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

Unruh unruh-spam at physics.ubc.ca
Wed Jan 23 16:25:40 UTC 2008

mayer at ntp.isc.org (Danny Mayer) writes:

>> And as Bill said, it would be Swell if there was a way to do this using, eg,
>> virtual machines so that we could test them that way.  Better yet, it would
>> be nice to have a simulator framework where we could run these tests faster
>> than in real-time.

>Virtual machines buys you the same problem as above. Even on a virtual 
>machine there's only one clock. You can have only one application 
>discipline that clock never mind how many virtual machines are running. 
>Don't be fooled by the technology.

Not if the virtual machines have a virtual clock-- Ie a little program
which intercepts all the clock routines and return the output of a little
program simulating a clock. Now intercepting the various adjtimex calls is
not that hard ( just rewrite the adjtimex and gettimeofday routine and and overload it for
your program) but chrony and ntp also use the clock as a scheduler, and
that is a lot more difficult to simulate and catch. 

>There are no simulators that I've ever seen that can run tests faster 
>than real-time. They are always many orders of magnitude slower, even 
>with hardware assist.
We are not asking for a machine simulator but a clock simulator and that
can run thousands of times faster than the real clock. You can run it at
any speed you want. And you can have a separate simualted clock with its
own theory of operation on each virtual machine. 

More information about the questions mailing list