Hello everybody.
I'm still on my MPI learning challenge :) Now i'm trying to learn to use the
MPI_Broadcast instruction.
I guess I must have a deep concept problem here. I'm trying to do a very simple
program in which node 0 calculates random numbers in a variable called "aux"
and broadcasts its value to all the nodes in the MPI World untill this "aux"
number reaches 5. I've done this:
------------------------------- broad.c ------------------------------
int main (int argc, char *argv[]){
int rank, size, node, aux=0, Counter=0, request;
MPI_Status status;
MPI_Init (&argc, &argv); /* starts MPI */
MPI_Comm_rank (MPI_COMM_WORLD, &rank); /* get current process id */
MPI_Comm_size (MPI_COMM_WORLD, &size); /* get number of processes */
srand (time(NULL));
while (aux != 5){
if (rank == 0){
aux = (rand()/200000000);
printf ("New Aux = %d\n", aux);
MPI_Bcast (&aux, 1, MPI_INT, 0, MPI_COMM_WORLD);
}
printf ("I'm node %d, and I've got an \'aux\' value of: %d\n", rank, aux);
}
printf ("Aux has reached %d in node %d\n", aux, rank);
return MPI_Finalize ();
}
---------------------------------------------------------------
So I thought that when node 0 calculates a new "aux" value would send it to all
the processes in the World (ehm... actually, only nodes 0 and 1) but it doesn't
work. If I execute this, node 1 doesn't update its "aux" value and the system
goes into an infinite loop, outputing:
New Aux = 8
I'm node 0, and I've got an 'aux' value of: 8
New Aux = 6
I'm node 0, and I've got an 'aux' value of: 6
New Aux = 5
I'm node 0, and I've got an 'aux' value of: 5
Aux has reached 5 in node 0
I'm node 1, and I've got an 'aux' value of: 0
I'm node 1, and I've got an 'aux' value of: 0
I'm node 1, and I've got an 'aux' value of: 0
I'm node 1, and I've got an 'aux' value of: 0
I'm node 1, and I've got an 'aux' value of: 0
I'm node 1, and I've got an 'aux' value of: 0
So it seems that node 0 calculates properly but then it's like if the
"Broadcast" message isn't received in the node 1.
I also tried to stop it somehow with an MPI_Barrier:
---------------------- barrier.c --------------------------
void stop(){
//sleep (1);
MPI_Barrier (MPI_COMM_WORLD);
}
int main (int argc, char *argv[]){
int rank, size, node, aux=0, Counter=0, request;
MPI_Status status;
MPI_Init (&argc, &argv); /* starts MPI */
MPI_Comm_rank (MPI_COMM_WORLD, &rank); /* get current process id */
MPI_Comm_size (MPI_COMM_WORLD, &size); /* get number of processes */
srand (time(NULL));
while (aux != 5){
if (rank == 0){
aux = (rand()/200000000);
printf ("New Aux = %d\n", aux);
MPI_Bcast (&aux, 1, MPI_INT, 0, MPI_COMM_WORLD);
}
printf ("I'm node %d, and I've got an \'aux\' value of: %d\n", rank, aux);
stop();
}
printf ("Aux has reached %d in node %d\n", aux, rank);
return MPI_Finalize ();
}
---------------------------------------------------
But then I also have the same problem (aux in node 1 is not updated, and when
aux in node 0 reaches 5, node 0 stops and then I've got the following error
Aux has reached 5 in node 0
I'm node 1, and I've got an 'aux' value of: 0
MPI_Recv: process in local group is dead (rank 1, MPI_COMM_WORLD)
Rank (1, MPI_COMM_WORLD): Call stack within LAM:
Rank (1, MPI_COMM_WORLD): - MPI_Recv()
Rank (1, MPI_COMM_WORLD): - MPI_Barrier()
Rank (1, MPI_COMM_WORLD): - main()
The only way to solve it is putting the MPI_Broadcast outside the if (rank==0):
------------------------ bcast2.c-----------------------------
int main (int argc, char *argv[]){
int rank, size, node, aux=0, Counter=0, request;
MPI_Status status;
MPI_Init (&argc, &argv); /* starts MPI */
MPI_Comm_rank (MPI_COMM_WORLD, &rank); /* get current process id */
MPI_Comm_size (MPI_COMM_WORLD, &size); /* get number of processes */
srand (time(NULL));
while (aux != 5){
if (rank == 0){
aux = (rand()/200000000);
printf ("New Aux = %d\n", aux);
}
MPI_Bcast (&aux, 1, MPI_INT, 0, MPI_COMM_WORLD);
printf ("I'm node %d, and I've got an \'aux\' value of: %d\n", rank, aux);
}
printf ("Aux has reached %d in node %d\n", aux, rank);
return MPI_Finalize ();
}
------------------------------------------------------
Then yes, it works properly:
New Aux = 9
I'm node 0, and I've got an 'aux' value of: 9
New Aux = 9
I'm node 1, and I've got an 'aux' value of: 9
I'm node 0, and I've got an 'aux' value of: 9
New Aux = 0
I'm node 1, and I've got an 'aux' value of: 9
I'm node 0, and I've got an 'aux' value of: 0
I'm node 1, and I've got an 'aux' value of: 0
New Aux = 5
I'm node 0, and I've got an 'aux' value of: 5
I'm node 1, and I've got an 'aux' value of: 5
Aux has reached 5 in node 0
Aux has reached 5 in node 1
What I would like to do with MPI_Broadcast would be something like what I can do
with a Send/Receive that looks like this:
---------------------- bcastWithoutBcast.c ------------------------
int main (int argc, char *argv[]){
int rank, size, node, aux=0, Counter=0, request;
MPI_Status status;
MPI_Init (&argc, &argv); /* starts MPI */
MPI_Comm_rank (MPI_COMM_WORLD, &rank); /* get current process id */
MPI_Comm_size (MPI_COMM_WORLD, &size); /* get number of processes */
srand (time(NULL));
while (aux != 5){
if (rank == 0){
aux = (rand()/200000000);
printf ("New Aux = %d\n", aux);
MPI_Send (&aux, 1, MPI_INT, 1, 100, MPI_COMM_WORLD);
}else {
MPI_Recv (&aux, 1, MPI_INT, 0, 100, MPI_COMM_WORLD, &status);
}
printf ("I'm node %d, and I've got an \'aux\' value of: %d\n", rank, aux);
}
printf ("Aux has reached %d in node %d\n", aux, rank);
return MPI_Finalize ();
}
-----------------------------------------------------
But if I try with MPI_Broadcast inside the if (rank==0) block, I can't, and I
don't understand why. Is like if node 1 did not listen to the broadcast
instruction...
Thank you very much for the help you could give me
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