Add managed memory allocator. (#10711)

tests/cpp/common/test_cuda_host_allocator.cu

@@ -0,0 +1,36 @@
+/**
+ * Copyright 2024, XGBoost Contributors
+ */
+#include <gtest/gtest.h>
+#include <xgboost/context.h>  // for Context
+
+#include <vector>
+
+#include "../../../src/common/cuda_pinned_allocator.h"
+#include "../../../src/common/device_helpers.cuh"  // for DefaultStream
+#include "../../../src/common/numeric.h"           // for Iota
+
+namespace xgboost {
+TEST(CudaHostMalloc, Pinned) {
+  std::vector<float, common::cuda_impl::pinned_allocator<float>> vec;
+  vec.resize(10);
+  ASSERT_EQ(vec.size(), 10);
+  Context ctx;
+  common::Iota(&ctx, vec.begin(), vec.end(), 0);
+  float k = 0;
+  for (auto v : vec) {
+    ASSERT_EQ(v, k);
+    ++k;
+  }
+}
+
+TEST(CudaHostMalloc, Managed) {
+  std::vector<float, common::cuda_impl::managed_allocator<float>> vec;
+  vec.resize(10);
+#if defined(__linux__)
+  dh::safe_cuda(
+      cudaMemPrefetchAsync(vec.data(), vec.size() * sizeof(float), 0, dh::DefaultStream()));
+#endif
+  dh::DefaultStream().Sync();
+}
+}  // namespace xgboost