Getting values in spawn process - c

Question

I'm trying to get values in my spawned process using collective MPI functions.
In this case I have a N*N matrix and I want to pass every row to each process. Get the values in each process and sum their values.
I'm using this example:
MPI_Scatter of 2D array and malloc
main
int main(int argc, char *argv[]){
int *n, range, i, j, dato, resultado;
int *matriz;
char *nombre_esclave="esclavo";
//MPI Section
int rank, size;
MPI_Comm hijos;
MPI_Status status;
MPI_Init(&argc, &argv);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
MPI_Comm_size(MPI_COMM_WORLD, &size);
matriz = createMatrix(N, N);
printArray(matriz, N * N);
//Child process
MPI_Comm_spawn("slave", MPI_ARGV_NULL, N, MPI_INFO_NULL, 0, MPI_COMM_SELF, &hijos, MPI_ERRCODES_IGNORE);
// received row will contain N integers
int *procRow = malloc(sizeof(int) * N);
MPI_Scatter(matriz, N, MPI_INT, // send one row, which contains N integers
procRow, N, MPI_INT, // receive one row, which contains N integers
MPI_ROOT, hijos);
MPI_Finalize();
return 0;
}
and in slave
slave
MPI_Init(&argc, &argv);
MPI_Comm_rank(MPI_COMM_WORLD, &pid);
MPI_Comm_size(MPI_COMM_WORLD, &size);
MPI_Comm_get_parent(&parent);
if (parent != MPI_COMM_NULL) {
printf("This is a child process\n");
}
//number of processes in the remote group of comm (integer)
MPI_Comm_remote_size(parent, &size);
int *procRow = malloc(sizeof(int) * N);
//UNABLE TO GET VALUES FROM THE PARENT
//I need to sum old the values y every portion of the matrix
//passed to every child process
MPI_Reduce(procRow, &resultado_global, N, MPI_INT, MPI_SUM, 0, parent);
UPDATE
With MPI_Comm_spawn I create 3 childs. In every child I want to get a row of matrix (I use scatter in master). Later I use MPI_Reduce to sum every row in child (that's why I say getting values).
UPDATE 2
On slave I have modified the code and I get the rows in every process.
if (parent != MPI_COMM_NULL) {
//number of processes in the remote group of comm (integer)
MPI_Comm_remote_size(parent, &size_remote);
int *matrix = malloc(sizeof(int) * size);
int *procRow = malloc(sizeof(int) * size);
MPI_Scatter(matrix, N, MPI_INT,procRow, N, MPI_INT,0, parent);
//procRow values correctly from each row of the matrix
if (procRow != NULL) {
printf("Process %d; %d %d %d \n", pid, procRow[0], procRow[1], procRow[2]);
}
//Unable to sum each row
MPI_Reduce(procRow, &resultado_global, size, MPI_INT, MPI_SUM, ROOT, parent);
//MPI_Reduce(procRow, &resultado_global, size, MPI_INT, MPI_SUM, ROOT, MPI_COMM_WORLD);
}
UPDATE 3 (SOLVED)
IN SLAVE
if (parent != MPI_COMM_NULL) {
//number of processes in the remote group of comm (integer)
MPI_Comm_remote_size(parent, &size_remote);
int *matrix = malloc(sizeof(int) * size);
int *procRow = malloc(sizeof(int) * size);
MPI_Scatter(matrix, N, MPI_INT, procRow, N, MPI_INT, 0, parent);
if (procRow != NULL) {
printf("Process %d; %d %d %d \n", pid, procRow[0], procRow[1], procRow[2]);
sumaParcial=0;
for (int i = 0; i < N; i++)
sumaParcial = sumaParcial + procRow[i];
}
MPI_Reduce(&sumaParcial, &resultado_global, 1, MPI_INT, MPI_SUM, ROOT, parent);
}
IN MASTER
// received row will contain N integers
int *procRow = malloc(sizeof(int) * N);
MPI_Scatter(matriz, N, MPI_INT, // send one row, which contains N integers
procRow, N, MPI_INT, // receive one row, which contains N integers
MPI_ROOT, hijos);
MPI_Reduce(&sumaParcial, &resultado_global, 1, MPI_INT, MPI_SUM, MPI_ROOT, hijos);
printf("\n GLOBAL RESULT :%d\n",resultado_global);
Any idea?
Thanks

Answer 1

From the edit I suppose that the scatter is working correctly.
You main confusion seems to be about MPI_Reduce. It does not do any local reduction. According to your graphic, you want to have the values 6, 15, 24at the ranks 0, 1, 2 in the slaves. That is done entirely without MPI, just by iterating over the local rows.
An MPI_Reduce on the rows would lead to the root having [12, 15, 18]. If you just want the total sum 45 at the root of the slaves, you should first summarize the values locally and then MPI_Reduce the single values from each rank to a single global value.