我应该在openMP并行区域内使用gnu并行模式函数（for-loop，tasks）

Question

I have a program accelerated by openMP , inside the parallel region, functions like std::nth_element , std::sort , std::partition are called. actually, these functions are used to process each openmp-thread's corresponding part of an array.

recently, I found g++ had implemented parallel version of above functions, So I wonder should I use function like __gnu_parallel::nth_element inside #pragma omp task or #pragma omp for region? if I used the parallel mode, would the total threads exceed the limit set by omp_set_num_threads() and lead to worse speedup?

Answer 1

Trivial (and best) answer: Benchmark and post your findings.

Less definitive: In my experience, the parallel versions of most algorithms are less efficient than the comparable serial ones, instead relying on multiple parallel processors to compensate in wall time. Regarding the number of threads, I don't think that OMP will spawn new threads if at the limit. I do remember that embedded #pragma omp for regions don't actually result in each of the outer threads spawning more "inner threads" without a specific flag (which I don't remember off the top of my head).