Reasons for omp_set_num_threads(1) slower than no openmp

Question

I believe everyone agree with the title of this post. Can someone point me the reason ? Any reference to that like book etc ? I have tried to find but no luck.

I believe

Answer 1

As Mystical explained, it's likely due to the OpenMP overhead. I have tried to get around this by doing for example:

#pragma omp parallel for if(nthreads>1)

I thought this would only use the OpenMP overhead if nthreads>1. However, at least in Visual Studio 2012, this also has significant overhead. Therefore, in order to properly compare single threaded and multi-threaded version of a function I define two versions of the functions with and without the OpenMP pragmas.