It's too clear just run the program and answer these question Why does half the processes send before receiving and vice versa? What would happen if all send before receiving?

------------------------------------------------------------------------------------------------------------------------ #include "mpi.h" #include #include #define MASTER 0

int main (int argc, char *argv[]) { int numtasks, taskid, len, partner, message; char hostname[MPI_MAX_PROCESSOR_NAME]; MPI_Status status;

MPI_Init(&argc, &argv); MPI_Comm_rank(MPI_COMM_WORLD, &taskid); MPI_Comm_size(MPI_COMM_WORLD, &numtasks);

/* need an even number of tasks */ if (numtasks % 2 != 0) { if (taskid == MASTER) printf("Quitting. Need an even number of tasks: numtasks=%d ", numtasks); } else { if (taskid == MASTER) printf("MASTER: Number of MPI tasks is: %d ",numtasks);

MPI_Get_processor_name(hostname, &len); printf ("Hello from task %d on %s! ", taskid, hostname);

/* determine partner and then send/receive with partner */ if (taskid < numtasks/2) { partner = numtasks/2 + taskid; MPI_Send(&taskid, 1, MPI_INT, partner, 1, MPI_COMM_WORLD); MPI_Recv(&message, 1, MPI_INT, partner, 1, MPI_COMM_WORLD, &status); } else if (taskid >= numtasks/2) { partner = taskid - numtasks/2; MPI_Recv(&message, 1, MPI_INT, partner, 1, MPI_COMM_WORLD, &status); MPI_Send(&taskid, 1, MPI_INT, partner, 1, MPI_COMM_WORLD); }

/* print partner info and exit*/ printf("Task %d is partner with %d ",taskid,message); }

MPI_Finalize(); }