Use a thread pool for external memory. (#10288)

tests/cpp/common/test_threadpool.cc

@@ -0,0 +1,49 @@
+/**
+ * Copyright 2024, XGBoost Contributors
+ */
+#include <gtest/gtest.h>
+
+#include <cstddef>  // for size_t
+#include <cstdint>  // for int32_t
+#include <future>   // for future
+#include <thread>   // for sleep_for, thread
+
+#include "../../../src/common/threadpool.h"
+
+namespace xgboost::common {
+TEST(ThreadPool, Basic) {
+  std::int32_t n_threads = std::thread::hardware_concurrency();
+  ThreadPool pool{n_threads};
+  {
+    auto fut = pool.Submit([] { return 3; });
+    ASSERT_EQ(fut.get(), 3);
+  }
+  {
+    auto fut = pool.Submit([] { return std::string{"ok"}; });
+    ASSERT_EQ(fut.get(), "ok");
+  }
+  {
+    std::vector<std::future<std::size_t>> futures;
+    for (std::size_t i = 0; i < static_cast<std::size_t>(n_threads) * 16; ++i) {
+      futures.emplace_back(pool.Submit([=] {
+        std::this_thread::sleep_for(std::chrono::milliseconds{10});
+        return i;
+      }));
+    }
+    for (std::size_t i = 0; i < futures.size(); ++i) {
+      ASSERT_EQ(futures[i].get(), i);
+    }
+  }
+  {
+    std::vector<std::future<std::size_t>> futures;
+    for (std::size_t i = 0; i < static_cast<std::size_t>(n_threads) * 16; ++i) {
+      futures.emplace_back(pool.Submit([=] {
+        return i;
+      }));
+    }
+    for (std::size_t i = 0; i < futures.size(); ++i) {
+      ASSERT_EQ(futures[i].get(), i);
+    }
+  }
+}
+}  // namespace xgboost::common