Parallelizing a series of independent sequential lines of code

Question

What is the best way to execute multiple lines of code in parallel if they are not dependent of each other? (I'm using OpenMP)

Pseudo code:

database->connect()
openfile("stuff.txt")
ping("stackoverflow.com")
x = 2;
y = a + b;

The only way I can come up with is:

#pragma omp parallel for
for(i = 0; i < 5; i++)
switch (i) {
case 0: database->connect(); break;
...

I haven't tried it, but I also remember that you're not supposed to break while using OpenMP

Answer 1

So I'm assuming that the indivdual things you listed as independant tasks were just examples. If they really are things like y=a+b, then as @chrisaycock and @ejd have said, they're too small for this sort of parallelism (eg thread based, as opposed to ILP or something) to actually take advantage of the concurrency due to overheads. But if they are bigger operations, the way to do task-based parallelism in OpenMP is with the task directive: eg,

#include <stdio.h>
#include <omp.h>
#include <unistd.h>

void work(int *v) {
    *v = omp_get_thread_num();
    sleep(1);
}

int main(int argc, char **argv)
{
    int a, b, c;
    #pragma omp parallel
    {
        #pragma omp single
        {
            #pragma omp task shared(a) default(none)
            work(&a);

            #pragma omp task shared(b) default(none)
            work(&b);

            #pragma omp task shared(c) default(none)
            work(&c);
        }
    }
    printf("a,b,c = %d,%d,%d\n", a, b, c);

    return 0;
}