It looks like the process launcher is broken in the OMPI trunk:
If you run any simple test (not necessarily including MPI calls) on 4 or
more nodes, the MPI processes won't be killed after the test finishes.
$ mpirun -host host_1,host_2,host_3,host_4 -np 4 --mca btl sm,tcp,self /bin/hostname
And test is hanging......
I have an older trunk (r25228), and everything is OK there.
Not sure if it means that something was broken after that, or the problem
existed before, but kicked in only now due to some other change.