Difficulty with MPI_Bcast: how to ensure that “correct” root is broadcasting

Question

I am relatively new to MPI (with C), and am having some trouble using MPI_Bcast to send an int to all processes.

In my code, I decide which rank is root within a for loop, where different processes are responsible for different element of the loop. Then, I want to Bcast a result from root to all processes, except all non-root processes do not know who to expect the bcast from, so do not receive it.

The code block looks something like this:

for (iX=start; iX<end; iX++)
//start and end are the starting row and ending row for each processes, defined earlier

  for (int iY=1; iY<nn; iY++)

    // do some calculations

    if (some condition)

      int bcastroot = rank;  // initialized above
      int X = 111;   // initialized above

    else
      //do other calculations

  end
end

MPI_Bcast(&X, 1, MPI_INT, bcastroot, comm);

// remainder of code and MPI_FINALIZE

When I execute this code, whatever the bcastroot default (value for all non-root processes) competes with root, so X is not broadcast correctly. I do not know the value of X nor can I predict the root beforehand, so I cannot define it in advance.

I have tried initializing bcastroot = -1, then setting it for rank, but this does not work. Is there a way I can Bcast this value without setting root for all processes?

Thanks, JonZor

If you do not know the root, you need to communicate it. Maybe bcast is not the right choice here? Did you have a look into MPI_Allreduce if it could solve your problem? — haraldkl

Jeremiah Willcock Jeremiah Willcock · Accepted Answer · 2012-03-10T21:20:38

There is no way to do an MPI_Bcast where the receivers don't know what the root is. If you know there will only be one root, you can first do an MPI_Allreduce to agree on it:

int root, maybe_root;
int i_am_root = ...;
maybe_root = (i_am_root ? rank : 0);
MPI_Allreduce(&maybe_root, &root, 1, MPI_INT, MPI_MAX, MPI_COMM_WORLD);

then every rank will know the same root and you can do your broadcast.

Difficulty with MPI_Bcast: how to ensure that “correct” root is broadcasting

1 Answers