A communication library between multiple sets of MPI processes for a MPMD model

Because current high-end parallel systems have more than several thousands of nodes, a new programming model is required to exploit different levels of parallelism. For example, a conventional master-worker program assumes that a worker is running as a single MPI process. In some cases, a worker may run as a set of MPI processes to exploit a different level of parallelism or solve large problems within each worker. As the number of nodes increases, communication may be needed between the multiple sets of MPI processes. On the other hand, in order to make use of a large-scale system more effectively, it has been recently common for researchers to couple parallel components to conduct simulations of multiple physics models. This method may also requires a way to integrate multiple MPI programs.