[ntp:questions] NTP vs chrony comparison (Was: oscillations in ntp clock synchronization)
mayer at ntp.isc.org
Wed Jan 23 18:38:33 UTC 2008
> mayer at ntp.isc.org (Danny Mayer) writes:
>> 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.
As a fellow physicist I would expect you to understand this better. It's
a basic principal in quantum mechanics: the observers influences the
observed results. In this case, it's not enough since you are directly
and deliberately affecting the clock itself and there really can only
be one clock. Multiple clocks lead to chaotic events. All "virtual
clocks" are driven off the real one which means that updating the clock
needs to update the real clock. You don't really have separate clocks,
it just looks like you do.
>> 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.
I've run many different simulators including hardware ones and I can
assure you nothing runs slower than a simulator. Like I said there is
only one real clock in a virtual machine, there just appears to be one
per virtual machine.
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