[OpenMP] Solution 6 : Histogram

This material is part of the course Practical Introduction to Parallel Programming and meant for educational purposes only.

Task

Write a program that generates a sequence of random numbers between \(0\) and \(N\) and counts how many times the integer part of that number equals \(0, 1, \dots, N\). This process is known as generating a histogram.

Implement two version, one with atomic the other with critical section, and compare the performance of both implementations for different \(N\) and a large number of sampling points.

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Câu hỏi 1: Histogram

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

#ifdef _OPENMP
#include <omp.h>
#else
#define omp_get_num_threads() 1
#define omp_get_max_threads() 1
#define omp_get_thread_num()  0
#endif

int main ( int argc, char** argv )
{
    const int N=1000;
    const int M=10;
    int hist[M];
    long int sum;

    double t0 = omp_get_wtime();

#pragma omp parallel shared(hist)
    {

        /* Initialize random number generator */
        unsigned int seed = omp_get_thread_num();

        /* Initialization */
#pragma omp for
        for (int i=0; i<M; i++)
            hist[i] = 0;

        /* Generate random numbers */
#pragma omp for
        for (int i=0; i<N; i++)
            {
                double r = rand_r(&seed) % M;

#pragma omp atomic
                hist[(int)r]++;
            }

        /* Count sum of hist (to check consistency) */
#pragma omp for reduction(+:sum)
        for (int i=0; i<M; i++)
            sum += hist[i];
    }

    double t1 = omp_get_wtime();

    /* Output */
    for (int i=0; i<M; i++)
        printf ( "hist[%3d] = %d\n", i, hist[i] );

    printf ( "sum       = %ld\n", sum );
    printf ( "in %g seconds\n", t1-t0 );

  return 0;
}

Câu hỏi 2: OpenMP environment variables

Specify the OpenMP environment variables that should be used for running your program (export OMP_NUM_THREADS=1 ...). Use semicolons to separate multiple variables

Câu hỏi 3: Command Line Argument

Specify the command line arguments that should be passed to your program when it is run. Arguments must be given in quotes and separated by commas. Leave this filed empty if you do not want to specify command line arguments.

Example: "1","int","arg=2" is interpreted as three arguments 1, int, and arg=2.

Tác giả	Erik-Jan Lingen, Matthias Möller
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