mpi时间测量 MPI_Wtime

https://mpitutorial.com/tutorials/mpi-broadcast-and-collective-communication/

Comparison of MPI_Bcast with MPI_Send and MPI_Recv

MPI_Wtime takes no arguments, and it simply returns a floating-point number of seconds since a set time in the past.

注意更新计时变量前的Barrier，确保所有的process都执行到了MPI_Barrier这一行之后，才让它们更新total_my_bcast_time

// Synchronize before starting timing
MPI_Barrier(MPI_COMM_WORLD);
total_my_bcast_time -= MPI_Wtime();
my_bcast(data, num_elements, MPI_INT, 0, MPI_COMM_WORLD);
// Synchronize again before obtaining final time
MPI_Barrier(MPI_COMM_WORLD);
total_my_bcast_time += MPI_Wtime();

注意在MPI_Init之后初始化的变量是每个process一个copy的，所以total_my_bcast_time是每个process一个copy的，不会有竞争

int main(int argc, char** argv) {
  if (argc != 3) {
    fprintf(stderr, "Usage: compare_bcast num_elements num_trials\n");
    exit(1);
  }

  int num_elements = atoi(argv[1]);
  int num_trials = atoi(argv[2]);

  MPI_Init(NULL, NULL);

  int world_rank;
  MPI_Comm_rank(MPI_COMM_WORLD, &world_rank);

  double total_my_bcast_time = 0.0;
  double total_mpi_bcast_time = 0.0;
  int i;
  int* data = (int*)malloc(sizeof(int) * num_elements);
  assert(data != NULL);

  for (i = 0; i < num_trials; i++) {
    // Time my_bcast
    // Synchronize before starting timing
    MPI_Barrier(MPI_COMM_WORLD);
    total_my_bcast_time -= MPI_Wtime();
    my_bcast(data, num_elements, MPI_INT, 0, MPI_COMM_WORLD);
    // Synchronize again before obtaining final time
    MPI_Barrier(MPI_COMM_WORLD);
    total_my_bcast_time += MPI_Wtime();

    // Time MPI_Bcast
    MPI_Barrier(MPI_COMM_WORLD);
    total_mpi_bcast_time -= MPI_Wtime();
    MPI_Bcast(data, num_elements, MPI_INT, 0, MPI_COMM_WORLD);
    MPI_Barrier(MPI_COMM_WORLD);
    total_mpi_bcast_time += MPI_Wtime();
  }

  // Print off timing information
  if (world_rank == 0) {
    printf("Data size = %d, Trials = %d\n", num_elements * (int)sizeof(int),
           num_trials);
    printf("Avg my_bcast time = %lf\n", total_my_bcast_time / num_trials);
    printf("Avg MPI_Bcast time = %lf\n", total_mpi_bcast_time / num_trials);
  }

  free(data);
  MPI_Finalize();
}