Sending blocks of 2D array rows using MPI in C - c

Question

I am trying to do a matrix multiplication using MPI in C. (c <= a*b)
I am running the following code on 4 nodes. All the matrices are 8*8 in size.
(num of rows in a matrix % num of nodes == 0)
matrix b[][] is broadcast so all the nodes get the same copy. For matrix a[][], instead of broadcasting, I want to send only the set of rows that is needed by each node.
But when I run the following code and print the matrix a[][] after MPI_Recv() worker nodes print 0s instead of the values assigned in the master node.
Can you point out what am I doing wrong here?
#include <stdio.h>
#include "mpi.h"
#include "matrix.c" // matrix definitions and matrix operation functions are here
int main(int argc, char *argv[])
{
MPI_Status status;
int num, rank, size, tag, high,low,i;
int offset, tmphigh,rows;
MPI_Init(&argc, &argv);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
MPI_Comm_size(MPI_COMM_WORLD, &size);
rows=MAX/size; // MAX is the length(=height) of the matrices
tag = 201;
low=rank*rows;
high=low+rows;
if (rank == 0) {
fillMatrix(b,MAX);
fillMatrix(a,MAX);
}
MPI_Bcast(&b[0][0],MAX*MAX,MPI_INT,0,MPI_COMM_WORLD);
if(rank==0){
for(i=1;i<size;i++){
offset=i*rows;
MPI_Send(&a[offset][0],rows*MAX,MPI_INT,i,tag,MPI_COMM_WORLD);
}
}else{
MPI_Recv(&a[low][0],rows*MAX,MPI_INT,0,tag,MPI_COMM_WORLD,&status);
}
printMatrix(a,MAX);
MPI_Finalize();
return 0;
}
here is how matrices are created
int a[MAX][MAX], b[MAX][MAX], c[MAX][MAX];
int len; //(edited after Jeremy W. Sherman's comment )
//this was the reason that caused this problem. changing this to int len=MAX; solved the problem
void fillMatrix(int (*matrix)[len], int len){
int i=0,j=0;
for(i=0;i<len;i++){
for(j=0;j<len;j++){
matrix[i][j]=j;
}
}
//printMatrix(matrix,len);
}
Thank You.

Answer 1

The problem might lie printMatrix() and fillMatrix(). clang refused to compile your definition of fillMatrix():
so_mpi.c:22:31: error: use of undeclared identifier 'len'
void fillMatrix(int (*matrix)[len], int len){
^
Dropping len from the prototype just creates another problem:
so_mpi.c:26:19: error: subscript of pointer to incomplete type 'int []'
matrix[i][j]=j;
~~~~~~^
What did work was this:
void fillMatrix(int *matrix, int len) {
int i, j;
for (i = 0; i < len; ++i) {
int *row = &matrix[i * len];
for(j = 0; j < len; ++j) {
row[j] = j;
}
}
}
fillMatrix((int *)a, MAX);
fillMatrix((int *)b, MAX);
With that change, everything seems to work fine. I used MAX = 5 and 5 nodes. I prefixed logging statements with the node's rank and added a few more logging statements. Here was the result:
$ mpirun -np 5 ./so_mpi
node 1 of 5
node 4 of 5
node 0 of 5
0: filling matrices
0: matrix B:
0 1 2 3 4
0 1 2 3 4
0 1 2 3 4
0 1 2 3 4
0 1 2 3 4
0: matrix A:
0 1 2 3 4
0 1 2 3 4
0 1 2 3 4
0 1 2 3 4
0 1 2 3 4
0: broadcast of B complete
0: sending 1 rows (5 elements) of A to node 1
0: sending 1 rows (5 elements) of A to node 2
0: sending 1 rows (5 elements) of A to node 3
0: sending 1 rows (5 elements) of A to node 4
0: matrix A:
0 1 2 3 4
0 1 2 3 4
0 1 2 3 4
0 1 2 3 4
0 1 2 3 4
1: broadcast of B complete
1: received portion of matrix
1: matrix A:
0 0 0 0 0
0 1 2 3 4
0 0 0 0 0
0 0 0 0 0
0 0 0 0 0
node 2 of 5
2: broadcast of B complete
2: received portion of matrix
2: matrix A:
0 0 0 0 0
0 0 0 0 0
0 1 2 3 4
0 0 0 0 0
0 0 0 0 0
node 3 of 5
3: broadcast of B complete
3: received portion of matrix
3: matrix A:
0 0 0 0 0
0 0 0 0 0
0 0 0 0 0
0 1 2 3 4
0 0 0 0 0
4: broadcast of B complete
4: received portion of matrix
4: matrix A:
0 0 0 0 0
0 0 0 0 0
0 0 0 0 0
0 0 0 0 0
0 1 2 3 4