MPI_SHM_REDUCESIZE

On SMPs, calling MPI_Reduce() causes all processors to participate in the reduce. Each processor will work on a piece of data equal to the MPI_SHM_REDUCESIZE setting. The default value is 256 bytes. Care must be taken when setting this variable because the system reserves MPI_SHM_REDUCESIZE * np * np memory to execute the reduce.