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How does the OpenMP runtime determine the best number of threads when omp_set_dynamic is used?
e.g. Are some sort of timing mechanisms used or does the compiler give hints to the runtime of how large the task size is?
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omp_get_num_threads() The omp_get_num_threads function returns the number of threads in the team currently executing the parallel region from which it is called.
omp_set_num_threads. Sets the number of threads in upcoming parallel regions, unless overridden by a num_threads clause.
Enables or disables dynamic adjustment of the number of threads available for execution of parallel regions.
omp_get_num_threads. Returns the number of threads in the parallel region. omp_get_max_threads. Returns an integer that is equal to or greater than the number of threads that would be available if a parallel region without num_threads were defined at that point in the code. omp_get_thread_num.
I don't think that the OpenMP does determine the 'best' number of threads for an application, in any likely sense of the word 'best'. As @aaa has commented, the runtime's behaviour when omp_set_dynamic is true is implementation specific.
I don't think that current Fortran/C/C++ compilers could provide information such as timings or task sizes to the runtime.
I believe that this function is available so that schedulers (and similar) can manage programs on machines, for throughput or similar.
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