CPU core oversubscription warning on macOS with OpenMP backend

[keichi]

I get the following warning when running a minimal Kokkos program with the OpenMP backend:

Kokkos::OpenMP::initialize WARNING: You are likely oversubscribing your CPU cores.
                                    Detected: -1 cores per node.
                                    Detected: 1 MPI_ranks per node.
                                    Requested: 8 threads per process.

• Kokkos 3.1.00
• GCC 9.3.0
• macOS 10.15.4 (19E266)

#include <Kokkos_Core.hpp>

int main(int argc, char* argv[])
{
    Kokkos::initialize(argc, argv);
    Kokkos::finalize();
    return 0;
}

[keichi]

It seems Impl:: processors_per_node() is hardcoded to return -1 on macOS for some reason.

kokkos/core/src/impl/Kokkos_CPUDiscovery.cpp

Lines 48 to 50 in 785d19f

	#elif !defined(__APPLE__)
	#include <unistd.h>
	#endif

[ndellingwood]

Cross reference #2477 (comment)

[dalg24]

@keichi the fix is in develop

[keichi]

Thanks!