OpenMP: parallel regions and loop parallelism

Overview

Teaching: 0 min
Exercises: 0 min

Questions

• Do we have to manually do everything?

Objectives

• Know how to use parallel for pragma

Loop parallelism

• Very common type of parallelism in scientific code
• In previous trapezoid example, we calculate the division of iteration manually.
• An alternative is to use parallel for pragma

Hands-on 1: Sum series implementation

• In the EXPLORER window, right-click on csc466/openmp and select New File.
• Type sum_series_for.c as the file name and hits Enter.
• Enter the following source code in the editor windows:
• Save the file when you are done:
  • Ctrl-S for Windows/Linux
  • Command-S for Macs
• Memorize your key-combos!.

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

int main (int argc, char *argv[]) {
  int N = atoi(argv[1]);
  int nthreads = atoi(argv[2]);
  int partial_sum[nthreads];
  clock_t start, end;

  omp_set_num_threads(nthreads);
  start = clock();
  #pragma omp parallel
  {
    int tid = omp_get_thread_num();
    partial_sum[tid] = 0;
    #pragma omp for
    for (int i = 0; i <  N; i++) {
      partial_sum[tid] += i;
    }
  }
  end = clock();

  int sum = 0;
  for (int i = 0; i < nthreads; i++) {
    sum += partial_sum[i];
  }
  printf("The integral is: %d\n", sum);
  printf("The run time is: %.4f\n", ((double) (end - start)) / CLOCKS_PER_SEC);
  return 0;
}

Hands-on 2: Improving sum series implementation

• In the EXPLORER window, right-click on csc466/openmp and select New File.
• Type sum_series_for_2.c as the file name and hits Enter.
• Enter the following source code in the editor windows:
• Save the file when you are done:
  • Ctrl-S for Windows/Linux
  • Command-S for Macs
• Memorize your key-combos!.

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

int main (int argc, char *argv[]) {
  int N = atoi(argv[1]);
  int nthreads = atoi(argv[2]);
  int partial_sum[nthreads];
  clock_t start, end;

  omp_set_num_threads(nthreads);
  start = clock();
  #pragma omp parallel
  {
    int tid = omp_get_thread_num();
    partial_sum[tid] = 0;
    int psum = 0;
    #pragma omp for
    for (int i = 0; i <  N; i++) {
      psum += i;
    }
    partial_sum[tid] = psum;
  }
  end = clock();

  int sum = 0;
  for (int i = 0; i < nthreads; i++) {
    sum += partial_sum[i];
  }
  printf("The integral is: %d\n", sum);
  printf("The run time is: %.4f\n", ((double) (end - start)) / CLOCKS_PER_SEC);
  return 0;
}

Key Points

• Parallel for allows simplification of code