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OpenMPI version 3.0 and node oversubscription #182

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@jrper

It see that OpenMPI version 3.0 has changed the default behaviour regarding oversubscription and mpirun. On this version, a plain call like mpirun -np 8 python -c "print('Hello')" will fail with an MPI error on systems with fewer than 8 slots available. This includes several of the Fluidity tests, which can request anything up to 16 processes.

The "right way" to do things is now to specify mpirun -np 8 -oversubscribe python -c "print('Hello')", on specific calls, or to define an environment variable, OMPI_MCA_rmaps_base_oversubscribe=1 mpirun -np 8 python -c "print('Hello')" to get back to the openmpi 2.0 behaviour. I'm not sure what the right thing to do for us is, possibly to use the last option inside of test harness itself?

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