gatherv/scatterv

gistfile1.c

#include <mpi.h>
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <omp.h>
#include <time.h>

#define N 10

int main(int argc, char** argv) {
  srand(time(NULL));

  // Initialize the MPI environment
  MPI_Init(NULL, NULL);
  // Find out rank, size
  int world_rank;
  MPI_Comm_rank(MPI_COMM_WORLD, &world_rank);
  int world_size;
  MPI_Comm_size(MPI_COMM_WORLD, &world_size);

  // We are assuming at least 2 processes for this task
  if (world_size < 2) {
    fprintf(stderr, "World size must be greater than 1 for %s\n", argv[0]);
    MPI_Abort(MPI_COMM_WORLD, 1);
  }

  int elements_per_proc = N / world_size;
  int difference = N - elements_per_proc * world_size;
  int* chunk_sizes = (int*)(malloc(sizeof(int) * world_size));
  int* displ = (int*)(malloc(sizeof(int) * world_size));

  for(int i = 0; i < world_size; ++i)
  {
    chunk_sizes[i] = elements_per_proc;
    displ[i] = i * elements_per_proc;
  }
  chunk_sizes[world_size - 1] += difference;

  float* rand_nums, *rand_nums_new;

  if (world_rank == 0) {
    rand_nums = (float*)malloc(sizeof(double) * N);
    rand_nums_new = (float*)malloc(sizeof(double) * N);

    for(int i = 0; i < N; ++i)
    {
      rand_nums[i] = rand() % 100 - 50;
      printf("%lf\n", rand_nums[i]);
    }

    printf("\n");
  }

  int current_recv_size = elements_per_proc + (world_rank == world_size - 1 ? difference : 0);
  // Create a buffer that will hold a subset of the random numbers
  float *sub_rand_nums = malloc(sizeof(float) * current_recv_size);

  // Scatter the random numbers to all processes
  MPI_Scatterv(rand_nums, chunk_sizes, displ, MPI_FLOAT, sub_rand_nums,
              current_recv_size, MPI_FLOAT, 0, MPI_COMM_WORLD);

  // printf("world_rank %d\n", world_rank);
  // for(int i = 0; i < elements_per_proc; ++i)
  // {
  //   printf("%lf\n", sub_rand_nums[i]);
  // }
  // printf("\n");

  for(int i = 0; i < current_recv_size; ++i)
  {
    if((elements_per_proc * world_rank + i) % 2 == 1)
    {
      sub_rand_nums[i] = 0;
    }
  }
  printf("\n");

  // printf("world_rank %d\n", world_rank);
  // for(int i = 0; i < elements_per_proc; ++i)
  // {
  //   printf("%lf\n", sub_rand_nums[i]);
  // }
  // printf("\n");

  MPI_Gatherv(sub_rand_nums, current_recv_size, MPI_FLOAT, rand_nums_new, chunk_sizes, displ, MPI_FLOAT, 0, MPI_COMM_WORLD);

  if (world_rank == 0) {
    for(int i = 0; i < N; ++i)
    {
      printf("%lf -> %lf\n", rand_nums[i], rand_nums_new[i]);
    }

    printf("\n");
  }

  MPI_Finalize();
}