Use environment variables to control OpenMP behavior in celer-sim (ce…

…leritas-project#1073)

app/celer-sim/Runner.cc

@@ -14,6 +14,10 @@
 #include <utility>
 #include <vector>
 
+#ifdef _OPENMP
+#    include <omp.h>
+#endif
+
 #include "corecel/cont/Span.hh"
 #include "corecel/io/Logger.hh"
 #include "corecel/io/OutputRegistry.hh"
@@ -73,29 +77,27 @@ namespace
 {
 //---------------------------------------------------------------------------//
 /*!
- * Get the number of streams from the OMP_NUM_THREADS environment variable.
- *
- * The value of OMP_NUM_THREADS should be a list of positive integers, each of
- * which sets the number of threads for the parallel region at the
- * corresponding nested level. The number of streams is set to the first value
- * in the list.
+ * Get the number of streams from the number of OpenMP threads.
  *
- * \note For a multithreaded CPU run, if OMP_NUM_THREADS is set to a single
- * value, the number of threads for each nested parallel region will be set to
- * that value.
+ * The OMP_NUM_THREADS environment variable can be used to control the number
+ * of threads/streams. The value of OMP_NUM_THREADS should be a list of
+ * positive integers, each of which sets the number of threads for the parallel
+ * region at the corresponding nested level. The number of streams is set to
+ * the first value in the list. If OMP_NUM_THREADS is not set, the value will
+ * be implementation defined.
  */
 size_type calc_num_streams(RunnerInput const& inp, size_type num_events)
 {
     size_type num_threads = 1;
 #if CELERITAS_USE_OPENMP
     if (!inp.merge_events)
     {
-        std::string const& nt_str = celeritas::getenv("OMP_NUM_THREADS");
-        if (!nt_str.empty())
+#    pragma omp parallel
         {
-            auto nt = std::stoi(nt_str);
-            CELER_VALIDATE(nt > 0, << "nonpositive num_streams=" << nt);
-            num_threads = static_cast<size_type>(nt);
+            if (omp_get_thread_num() == 0)
+            {
+                num_threads = omp_get_num_threads();
+            }
         }
     }
 #else

app/celer-sim/celer-sim.cc

@@ -126,13 +126,7 @@ void run(std::istream* is, std::shared_ptr<OutputRegistry> output)
                           << " on " << num_streams << " threads";
         MultiExceptionHandler capture_exception;
 #ifdef _OPENMP
-        // Set the maximum number of nested parallel regions
-        // TODO: Enable nested OpenMP parallel regions for multithreaded CPU
-        // once the performance issues have been resolved. For now, limit the
-        // level of nesting to a single parallel region (over events) and
-        // deactivate any deeper nested parallel regions.
-        omp_set_max_active_levels(1);
-#    pragma omp parallel for num_threads(num_streams)
+#    pragma omp parallel for
 #endif
         for (size_type event = 0; event < run_stream.num_events(); ++event)
         {