This post talks about Reduction operation in MPI using MPI_Reduce. In reduction operation, each process contributes its local data, and these values are aggregated according to the specified operation such as summation, finding the maximum, or custom-defined functions.

Syntax for MPI_Reduce

int MPI_Reduce(const void *sendbuf, void *recvbuf, int count, MPI_Datatype datatype, MPI_Op op, int root, MPI_Comm comm)

Input Parameters

• sendbuf : Pointer to the data/array to be sent (for example : int *mydata)
• count : number of elements/values to be sent (for example : 10)
• datatype : data type of elements/values to be sent (for example : MPI_INT)
• op : reduction operation to be performed (for example – MPI_SUM)
• root : rank of root process (for example : 0)
• comm : communicator (for example : MPI_COMM_WORLD)

Output Parameters

• recvbuf : Pointer to the data/array to be received (for example : int *myarr)

Example code –

#include"stdio.h"
#include"mpi.h"

int main(int argc, char **argv)
{
	int myid, size;
	int sum;
	
	sum = 100;
	 
	MPI_Init(&argc,&argv);
	MPI_Comm_size(MPI_COMM_WORLD, &size);
	MPI_Comm_rank(MPI_COMM_WORLD, &myid);

        MPI_Reduce(&myid, &sum, 1, MPI_INT, MPI_SUM, 0, MPI_COMM_WORLD);	
	
	if(myid==0)
	{
	    printf("\n sum = %d\n",sum);
	}
	
        MPI_Finalize();	
}	

To compile this code, use following command –

mpicc mpi_reduce.c

To execute this code, run following command –

mpiexec -n 4 ./a.out

Output of this code will be something similar to the following –

 sum = 6

To know more about MPI, visit our dedicated page for MPI here.

If you are new to Parallel Programming / HPC, visit our dedicated page for beginners.

References :

• MPI Standard Website
• MPI_Reduce Syntax and other information