The mpi Roll provides MPICH2, MVAPICH2 and OpenMPI for a Rocks cluster. It installs the software in /opt/mpich2, /opt/mvapich2, and /opt/openmpi on both client and server hosts. Both static and shared MPI libraries are included with the MPICH2 and MVAPICH2 installations; for OpenMPI, only static libraries are provided. Binaries and libraries produced with the GNU compilers are always included. If the PGI and/or Intel compilers are available, compatible versions of the MPI binaries and libraries are also installed in subdirectories of each implementation installation directories.