Limit the maximum number of threads. (#10872)

src/common/threading_utils.cc

@@ -3,7 +3,7 @@
  */
 #include "threading_utils.h"
 
-#include <algorithm>   // for max
+#include <algorithm>   // for max, min
 #include <exception>   // for exception
 #include <filesystem>  // for path, exists
 #include <fstream>     // for ifstream
@@ -105,17 +105,18 @@ std::int32_t GetCfsCPUCount() noexcept {
   return -1;
 }
 
-std::int32_t OmpGetNumThreads(std::int32_t n_threads) {
+std::int32_t OmpGetNumThreads(std::int32_t n_threads) noexcept(true) {
   // Don't use parallel if we are in a parallel region.
   if (omp_in_parallel()) {
     return 1;
   }
+  // Honor the openmp thread limit, which can be set via environment variable.
+  auto max_n_threads = std::min({omp_get_num_procs(), omp_get_max_threads(), OmpGetThreadLimit()});
   // If -1 or 0 is specified by the user, we default to maximum number of threads.
   if (n_threads <= 0) {
-    n_threads = std::min(omp_get_num_procs(), omp_get_max_threads());
+    n_threads = max_n_threads;
   }
-  // Honor the openmp thread limit, which can be set via environment variable.
-  n_threads = std::min(n_threads, OmpGetThreadLimit());
+  n_threads = std::min(n_threads, max_n_threads);
   n_threads = std::max(n_threads, 1);
   return n_threads;
 }

src/common/threading_utils.h

@@ -258,9 +258,9 @@ inline std::int32_t OmpGetThreadLimit() {
 std::int32_t GetCfsCPUCount() noexcept;
 
 /**
- * \brief Get the number of available threads based on n_threads specified by users.
+ * @brief Get the number of available threads based on n_threads specified by users.
  */
-std::int32_t OmpGetNumThreads(std::int32_t n_threads);
+std::int32_t OmpGetNumThreads(std::int32_t n_threads) noexcept(true);
 
 /*!
  * \brief A C-style array with in-stack allocation. As long as the array is smaller than

tests/cpp/common/test_threading_utils.cc

@@ -1,5 +1,5 @@
 /**
- * Copyright 2019-2023 by XGBoost Contributors
+ * Copyright 2019-2024, XGBoost Contributors
  */
 #include <gtest/gtest.h>
 
@@ -9,9 +9,7 @@
 #include "dmlc/omp.h"                             // omp_in_parallel
 #include "xgboost/context.h"                      // Context
 
-namespace xgboost {
-namespace common {
-
+namespace xgboost::common {
 TEST(ParallelFor2d, CreateBlockedSpace2d) {
   constexpr size_t kDim1 = 5;
   constexpr size_t kDim2 = 3;
@@ -102,5 +100,14 @@ TEST(ParallelFor, Basic) {
   });
   ASSERT_FALSE(omp_in_parallel());
 }
-}  // namespace common
-}  // namespace xgboost
+
+TEST(OmpGetNumThreads, Max) {
+#if defined(_OPENMP)
+  auto n_threads = OmpGetNumThreads(1 << 18);
+  ASSERT_LE(n_threads, std::thread::hardware_concurrency());  // le due to container
+  n_threads = OmpGetNumThreads(0);
+  ASSERT_GE(n_threads, 1);
+  ASSERT_LE(n_threads, std::thread::hardware_concurrency());
+#endif
+}
+}  // namespace xgboost::common