Make 2 threads default and fix -1 for automatic

opm/models/discretization/common/fvbaseparameters.hh

@@ -125,7 +125,13 @@ struct MinTimeStepSize { static constexpr Scalar value = 0.0; };
 struct OutputDir { static constexpr auto value = ""; };
 
 //! \brief Number of threads per process.
-struct ThreadsPerProcess { static constexpr int value = 1; };
+struct ThreadsPerProcess {
+#if _OPENMP
+  static constexpr int value = 2;
+#else
+  static constexpr int value = 1;
+#endif // _OPENMP
+};
 
 } // namespace Opm::Parameters
 

opm/simulators/flow/BlackoilModelParameters.cpp

@@ -261,9 +261,9 @@ void BlackoilModelParameters<Scalar>::registerParameters()
 
     Parameters::Hide<Parameters::DebugEmitCellPartition>();
 
-    // if openMP is available, determine the number threads per process automatically.
+    // if openMP is available, use two threads per mpi rank by default
 #if _OPENMP
-    Parameters::SetDefault<Parameters::ThreadsPerProcess>(-1);
+    Parameters::SetDefault<Parameters::ThreadsPerProcess>(2);
 #endif
 }
 

opm/simulators/flow/FlowMain.hpp

@@ -310,10 +310,10 @@ namespace Opm {
                 }
             }
 
-            // We are not limiting this to the number of processes
-            // reported by OpenMP as on some hardware (and some OpenMPI
-            // versions) this will be 1 when run with mpirun
-            omp_set_num_threads(threads);
+            // Requesting -1 thread will let OMP automatically deduce the number
+            if (requested_threads != -1) {
+                omp_set_num_threads(threads);
+            }
 #endif
 
             using TM = GetPropType<TypeTag, Properties::ThreadManager>;