Many ``dynamic'' MPI applications are expected to exist in a static runtime environment, in which resources have been allocated before the application is run. When a user (or possibly a batch system) runs one of these quasi-static applications, she will usually specify a number of processes to start and a total number of processes that are expected. An application simply needs to know how many slots there are, i.e., how many processes it should spawn.
MPI provides an attribute on MPI_COMM_WORLD, MPI_UNIVERSE_SIZE, that allows the application to obtain this information in a portable manner. This attribute indicates the total number of processes that are expected. In Fortran, the attribute is the integer value. In C, the attribute is a pointer to the integer value. An application typically subtracts the size of MPI_COMM_WORLD from MPI_UNIVERSE_SIZE to find out how many processes it should spawn. MPI_UNIVERSE_SIZE is initialized in MPI_INIT and is not changed by MPI. If defined, it has the same value on all processes of MPI_COMM_WORLD. MPI_UNIVERSE_SIZE is determined by the application startup mechanism in a way not specified by MPI. (The size of MPI_COMM_WORLD is another example of such a parameter.)
Possibilities for how MPI_UNIVERSE_SIZE might be set include
MPI_UNIVERSE_SIZE is a recommendation, not necessarily a hard limit. For instance, some implementations may allow an application to spawn 50 processes per processor, if they are requested. However, it is likely that the user only wants to spawn one process per processor.
MPI_UNIVERSE_SIZE is assumed to have been specified when an application was started, and is in essence a portable mechanism to allow the user to pass to the application (through the MPI process startup mechanism, such as mpiexec) a piece of critical runtime information. Note that no interaction with the runtime environment is required. If the runtime environment changes size while an application is running, MPI_UNIVERSE_SIZE is not updated, and the application must find out about the change through direct communication with the runtime system.