parallel for and threadpools

Hi,

if I use an openMP parallel for construct like

Code:

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#pragma omp parallel for num_threads(4)
for(int i=0; i<10; ++i){}

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when are the 4 threads created, after the pragma and therefore eventually very often during process live time or is the run time clever enough to create a ready to use thread pool prior process start up?

Thanks in advance!

That depends on the OMP implementation. I would say that all implementations keep a thread pool handy, since launching and stopping threads is expensive.