate.
 	If you have no bound on the slew rate, then
 	you cannot make mathematical statements about
    	the response to transient inputs - such as the time
 	to amortize an offset.
 	If one cannot bound, via mathematics, the transient
 	response, then how can one constrain the loop
 	And if one cannot bound or define the loop stability,
 	then its use in a large network of like performers
 	becomes problematic. How does one bound the
 	oscillation between partners, under these conditions?

 	It may be that these questions have practical answers,
 	disjunct from the analysis, that are perfectly adequate
 	in practice. I have seen no such demonstration, so I
 	prefer the bounds within NTP. These have been supported
 	by extensive analysis, and practical deployment.
 	Perhaps not perfect for all purposes, but usable.

 	I would expect that for leaf nodes, Chrony would be
 	a totally acceptable solution, with characteristics
 	that would make it attractive to a large number of
 	system administrators.

 	Does this answer address your concerns?


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