[ntp:questions] Visualization of clock control

Dennis Ferguson dennis.c.ferguson at gmail.com
Thu Jan 5 03:40:25 UTC 2012


On 4 Jan, 2012, at 22:54 , Miroslav Lichvar wrote:
> Here are some example runs of the tool captured to animated gifs:
> http://mlichvar.fedorapeople.org/clknetsim/chrony_ntp/vis/visclocks_10us.gif
> http://mlichvar.fedorapeople.org/clknetsim/chrony_ntp/vis/visclocks_100us.gif
> http://mlichvar.fedorapeople.org/clknetsim/chrony_ntp/vis/visclocks_1000us.gif
> 
> The simulations were done with a clock wandering at 1 ppb/s,
> 10/100/1000us network jitter with exponential distribution and the NTP
> clients were configured to use 64s polling interval.

That's pretty neat.  I think, however, that the clock wander of 1 ppb/s
is about an order of magnitude too large for real life, at least for machines
kept in an air conditioned room (and the behavior of clocks in machines
subject to environmental variations probably can't be modeled by "wander" at
all).  My measurements against precise hardware tended towards a value of
1ppb/10s, which is also consistent with the 10^-8/1000s which sometimes shows
up on Allan variance plots (I think there's a square root relationship in there
if the wander is a truly random walk).

The other difficulty with respect to real life may be modeling network jitter
as exponential, since I believe the probability distribution for network delays
is heavy-tailed (i.e. with extreme values way over-represented; this is a problem
when using statistics which assume the underlying error distribution is gaussian).
I don't know how to fix that, though.

Dennis Ferguson


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