Most massively parallel processors (MPPs) provide a way to start a program on a requested number of processors; mpirun makes use of the appropriate command whenever possible. In contrast, workstation clusters require that each process in a parallel job be started individually, though programs to help start these processes exist (see Using the secure server below). Because workstation clusters are not already organized as an MPP, additional information is required to make use of them. mpich should be installed with a list of participating workstations in the file machines.<arch> in the directory /usr/local/mpi/bin/machines. This file is used by mpirun to choose processors to run on. (Using heterogeneous clusters is discussed below.) The rest of this section discusses some of the details of this process, and how you can check for problems. These instructions apply to only the ch_p4 and ch_nexus devices. There are some differences between them that will be noted; however, most options are the same for both devices.