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mpi_reduce_allreduce.cpp
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mpi_reduce_allreduce.cpp
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// Author: Wes Kendall
// Copyright 2013 www.mpitutorial.com
// This code is provided freely with the tutorials on mpitutorial.com. Feel
// free to modify it for your own use. Any distribution of the code must
// either provide a link to www.mpitutorial.com or keep this header intact.
//
// Program that computes the standard deviation of an array of elements in parallel using
// MPI_Reduce.
//
#include <stdio.h>
#include <stdlib.h>
#include <mpi.h>
#include <math.h>
#include <assert.h>
// Creates an array of random numbers. Each number has a value from 0 - 1
float *create_rand_nums(int num_elements) {
float *rand_nums = (float *)malloc(sizeof(float) * num_elements);
assert(rand_nums != NULL);
int i;
for (i = 0; i < num_elements; i++) {
rand_nums[i] = (rand() / (float)RAND_MAX);
}
return rand_nums;
}
int main(int argc, char** argv) {
if (argc != 2) {
fprintf(stderr, "Usage: avg num_elements_per_proc\n");
exit(1);
}
int num_elements_per_proc = atoi(argv[1]);
MPI_Init(NULL, NULL);
int world_rank;
MPI_Comm_rank(MPI_COMM_WORLD, &world_rank);
int world_size;
MPI_Comm_size(MPI_COMM_WORLD, &world_size);
// Create a random array of elements on all processes.
srand(time(NULL)*world_rank); // Seed the random number generator of processes uniquely
float *rand_nums = NULL;
rand_nums = create_rand_nums(num_elements_per_proc);
// Sum the numbers locally
float local_sum = 0;
int i;
for (i = 0; i < num_elements_per_proc; i++) {
local_sum += rand_nums[i];
}
// Reduce all of the local sums into the global sum in order to
// calculate the mean
float global_sum;
MPI_Allreduce(&local_sum, &global_sum, 1, MPI_FLOAT, MPI_SUM,
MPI_COMM_WORLD);
float mean = global_sum / (num_elements_per_proc * world_size);
// Compute the local sum of the squared differences from the mean
float local_sq_diff = 0;
for (i = 0; i < num_elements_per_proc; i++) {
local_sq_diff += (rand_nums[i] - mean) * (rand_nums[i] - mean);
}
// Reduce the global sum of the squared differences to the root process
// and print off the answer
float global_sq_diff;
MPI_Reduce(&local_sq_diff, &global_sq_diff, 1, MPI_FLOAT, MPI_SUM, 0,
MPI_COMM_WORLD);
// The standard deviation is the square root of the mean of the squared
// differences.
if (world_rank == 0) {
float stddev = sqrt(global_sq_diff /
(num_elements_per_proc * world_size));
printf("Mean - %f, Standard deviation = %f\n", mean, stddev);
}
// Clean up
free(rand_nums);
MPI_Barrier(MPI_COMM_WORLD);
MPI_Finalize();
}