I am currently using the MPI C library, but coding c++, I know that MPI_Barrier(MPI_COMM_WORLD) function blocks the caller until all processes in the communicator have called it, as in the documentation. Here is my code, running on 4 processes.
int WORLD_SIZE = 0;
int WORLD_RANK = 0;
{
MPI_Init(&argc, &argv);
MPI_Comm_size(MPI_COMM_WORLD, &WORLD_SIZE);
MPI_Comm_rank(MPI_COMM_WORLD, &WORLD_RANK);
MPI_Barrier(MPI_COMM_WORLD);
}
// everything works up till here
// WORLD_SIZE is 4, WORLD_RANK is the current process rank
{
int j = 1;
while (j <= log2(WORLD_SIZE)) {
printf("rank%d at iteration %d\n", WORLD_RANK, j);
MPI_Barrier(MPI_COMM_WORLD);
j++;
}
}
{
MPI_Finalize();
}
The program gives me the output.
rank0 at iteration 1
rank0 at iteration 2
rank1 at iteration 1
rank1 at iteration 2
rank2 at iteration 1
rank2 at iteration 2
rank3 at iteration 1
rank3 at iteration 2
Where I am expecting the following due to the barrier.
rank0 at iteration 1
rank1 at iteration 1
rank2 at iteration 1
rank3 at iteration 1
rank0 at iteration 2
rank1 at iteration 2
rank2 at iteration 2
rank3 at iteration 2
Any help appreciated. I can post more code if needed.
Is the current program execution order really the same as the current std output? If no how do I tell the real execution order? If yes then how do I correctly use barrier?
