How to parallelize the below for loop - c

Question

I am trying to integrate a function of curve, and convert the serial code to parallel program, I am using openMP for the same.
I have parallelized the for loop using openMP parallel for and have achieved lesser program time, but the problem is the result is not the expected one, there is something which get messed up in the threads, I want to know how to parallelize the for loop for N number of threads.
#include <stdio.h>
#include <omp.h>
#include <math.h>
double f(double x){
return sin(x)+0.5*x;
}
int main(){
int n=134217728,i;
double a=0,b=9,h,x,sum=0,integral;
double start = omp_get_wtime();
h=fabs(b-a)/n;
omp_set_dynamic(0);
omp_set_num_threads(64);
#pragma omp parallel for reduction (+:sum) shared(x)
for(i=1;i<n;i++){
x=a+i*h;
sum=sum+f(x);
}
integral=(h/2)*(f(a)+f(b)+2*sum);
double end = omp_get_wtime();
double time = end - start;
printf("Execution time: %2.3f seconds\n",time);
printf("\nThe integral is: %lf\n",integral);
}
The expected output is 22.161130 but it is getting varied each time the program is ran.

Answer 1

The loop you are trying to parallelise modifies the same variables x and sum in each iteration, this is very cumbersome to parallelize.
You could rewrite the code to make the path to parallelisation more obvious:
#include <stdio.h>
#include <omp.h>
#include <math.h>
double f(double x) {
return sin(x) + 0.5 * x;
}
int main() {
int n = 1 << 27, i, j;
double a = 0, b = 9, h, x, sum, integral;
double sums[64] = { 0 };
double start = omp_get_wtime();
h = fabs(b - a) / n;
omp_set_dynamic(0);
omp_set_num_threads(64);
#pragma omp parallel for
for (j = 0; j < 64; j++) {
for (i = 0; i < n; i += 64) {
sums[j] += f(a + i * h + j * h);
}
}
sum = 0;
for (j = 0; j < 64; j++) {
sum += sums[i];
}
integral = (h / 2) * (f(a) + f(b) + 2 * sum);
double end = omp_get_wtime();
double time = end - start;
printf("Execution time: %2.3f seconds\n", time);
printf("\nThe integral is: %lf\n", integral);
return 0;
}