[yt-users] Change in parallel execution policy

[Matthew Turk]

Hi guys,

I've changed the policy in trunk.  After talking with Stephen Skory
about this, and going back and forth, it became obvious that the
original policy of running in parallel "whenever possible" via
autodiscovery of the mpi4py module & job size was not a viable option.
 You now have to use the executable 'mpi4py' instead of the standard
'python' if you want to notify yt to run in parallel.  (The executable
mpi4py is part of the mpi4py distribution, and is available by running
the 'install_exe' task on their setup.py.)

So, instead of:

mpirun -np 4 python2.5 some_script.py

you'd run:

mpirun -np 4 mpi4py some_script.py

By avoiding global variables, we are able to preserve the usage of
identical scripts in parallel and in serial.  (From the yt
perspective.)

I'm still working on parallelizing other objects (slices & cutting
planes) besides projections, quantities & profiles; Stephen Skory has
begun work on parallelizing HOP, which would be a pretty awesome
addition.  More updates as they come.

-Matt