Hello, I am using mpi4py to call MPI from python, and when I increase the number of ranks at the command line so something like: [jlarson@mintthinkpad test]$ mpiexec -n 30 python script.py I receive the following error messages: [mpiexec@mintthinkpad] handle_pmi_cmd (pm/pmiserv/pmiserv_cb.c:52): Unrecognized PMI command: | cleaning up processes [mpiexec@mintthinkpad] control_cb (pm/pmiserv/pmiserv_cb.c:280): unable to process PMI command [mpiexec@mintthinkpad] HYDT_dmxu_poll_wait_for_event (tools/demux/demux_poll.c:76): callback returned error status [mpiexec@mintthinkpad] HYD_pmci_wait_for_completion (pm/pmiserv/pmiserv_pmci.c:198): error waiting for event [mpiexec@mintthinkpad] main (ui/mpich/mpiexec.c:336): process manager error waiting for completion When I search online for help with these error messages, I am only pointed towards the repository containing the files that are themselves throwing the error. For example: Googling for "handle_pmi_cmd Unrecognized PMI command" only returns: https://svn.mcs.anl.gov/repos/mpi/mpich2/tags/release/mpich2-1.3a1/src/pm/hy... Note that my script is using very few resources; each rank is spawning a simple external function. The function takes a vector and then sums all the entries, squares them, and returns the number to each rank. The script works as expected when the number of initial ranks is small. Thank you for your help. Jeffrey Larson