#include <mpi.h>

#include <stdlib.h>

#include <stdio.h>

#include <time.h>

#define items (1024*1024)

int array[items]; // Goal of the program: Summing up this array. (Instead use allocated memory in real programs!)

long sum = 0; // The result of the computation

long sum__sequential_reference_implementation() { // Non-parallel reference implementation

long s = 0;

for(int item = 0; item < items; ++item)

s += array[item];

return s;

}

int main(int argc, char** argv) {

MPI_Init(&argc, &argv);

int my_rank; // Number of the node

MPI_Comm_rank(MPI_COMM_WORLD, &my_rank);

int node_count; // Total number of nodes

MPI_Comm_size(MPI_COMM_WORLD, &node_count);

// The root must load the input data to distribute to the other nodes

if(my_rank == 0) {

// In our case it generates a random array as input data

srand(time(NULL));

for(int item = 0; item < items; ++item)

array[item] = rand();

}

int items_per_rank = items / node_count;

int remainder_items = items % node_count;

int* my_work;

MPI_Alloc_mem(items_per_rank * sizeof(int), MPI_INFO_NULL, &my_work);

// MPI_Scatter is a collective operation which distributes an equal-sized part of the given array to each node.

MPI_Scatter(&array[remainder_items] /* send buffer */, items_per_rank /* send count per node */, MPI_INT /* send type */,

my_work /* receive buffer on each node */, items_per_rank /* receive count */ , MPI_INT /* receive type */,

0 /* send buffer is stored on this rank */, MPI_COMM_WORLD /* communication channel */);

// This is the actual working-loop

long sub_sum = 0;

for(int i=0; i < items_per_rank; i++)

sub_sum += my_work[i];

if(my_rank == 0) { // Scatter cannot deal with a division remainder so we manually deal with it

while(remainder_items > 0)

sub_sum += array[--remainder_items];

}

MPI_Free_mem(my_work);

// MPI_Reduce with op-code MPI_SUM is a collective operation which sums up the input sub_sum of each node

// into single a resulting output sum on the master.

MPI_Reduce(&sub_sum /* input to sum up */, &sum /* output */, 1 /* input count */, MPI_LONG /* input type */,

MPI_SUM /* operation */, 0 /* output is stored on this rank */, MPI_COMM_WORLD /* communication channel */);

if(my_rank == 0) {

// The result of the computation now is available on rank 0.

// We compare it with the sequential reference implementation to test our parallel implementation.

if(sum == sum__sequential_reference_implementation())

fprintf(stderr, "Test OK.\n");

else

fprintf(stderr, "Test FAILED!\n");

}

MPI_Barrier(MPI_COMM_WORLD);

MPI_Finalize();

return EXIT_SUCCESS;

}