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