The following code:

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#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(); } 

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TASKS:

1. Why does half the processes send before receiving and vice versa?

2. What would happen if all send before receiving?

3. The MPI send and MPI receive are blocking operations. Change the program to use the non-blocking communication operations, and make all tasks send before receiving. Hint: You must use MPI Waitall to wait for communication to complete.