Increasing contention for shared resources prevents linear scale-up with increasing parallelism. In a shared MEMORY system, contention for memory (which implies bus contention) will result in falling scale-up with increasing parallelism. In a shared disk system, it is contention for disk and bus access which affects scale-up. In a shared-nothing system, inter-process COMMUNICATION overheads will be the MAIN impeding factor. Since there is no shared memory, acquiring locks, and other activities requiring message PASSING between processes will take more time with increased parallelism.

Increasing contention for shared resources prevents linear scale-up with increasing parallelism. In a shared memory system, contention for memory (which implies bus contention) will result in falling scale-up with increasing parallelism. In a shared disk system, it is contention for disk and bus access which affects scale-up. In a shared-nothing system, inter-process communication overheads will be the main impeding factor. Since there is no shared memory, acquiring locks, and other activities requiring message passing between processes will take more time with increased parallelism.