Unify thread configuration. (#6186)

src/common/threading_utils.h

@@ -144,6 +144,22 @@ void ParallelFor(size_t size, size_t nthreads, Func fn) {
   omp_exc.Rethrow();
 }
 
+/* \brief Configure parallel threads.
+ *
+ * \param p_threads Number of threads, when it's less than or equal to 0, this function
+ *        will change it to number of process on system.
+ *
+ * \return Global openmp max threads before configuration.
+ */
+inline int32_t OmpSetNumThreads(int32_t* p_threads) {
+  auto& threads = *p_threads;
+  int32_t nthread_original = omp_get_max_threads();
+  if (threads <= 0) {
+    threads = omp_get_num_procs();
+  }
+  omp_set_num_threads(threads);
+  return nthread_original;
+}
 }  // namespace common
 }  // namespace xgboost
 

src/data/data.cc

@@ -19,6 +19,7 @@
 #include "../common/math.h"
 #include "../common/version.h"
 #include "../common/group_data.h"
+#include "../common/threading_utils.h"
 #include "../data/adapter.h"
 #include "../data/iterative_device_dmatrix.h"
 
@@ -843,10 +844,7 @@ void SparsePage::Push(const SparsePage &batch) {
 template <typename AdapterBatchT>
 uint64_t SparsePage::Push(const AdapterBatchT& batch, float missing, int nthread) {
   // Set number of threads but keep old value so we can reset it after
-  const int nthreadmax = omp_get_max_threads();
-  if (nthread <= 0) nthread = nthreadmax;
-  const int nthread_original = omp_get_max_threads();
-  omp_set_num_threads(nthread);
+  int nthread_original = common::OmpSetNumThreads(&nthread);
   auto& offset_vec = offset.HostVector();
   auto& data_vec = data.HostVector();
 
@@ -865,7 +863,7 @@ uint64_t SparsePage::Push(const AdapterBatchT& batch, float missing, int nthread
     }
   }
   size_t batch_size = batch.Size();
-  const size_t thread_size = batch_size/nthread;
+  const size_t thread_size = batch_size / nthread;
   builder.InitBudget(expected_rows+1, nthread);
   uint64_t max_columns = 0;
   if (batch_size == 0) {

src/data/simple_dmatrix.cc

@@ -15,6 +15,7 @@
 #include "simple_dmatrix.h"
 #include "./simple_batch_iterator.h"
 #include "../common/random.h"
+#include "../common/threading_utils.h"
 #include "adapter.h"
 
 namespace xgboost {
@@ -92,10 +93,7 @@ BatchSet<EllpackPage> SimpleDMatrix::GetEllpackBatches(const BatchParam& param)
 template <typename AdapterT>
 SimpleDMatrix::SimpleDMatrix(AdapterT* adapter, float missing, int nthread) {
   // Set number of threads but keep old value so we can reset it after
-  const int nthreadmax = omp_get_max_threads();
-  if (nthread <= 0) nthread = nthreadmax;
-  int nthread_original = omp_get_max_threads();
-  omp_set_num_threads(nthread);
+  int nthread_original = common::OmpSetNumThreads(&nthread);
 
   std::vector<uint64_t> qids;
   uint64_t default_max = std::numeric_limits<uint64_t>::max();

src/learner.cc

@@ -43,6 +43,7 @@
 #include "common/timer.h"
 #include "common/charconv.h"
 #include "common/version.h"
+#include "common/threading_utils.h"
 
 namespace {
 
@@ -287,9 +288,7 @@ class LearnerConfiguration : public Learner {
     generic_parameters_.CheckDeprecated();
 
     ConsoleLogger::Configure(args);
-    if (generic_parameters_.nthread != 0) {
-      omp_set_num_threads(generic_parameters_.nthread);
-    }
+    common::OmpSetNumThreads(&generic_parameters_.nthread);
 
     // add additional parameters
     // These are cosntraints that need to be satisfied.

tests/cpp/common/test_threading_utils.cc

@@ -88,6 +88,22 @@ TEST(ParallelFor2dNonUniform, Test) {
 
   omp_set_num_threads(old);
 }
+#if defined(_OPENMP)
+TEST(OmpSetNumThreads, Basic) {
+  auto nthreads = 2;
+  auto orgi = OmpSetNumThreads(&nthreads);
+  ASSERT_EQ(omp_get_max_threads(), 2);
+  nthreads = 0;
+  OmpSetNumThreads(&nthreads);
+  ASSERT_EQ(omp_get_max_threads(), omp_get_num_procs());
+  nthreads = 1;
+  OmpSetNumThreads(&nthreads);
+  nthreads = 0;
+  OmpSetNumThreads(&nthreads);
+  ASSERT_EQ(omp_get_max_threads(), omp_get_num_procs());
 
+  omp_set_num_threads(orgi);
+}
+#endif  // defined(_OPENMP)
 }  // namespace common
 }  // namespace xgboost