From cc9bd8a2851ce620c96c430c67f5fb8e535498c7 Mon Sep 17 00:00:00 2001
From: Jiaming Yuan <jm.yuan@outlook.com>
Date: Sun, 19 Jan 2025 05:21:44 +0800
Subject: [PATCH] don't use dmlc param

---
 include/xgboost/global_config.h |  2 +-
 src/c_api/c_api.cc              | 20 +++++++++++++++-----
 2 files changed, 16 insertions(+), 6 deletions(-)

diff --git a/include/xgboost/global_config.h b/include/xgboost/global_config.h
index bb6c211f1089..1a3d1b711d78 100644
--- a/include/xgboost/global_config.h
+++ b/include/xgboost/global_config.h
@@ -16,6 +16,7 @@ namespace xgboost {
 struct GlobalConfiguration : public XGBoostParameter<GlobalConfiguration> {
   std::int32_t verbosity{1};
   bool use_rmm{false};
+  // This is not a dmlc parameter to avoid conflict with the context class.
   std::int32_t nthread{0};
   DMLC_DECLARE_PARAMETER(GlobalConfiguration) {
     DMLC_DECLARE_FIELD(verbosity)
@@ -24,7 +25,6 @@ struct GlobalConfiguration : public XGBoostParameter<GlobalConfiguration> {
         .describe("Flag to print out detailed breakdown of runtime.");
     DMLC_DECLARE_FIELD(use_rmm).set_default(false).describe(
         "Whether to use RAPIDS Memory Manager to allocate GPU memory in XGBoost");
-    DMLC_DECLARE_FIELD(nthread).set_lower_bound(-1).set_default(0);
   }
 };
 
diff --git a/src/c_api/c_api.cc b/src/c_api/c_api.cc
index bb2324fec91b..008c2e4aff67 100644
--- a/src/c_api/c_api.cc
+++ b/src/c_api/c_api.cc
@@ -143,7 +143,19 @@ XGB_DLL int XGBSetGlobalConfig(const char* json_str) {
   xgboost_CHECK_C_ARG_PTR(json_str);
   Json config{Json::Load(StringView{json_str})};
 
-  for (auto& items : get<Object>(config)) {
+  // handle nthread, it's not a dmlc parameter.
+  auto& obj = get<Object>(config);
+  auto it = obj.find("nthread");
+  if (it != obj.cend()) {
+    auto nthread = OptionalArg<Integer>(config, "nthread", Integer::Int{0});
+    if (nthread > 0) {
+      omp_set_num_threads(nthread);
+      GlobalConfigThreadLocalStore::Get()->nthread = nthread;
+    }
+    get<Object>(config).erase("nthread");
+  }
+
+  for (auto &items : obj) {
     switch (items.second.GetValue().Type()) {
     case xgboost::Value::ValueKind::kInteger: {
       items.second = String{std::to_string(get<Integer const>(items.second))};
@@ -183,10 +195,7 @@ XGB_DLL int XGBSetGlobalConfig(const char* json_str) {
     }
     LOG(FATAL) << ss.str()  << " }";
   }
-  // The default is 0, we call omp rt only if the default is modified.
-  if (GlobalConfigThreadLocalStore::Get()->nthread > 0) {
-    omp_set_num_threads(GlobalConfigThreadLocalStore::Get()->nthread);
-  }
+
   API_END();
 }
 
@@ -220,6 +229,7 @@ XGB_DLL int XGBGetGlobalConfig(const char** json_str) {
     }
   }
 
+  config["nthread"] = GlobalConfigThreadLocalStore::Get()->nthread;
   auto& local = *GlobalConfigAPIThreadLocalStore::Get();
   Json::Dump(config, &local.ret_str);