How are number of cores defined in OpenMP, how does this API know about the existence of cores.
How does OpenMP determine the Number of cores in the system?
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OpenMP defines an Internal Control Variable (ICV) nthreads-var, which basically controls the number of threads of a parallel region.
The initial value of nthreads-var (if not defined by the environment Variable
OMP_NUM_THREADS), is implementation defined. The number of cores might seem like a sensible default. GCCs (libgomp) implements it for BSD, Linux, mingw32, POSIX, rtems ingomp_init_num_threads. You can check out the specific implementations underlibgomp/config/<platform>/proc.c.Linux checks the thread affinity via
pthread_getaffinity_npand sets the number of threads to the number of CPUs the process is allowed to run on. MinGW similarly viaGetProcessAffinityMask. The remaining platforms basically usesysconf(_SC_NPROCESSORS_ONLN).