cheap parallel prime number tester. region division with remainder(calculate istart and iend for each process) could be useful for some other cases...? $ make $ time ./seri 2147483647 val: 2147483647 this is a prime number. real 0m11.653s user 0m11.641s sys 0m0.001s $ time ./omp 2147483647 val: 2147483647 this is a prime number. real 0m1.124s user 0m14.925s sys 0m0.258s $ time srun --mpi=pmi2 -n64 -c1 --cpu_bind=cores -m block:block ./mpi 2147483647 val: 2147483647 this is a prime number. real 0m0.897s user 0m0.011s sys 0m0.015s CPU: Intel(R) Xeon(R) CPU E5-2680 0 @ 2.70GHz, 2 sockets/8 cores per socket MPI version: 4 nodes, 64 cores, intelmpi 2018u0