Question

In the following codes:

if (rank==0) master();
else slave();

...

void master()
{
int i=0;
}

...

void slave()
{
int i=1;
MPI_BCAST(&i,1,MPI_INT,0,COMM);
}

Will the slave node broadcast “i(==0)” in the master node and set the “i” values in all slave nodes to be 0?

Answer 1

It’s a little unclear from what you’ve posted that you have the semantics right — all processes in the communicator have to call MPI_BCAST, it’s one of MPI’s collective operations. Your program would then behave as if the process designated the root in the call to MPI_BCAST sends the message to all the other processes in the designated communicator which, in turn, receive the message.

Your snippet suggests that you think that the call to MPI_BCAST would be successful if called only on what you call the ‘slave’ process(es), which would be incorrect.

Your syntax for the call is, however, correct.

EDIT in response to comment

I believe that all processes have to execute the piece of code which calls MPI_BCAST. If, as you suggest, the pseudo-code is like this:

if (myrank == master) then
   do_master_stuff ...
   call mpi_bcast(...)
end if

if (myrank /= master) then
   call mpi_bcast(...)
   do_worker_stuff ...
end if

then the call will fail; it’s likely that your program will stall until the job management or operating system notices and chucks it out. There is no mechanism within MPI for ‘matching’ calls to MPI_BCAST (or any of the other collective communications routines) across scopes.

Your pseudo-code ought to be like this

if (myrank == master) then
   do_master_stuff ...
end if

if (myrank /= master) then
   do_worker_stuff ...
end if

! all together now
call mpi_bcast(...)

or whatever variant your program requires