Spawn MPI in x-platform way

[Jacob Hinkle]

This is related to the discussion on the PyCA issues list here. Essentially the problem is that currently, Compute.Compute spawns mpi processes using the mpiexec executable along with a call to os.system. However, that executable is not always close at hand, for instance on windows machines, in which case the system call craps out.

mpi4py has a facility to spawn mpi processes, but I don't know how to give it a host list and constrain the number of processes. Basically I need to better understand that aspect of mpi4py and replace the current crappy system call with the builtin support for spawning.

[Jacob Hinkle]

ok, so you can do the following, for example (from the docs)

comm = MPI.COMM_SELF.Spawn(sys.executable,
                           args=['cpi.py'],
                           maxprocs=5)

I'm not sure how to also supply a nodehosts list like you can with mpiexec, but for now I will launch using the above unless multiple other hosts are provided.