warning #2901: [omp] OpenMP is not active; all OpenMP directives will be ignored

In order to get OpenMP support you need to explicitly tell your compiler.

• g++, gcc and clang need the option -fopenmp
• mvsc needs the option /openmp (more info here if you use visual studio)

Aside from the obvious having to compile with -fopenmp flag your code has some problem worth pointing out, namely:

To measure time use omp_get_wtime() instead of clock() (it will give you the number of clock ticks accumulated across all threads).

The other problem is:

#pragma omp parallel num_threads(4) #pragma omp parallel for for (int i = 0; i<pow(10,7);i++) { something=sqrt(123456); } 

the iterations of the loop are not being assigned to threads as you wanted. Because you have added again the clause parallel to #pragma omp for, and assuming that you have nested parallelism disabled, which by default it is, each of the threads created in the outer parallel region will execute "sequentially" the code within that region. Consequently, for a n = 6 (i.e., pow(10,7) = 6) and number of threads = 4, you would have the following block of code:

for (int i=0; i<n; i++) { something=sqrt(123456); } 

being executed 6 x 4 = 24 times (i.e., the total number of loop iterations multiple by the total number of threads). For a more in depth explanation check this SO Thread about a similar issue. Nevertheless, the image below provides a visualization of the essential:

To fix this adapt your code to the following:

#pragma omp parallel for num_threads(4) for (int i = 0; i<pow(10,7);i++) { something=sqrt(123456); }