MPI dynamic communication between 2 Processes - c

Question

I have a process that manages a list of 2d arrays and passes different 2d arrays to each process. There is a possibility that I have not enough processes for all 2d arrays So I need to ask process 0 which manages the list of 2d arrays if there are any arrays left after any process which is not 0 finishes working with the first received array. I dont know how to implement this.
(...)
if(rank == 0)
//check if we have elements in 2d array left
while (ptr != NULL)
{
MPI_Status status;
int sig;
// Wait for a process to ask for a 2d array
MPI_Recv(&sig, 1, MPI_INT, MPI_ANY_SOURCE, 0, MPI_COMM_WORLD, &status);
// send it to them
MPI_Send(&ptr->grid.sudoku, N * N, MPI_INT, status.MPI_SOURCE, 0, MPI_COMM_WORLD);
ptr = ptr->next;
}
// free memory
delete_grids(list);
elementsAvailable = 0;
}
// rank != 0
else
{
lookForSolution(recvGrid(), rank); // recvGrid calls MPI_Recv and passes
//the given array to a function to calculate something
}
MPI_Bcast(&sudokusAvailable, 1, MPI_INT, 0, MPI_COMM_WORLD); // Here I thought I make a MPI_Bcast
//to tell other processes that process 0 has no arrays left but If I put it here
//the next if statement will never be called in the first place
if (rank != 0 && elementsAvailable == 1)
{
MPI_Status status;
// Send process 0 that ready for new 2d array
MPI_Send(1, 1, MPI_INT, 0, 0, MPI_COMM_WORLD);
lookForSolution(recvGrid(), rank);
}
(...)

Answer 1

The simplest solution is to let process zero send data to another process, and then post an MPI_Irecv for the result. When all the processes have work, the manager does an MPI_Waitany to see if any process is returning results, if so, accept, and send new work. If the manager runs out of work, it sends a special message to the workers, and they quit.
From here on it's an exercise to the reader.
If you want the manager also to participate in the work, it becomes a little more complicated, and you could solve the whole thing with one-sided communication, creating in effect a shared object with all the work.