[ntp:questions] chrony and ntp comparison-- ADSL hookup

Bill Unruh unruh at physics.ubc.ca
Tue Feb 19 00:57:26 UTC 2008

I have now run some absolute time discipline tests using chrony and ntp
The server is a GPS PPM disciplined clock using ntp. The client is a
machine on an ADSL line. The round trip between server and client is about
16ms, which is about 100 times longer than the round trip times used
earlier in the previous tests I posted. 

In each case I run the GPS PPM to measure the offset of the computer clock
vs the GPS (It is a parallel port interrupt routine which on each interrupt
reads the system clock and saves it in a buffer readable from /dev/gpsint.
Ie, I measure the system time each time it receives a pulse from the GPS
Garmin 18LVC receiver. Since this has an mean accuracy of about 2usec, its
noise is negligible. 

The mean in both cases is about -.3ms. Ie the adsl trip is assymetric as to
outgoing and return but about .6ms.
In the case of ntp the variance in the readings is about .32ms. For chrony,
the variance is about .11 msec, 1/3 of the variance of ntp. This is in
confomity with what I found for the client on the same subnet as the clock. 

Now, chrony is much more aggresive in decreasing the poll interval when the
statistics gets bad, so the mean poll time for chrony was 36sec while for
ntp it was 127 sec (in both cases minpoll was 4 and maxpoll was 7). 
Since the noise is dominated by the random transmission times, and not the
frequency drift of the osillator in the computer, this should not make any
difference.  (the variations in the drift are about 1ppm, and in 128 sec,
that is only .1ms ) However, I will alter chrony to make it less agressive in
decreasing the poll interval, and I hope increasing its mean poll time. If that
does not work I will set minpoll to 6 for chrony.

However the indications are that chrony is again more accurate than is ntp
in disciplining the clock.

Note that seems a reasonably stringent test of the two programs in a situation in
which the transmission noise dominates the error budget, rather than the
frequency drift noise as in the test on the same subnet. 

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