How to get 100% CPU usage from a C program

Question

This is quite an interesting question so let me set the scene. I work at The National Museum of Computing, and we have just managed to get a Cray Y-MP EL super computer from

Answer 1

If you want 100% CPU, you need to use more than 1 core. To do that, you need multiple threads.

Here's a parallel version using OpenMP:

I had to increase the limit to 1000000 to make it take more than 1 second on my machine.

#include 
#include 
#include 

int main() {
    double start, end;
    double runTime;
    start = omp_get_wtime();
    int num = 1,primes = 0;

    int limit = 1000000;

#pragma omp parallel for schedule(dynamic) reduction(+ : primes)
    for (num = 1; num <= limit; num++) { 
        int i = 2; 
        while(i <= num) { 
            if(num % i == 0)
                break;
            i++; 
        }
        if(i == num)
            primes++;
//      printf("%d prime numbers calculated\n",primes);
    }

    end = omp_get_wtime();
    runTime = end - start;
    printf("This machine calculated all %d prime numbers under %d in %g seconds\n",primes,limit,runTime);

    return 0;
}

Output:

This machine calculated all 78498 prime numbers under 1000000 in 29.753 seconds

Here's your 100% CPU: