[Beowulf] ...Re: Benchmark results

James Cownie jcownie at etnus.com
Fri Jan 9 04:44:48 EST 2004


Greg Lindahl wrote :-
> On Thu, Jan 08, 2004 at 04:14:38PM -0800, Jim Lux wrote:
> 
> > >Perhaps I'm overly optimistic, but writing a kernel module to achieve 
> > >cluster-wide microsecond-level clock synchronization just might be 
> > >feasible...
> > 
> > I am entirely certain that this can be done, and may have already been 
> > done. It's such a useful thing to "know, exactly, what time it is"....
> 
> Octiga Bay claimed to have done it at SC. However, they made it sound
> like they did a lot of work to get it going, as what they were doing
> was enabling MPI programs to run in lock-step on microsecond
> timescales.  That's much harder than just getting global times.

But the OctigaBay system has a single network clock from which all the
machine clocks are derived, so all of the processors are already
synchronised.

Given than architecture they can then easily ensure that timer
interrupts are also synchronised on all nodes, and it's no big leap
from there to co-scheduling the MPI processes.

That's rather different from the problem being discussed here, which
is how to get clock synchonisation in a Beowulf where you do _not_ have
any common clock. (Indeed many of the solutions to that problem which
are being suggested are basically to provide a common timing signal
(clock input) to all the nodes...)

-- Jim

James Cownie	<jcownie at etnus.com>
Etnus, LLC.     +44 117 9071438
http://www.etnus.com

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