[ntp:questions] NTP vs chrony comparison (Was: oscillations in ntp clock synchronization)

Unruh unruh-spam at physics.ubc.ca
Tue Jan 22 23:14:21 UTC 2008

Harlan Stenn <stenn at ntp.org> writes:

>Hey Danny!

>>>> In article <2Fqlj.12866$vp3.5778 at edtnps90>, Unruh <unruh-spam at physics.ubc.ca> writes:

>Unruh> Unfortunately I cannot run both ntp and chrony on the same system at
>Unruh> the same time.


>Exactly why can you not run ntpd and chrony on the same system at the same

The key problem is that there is a feedback between the clock control
algorithm and the clock loop routines. That feedback is missing if chrony
or whatever does not control the clock. It may tell  you a bit but removing
such a crucial part of the feedback loop would totally change the behaviour
of the loop. 

What would be great to have would be the ability to run them on a fake
clock machine, where it is a program which responds to the output (clock
control) and the input ( the network stuff.) In particular chrony uses
system clock stuff to schedule various events-- like sending out the
packets, etc. One would have to have the system feed those routines as
well. HOwever if one could do that then one could look at how both ntp and
chrony reacted to exactly the same input/output. It is however a massive
rewrite of the code, unfortunately, as far as i can see. 

>I want the ability to run multiple instances of ntpd where at most 1
>instance of ntpd is actually controlling the clock, specifically to make it
>easy to (more quickly) analyze the performance/behavior of different
>configurations of ntpd.  I understand that the boat is rocking while this is
>going on, but I suspect this capability would be a useful one in at least
>some cases.

Unfortunately I do not think that will give much info as to how the
different configurations behave. It would be like disconnecting the
feedback in an amplifier-- the amp behaves very very differently.


More information about the questions mailing list