OmpSystem.cpp

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*/

#include <algorithm>

#include "CvarSystem.h"
#include "OmpSystem.h"

#if defined(_OPENMP)
#include "omp.h"
#endif

#if defined(_OPENMP)
static Cvar::Range<Cvar::Cvar<int>> common_ompThreads(
	"common.ompThreads", "OpenMP threads", Cvar::NONE, 0, 0, 32 );
#endif

namespace Omp {
#if defined(_OPENMP)
static int ompMaxThreads = 1;
#endif

static int ompThreads = 1;

static void ReadMaxThreads()
{
#if defined(_OPENMP)
	ompMaxThreads = omp_get_max_threads();
#endif
}

void EnlistThreads()
{
#if defined(_OPENMP)
	omp_set_num_threads( ompThreads );
#endif
}

void SetupThreads()
{
#if defined(_OPENMP)
	if ( common_ompThreads.Get() )
	{
		ompThreads = common_ompThreads.Get();
	}
	else
	{
		/* Only GCC and Clang on Linux and MinGW have been tested for performance yet,
		We currently not enable OpenMP in releases on other systems yet. */
		#if defined(__MINGW32__)
		/* On Windows with a MinGW build, using all available threads is known to
		work best with:

		- 4 threads on low-end 4-cores (no SMT) Ryzen 3 3200G.

		It has not been tested yet on a Windows system where ompMaxThreads > 4.

		At least on Wine over Linux, using a few amount of threads is known to work
		best with:

		- 4 threads on high-end 16-cores/32-threads Ryzen Threadripper PRO 3955WX.

		Performance regression has been observed above 4 threads on Wine on Linux. */
		constexpr int threadDivisor = 1;
		constexpr int threadCap = 4;
		#else
		/* On Linux, using half threads than available is known to work best with:

		- 2 threads on low-end 4-core (no SMT) Ryzen 3 3200G,
		- 16 threads on high-end 16-cores/32-threads Ryzen Threadripper PRO 3955WX.

		It has not been tested yet on a system where ompMaxThreads / 2 > 16. */
		constexpr int threadDivisor = 2;
		constexpr int threadCap = 16;
		#endif

		switch( ompMaxThreads )
		{
			case 1:
				ompThreads = 1;
				break;
			case 2:
			case 3:
				ompThreads = 2;
				break;
			default:
				ompThreads = std::min( ompMaxThreads / threadDivisor, threadCap );
		}
	}
#endif

	EnlistThreads();
}

void Init()
{
	ReadMaxThreads();
	SetupThreads();
}

int GetThreads()
{
	return ompThreads;
}
}