[ntp:questions] Red Hat vote for chrony

David Woolley david at ex.djwhome.demon.invalid
Fri Dec 5 16:06:32 UTC 2014


On 05/12/14 08:42, William Unruh wrote:

> Nope. ntp changes the rate of the local clock to correct offsets. That
> is all it does. It does not make the rate correct, and then the offset.

Not true.  It applies a correction for the offset, such that if that 
were the only error, it would be eliminated exponentially with a time 
constant that is related to the poll rate.  I believe it expects to 
update this correction well before it is fully applied.

At the same time, it applies a correction to the long term estimate of 
the frequency, with a much longer time constant.


> much more quickly than ntpd does, for the same poll. Remember that ntpd
> throws out 85% of the measurements it makes, in order to try (poorly) to

If the round trip variability was sufficiently low that this pre-filter 
were not needed, any variations in the samples it is ignoring would be 
largely suppressed by the loop filters, which have time constants that 
are significantly longer than even eight times the poll interval.  ntpd 
is heavily oversampling, which means that the min and maxpols quoted for 
chrony imply an even higher excess rate.


>
> Actually not true. How do you think the standards of the various
> contries determine the accuracy of their clocks. They have no better
> time standard to compare them with. And yet they confidently will quote
> accuracy figures for their clocks. Study that.

I presume that whatever time transfer mechanism they use has well 
characterised errors.  NTP operates in an environment where the errors 
are not well characterised.



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