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Subject: Re: [OMPI users] Any scientific application heavilyusing MPI_Barrier?
From: Joe Landman (landman_at_[hidden])
Date: 2009-03-05 10:50:11

Jeff Squyres wrote:
> On Mar 5, 2009, at 10:33 AM, Gerry Creager wrote:
>> We've been playing with it in a coupled atmosphere-ocean model to allow
>> the two to synchronize and exchange data. The models have differing
>> levels of physics complexity and the time step requirements are
>> significantly different. To sync them up we have to know where the
>> timesteps are identical, stop the process, exchange data and continue.
>> We've been playing with barrier to help.
> I'm not sure I understand -- "help" meaning what?
> If you're exchanging data at the end of an iteration, then you
> effectively have a synchronization anyway -- no need for an extra
> barrier synchronization.

Hi Jeff and Gerry:

   I think what Gerry means is that they have 2 distinct models running
as different processes, and every now and then, the time steps of the
models agree ... so both models register the same time to within some
uncertainty or pre-defined accuracy. At that point, they exchange
information. So it is not necessarily a synchronization at every time
step, its likely at every Nth time step for one code, and every Mth for
the other code. Of course this assumes fixed size time steps which
might not be the case.

   Regardless, at the point where the times are determined to be equal,
the models need to exchange data. Which means if one got there sooner
than the other, that one has to wait for the other. Continuing
calculating without the data exchange would be, I am guessing, a waste
of processor cycles as the models are coupled, and continuing without
the coupling would be incorrect.



Joseph Landman, Ph.D
Founder and CEO
Scalable Informatics LLC,
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