Installing Paraview after Firedrake clobbers mpich with openmpi for mpirun

[dorugeber]

(This is on a clean Ubuntu 18.04.1)

This has a nasty failure mode: mpirun -n X python foo.py simply runs X independent copies of foo.py, with no obvious indication to the user that the wrong thing is happening.

Can be fixed with sudo update-alternatives --config mpirun, and choosing the mpich option, at least if you have admin rights.

Two suggestions:

1. Ideally, the right mpirun should be used automatically (within the virtualenv, if not system-wide)
2. Some way of (automatically) verifying that mpirun -n X isn't just running X serial copies.

[dorugeber]

Definitely not all @jandrej's fault ;-)

[wence-]

What installed mpirun?
In the install script we can do:

mpirun -n 2 python -c "import mpi4py; from mpi4py import MPI; if MPI.COMM_WORLD.size != 2: raise RuntimeError" or similar?

[wence-]

These days our preferred way of fixing this is telling
firedrake-install what mpicc, mpicxx, mpirun, etc... you are going
to use. This then puts symlinks in the virtualenv bin directory, so
when you activate it, you get the right versions of everything.