Selecting a Process Manager

The mpirun script uses the process manager specified by the I_MPI_PROCESS_MANAGER variable. By default, mpirun selects the Hydra Process Manager. Setting I_MPI_PROCESS_MANAGER to hydra will explicitly select Hydra, and setting it to mpd will explicitly select MPD. The process manager can also be selected by directly calling the appropriate mpiexec file: mpiexec.hydra for Hydra or mpiexec for MPD.