[ntp:questions] Red Hat vote for chrony

Charles Swiger cswiger at mac.com
Fri Dec 5 05:45:41 UTC 2014


On Dec 4, 2014, at 7:00 PM, William Unruh <unruh at invalid.ca> wrote:
[ ... ]
> Actually Miroslav Lichvar IS an expert. He is the chrony maintainer, has
> done a lot of testing comparing chrony to ntpd ( which showed that
> chrony controlled the clock a factor of 2 to 20 times better than ntpd
> did), and is with Redhat. 

The data I've seen for chrony suggests it handles broken clocks such as
commonly found in VMs better than ntpd does.  The tradeoff is that
chrony prioritizes chasing the reference time over first trying to ensure
that the local clock frequency is stable, whereas ntpd really wants
to make sure that the local clock counts 3600 seconds in each hour of
wall-clock time and then worries about slewing the local time to match
up with the reference time.

It's informative to note that the chrony docs (section 5.3.4) recommend
using minpoll=2 and maxpoll=4!  With those settings chrony will send 225
polls per hour, versus 3.5 polls per hour for ntpd with its maxpoll=10.
Assuming arguendo the claim of "a factor of 20 times better" is true, I
still don't care to pay the price of a factor of 64 times more network polls.

Furthermore-- unfortunately-- I have yet to see data on the accuracy of
chrony measured against high-quality TCXO or Rb/Cs reference clocks,
such as the PRS-10 that PHK used:

http://www.thinksrs.com/products/PRS10.htm

...the current version of which claims to have a +/- 10 ns accuracy for
the PPS signal.  Instead, most of the data I've seen provided for chrony
has involved comparing local clock timestamps to the reference timesource
or to some other network timesource, without detailed information as to the
accuracy of those references.

Of course you're not going to see much delta between the local clock and the
reference that you're polling every 16 seconds.  Without measuring the
local clock against some other clock or oscillator which is known to be
accurate to sub-microsecond levels, one doesn't have the data needed to draw
conclusions about the actual timekeeping precision.

Regards,
-- 
-Chuck



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