CODEWITHSUNDEEP
cparallel-processingmpi
MattSt
MattSt

Reputation: 1192

Using MPI_Gather to gather the sums calculated from all processes in one array

I' m trying to have each process calculate a sum and then send the sums back to the root process.

However, the printf("\nSUMS[%d] = %d",i,sums[i]); line prints out different sums than those printed at printf("\nI am process %d and my sum is %d", my_rank, sum);

Only the sum of root is the same with sums[0].

Can someone explain why this happens? Excuse me if this question has an obvious answer but I'm new to MPI.

sum = 0;
for ( i=0 ; i < np; i++)
    sum += y[i];

printf("\nI am process %d and my sum is %d", my_rank, sum);



if (!my_rank)
    sums = (int *)malloc(sizeof(int) * comm_size);

MPI_Gather(&sum, 1, MPI_INT, sums, comm_size, MPI_INT, 0, MPI_COMM_WORLD);

if (!my_rank)
    for ( i = 0; i < comm_size; i++)
        printf("\nSUMS[%d] = %d",i,sums[i]);

Upvotes: 0

Views: 1683

Answers (1)

francis
francis

Reputation: 9817

According to the documentation of mpich on MPI_Gather() the fifth argument of this function recvcount is the number of elements for any single receive. In the code you posted, comm_size is used, but it is the total number of elements to be received. Hence, the following line could solve your problem:

MPI_Gather(&sum, 1, MPI_INT, sums, 1, MPI_INT, 0, MPI_COMM_WORLD);

If it does not work, please let us know!

Upvotes: 1

Related Questions