Коллективная

#include <iostream>
#include "mpi.h"

int main(int argc, char* argv[]) {
    MPI_Init(&argc, &argv);

    double starttime, endtime;
    int world_rank, count_ranks, total;
    MPI_Comm_size(MPI_COMM_WORLD, &count_ranks);
    MPI_Comm_rank(MPI_COMM_WORLD, &world_rank);
    /*
    // scan
    starttime = MPI_Wtime();

    MPI_Scan(&world_rank, &total, 1, MPI_INT, MPI_SUM, MPI_COMM_WORLD);

    endtime = MPI_Wtime();

    printf("[MPI process %d library] Total = %d.\n", world_rank, total);
    printf("[MPI process %d library] Time = %g ms\n", world_rank, (endtime - starttime));
    */
    // custom
    starttime = MPI_Wtime();

    if (world_rank == 0) {
        total = world_rank;
        MPI_Send(&total, 1, MPI_INT, 1, 0, MPI_COMM_WORLD);
    }
    else if (world_rank < count_ranks - 1) {
        MPI_Recv(&total, 1, MPI_INT, (world_rank - 1), 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
        total += world_rank;
        MPI_Send(&total, 1, MPI_INT, (world_rank + 1), 0, MPI_COMM_WORLD);
    }
    else {
        MPI_Recv(&total, 1, MPI_INT, (world_rank - 1), 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
        total += world_rank;
    }

    endtime = MPI_Wtime();

    printf("[MPI process %d] Total = %d.\n", world_rank, total);
    printf("[MPI process %d] Time = %g ms\n", world_rank, (endtime - starttime));

    MPI_Finalize();
    return 0;
}