Why is OpenMP outperforming threads?

Question

I\'ve been calling this in OpenMP

#pragma omp parallel for num_threads(totalThreads)
for(unsigned i=0; i

        

Answer 1

Where does totalThreads come from in your OpenMP version? I bet it's not startIndex.size().

The OpenMP version queues the requests onto totalThreads worker threads. It looks like the C++11 version creates, startIndex.size() threads, which involves a ridiculous amount of overhead if that's a big number.

Answer 2

Consider the following code. The OpenMP version runs in 0 seconds while the C++11 version runs in 50 seconds. This is not due to the function being doNothing, and it's not due to vector being within the loop. As you can imagine, the c++11 threads are created and then destroyed in each iteration. On the other hand, OpenMP actually implements threadpools. It's not in the standard, but it's in Intel's and AMD's implementations.

for(int j=1; j<100000; ++j)
{
    if(algorithmToRun == 1)
    {
        vector<thread> threads;
        for(int i=0; i<16; i++)
        {
            threads.push_back(thread(doNothing));
        }
        for(auto& thread : threads) thread.join();
    }
    else if(algorithmToRun == 2)
    {
        #pragma omp parallel for num_threads(16)
        for(unsigned i=0; i<16; i++)
        {
            doNothing();
        }
    }
}