MPI_Gather doesn't receive data - c

Question

The program should take two cmd line arguments, N = the number of items each worker should generate, and H = the highest value in the range of random numbers generate by each worker. Each worker makes a list of those random values and then the BigList is where I'm trying to gather them all back to but nothing shows up in the array of BigList. So for example:
Running mpirun -np 3 a.out 4 20 gets:
RANK: 1 --- NUM: 18
RANK: 1 --- NUM: 6
RANK: 1 --- NUM: 12
RANK: 1 --- NUM: 10
RANK: 2 --- NUM: 9
RANK: 2 --- NUM: 3
RANK: 2 --- NUM: 6
RANK: 2 --- NUM: 5
and BigList is empty when I'd expect it to get composed of every num listed above.
#include <mpi.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int main(int argc, char* argv[]){
double t1, t2;
MPI_Init(&argc, &argv);
int rank;
int wsize;
int N = 10, H = 5;
int num, k, i;
int locarr[25];
int bigList[300];
srand(time(NULL));
if(argc > 1){
N = atoi(argv[1]);
H = atoi(argv[2]);
}
t1 = MPI_Wtime();
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
MPI_Comm_size(MPI_COMM_WORLD, &wsize);
if( rank == 0){
MPI_Bcast(&N, 1, MPI_INT, 0, MPI_COMM_WORLD);
MPI_Bcast(&H, 1, MPI_INT, 0, MPI_COMM_WORLD);
}
else{
for(i = 0; i < N; i++){
locarr[i] = (((rand() % H) + 1) / rank);
printf("RANK: %d --- NUM: %d\n", rank, locarr[i]);
}
}
MPI_Gather(&locarr, N, MPI_INT, bigList, N, MPI_INT, 0, MPI_COMM_WORLD);
if( rank == 0){
printf("BigList: ");
for(k = 0; k < (rank * N); k++){
printf(" %d", bigList[k]);
}
printf("\n");
}
t2 = MPI_Wtime();
// printf("\nMPI_Wtime(): %f\n", t2 - t1);
MPI_Finalize();
return 0;
}

Answer 1

Let me expand the comment of Gilles Gouaillardet,
MPI_Gather call is written correctly. To get the expected results, two changes need to be done.
MPI_Bcast is a collective operation. All processes should call it. So the code should be:
if( rank == 0){
MPI_Bcast(&N, 1, MPI_INT, 0, MPI_COMM_WORLD);
MPI_Bcast(&H, 1, MPI_INT, 0, MPI_COMM_WORLD);
}
else{
MPI_Bcast(&N, 1, MPI_INT, 0, MPI_COMM_WORLD);
MPI_Bcast(&H, 1, MPI_INT, 0, MPI_COMM_WORLD);
for(i = 0; i < N; i++){
locarr[i] = (((rand() % H) + 1) / rank);
printf("RANK: %d --- NUM: %d\n", rank, locarr[i]);
}
}
Also, rank 0 prints the contents of the bigList. But in for loop, the loop condition is k<rank*N, for rank 0 this will always be false (k<0*N) as a result loop won't be executed and no value will be printed. So it should be world size (wsize) instead of rank.
if( rank == 0){
printf("BigList: ");
for(k = 0; k < wsize*N; k++){
printf(" %d", bigList[k]);
}
printf("\n");
}

Answer 2

The print function was printing some extra garbage values in the BigList array. Instead replacing the print part of the code with this would solve the error
if (rank == 0)
{
printf("BigList: ");
for (k = N; k < wsize * N; k++)
{
printf(" %d", bigList[k]);
}
printf("\n");
}
The full code for the problem is-
#include <mpi.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int main(int argc, char *argv[])
{
double t1, t2;
MPI_Init(&argc, &argv);
int rank;
int wsize;
int N = 10, H = 5;
int num, k, i;
int locarr[25];
int bigList[300];
srand(time(NULL));
if (argc > 1)
{
N = atoi(argv[1]);
H = atoi(argv[2]);
}
t1 = MPI_Wtime();
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
MPI_Comm_size(MPI_COMM_WORLD, &wsize);
if (rank == 0)
{
MPI_Bcast(&N, 1, MPI_INT, 0, MPI_COMM_WORLD);
MPI_Bcast(&H, 1, MPI_INT, 0, MPI_COMM_WORLD);
}
else
{
MPI_Bcast(&N, 1, MPI_INT, 0, MPI_COMM_WORLD);
MPI_Bcast(&H, 1, MPI_INT, 0, MPI_COMM_WORLD);
for (i = 0; i < N; i++)
{
locarr[i] = (((rand() % H) + 1) / rank);
printf("RANK: %d --- NUM: %d\n", rank, locarr[i]);
}
}
MPI_Gather(&locarr, N, MPI_INT, bigList, N, MPI_INT, 0, MPI_COMM_WORLD);
if (rank == 0)
{
printf("BigList: ");
for (k = N; k < wsize * N; k++)
{
printf(" %d", bigList[k]);
}
printf("\n");
}
MPI_Finalize();
return 0;
}