Not sure if this question is more appropriate for StackOverload, but I'm guessing a bunch of people here have experience with MPI2, so thought I would try here first. I have some large dense complex symmetric matrices that I would like to factor and wanted to try out Elemental for its MPI support. I compiled Open MPI 1.8.1 and a simple "hello world" example, but for some reason when I try to run: mpirun -n 2 -host localhost,machine-2 hello_world it just exits without outputting anything or even starting the hello_world program. If I run it on just localhost (i.e. mpirun -n 2 -host localhost hello_world), it works just fine. Do you use Open MPI or a different flavor of MPI2? I've searched everywhere online and can't find a working solution or even just a cause for this problem. (I have ssh setup between all the machines such that I do not need to use a password.) The machines are running RedHat 5, and I do not have root access to them. I have read that it might be a firewall problem (what ports does OpenMPI use outside of ssh?). Do I need to have anything placed in /etc/hosts? Thanks so much in advance!