Reduce device synchronisation (#5631)

* Reduce device synchronisation

* Initialise pinned memory

tests/cpp/tree/gpu_hist/test_driver.cu

@@ -0,0 +1,59 @@
+#include <gtest/gtest.h>
+#include "../../../../src/tree/gpu_hist/driver.cuh"
+
+namespace xgboost {
+namespace tree {
+
+TEST(GpuHist, DriverDepthWise) {
+  Driver driver(TrainParam::kDepthWise);
+  EXPECT_TRUE(driver.Pop().empty());
+  DeviceSplitCandidate split;
+  split.loss_chg = 1.0f;
+  ExpandEntry root(0, 0, split);
+  driver.Push({root});
+  EXPECT_EQ(driver.Pop().front().nid, 0);
+  driver.Push({ExpandEntry{1, 1, split}});
+  driver.Push({ExpandEntry{2, 1, split}});
+  driver.Push({ExpandEntry{3, 2, split}});
+  // Should return entries from level 1
+  auto res = driver.Pop();
+  EXPECT_EQ(res.size(), 2);
+  for (auto &e : res) {
+    EXPECT_EQ(e.depth, 1);
+  }
+  res = driver.Pop();
+  EXPECT_EQ(res[0].depth, 2);
+  EXPECT_TRUE(driver.Pop().empty());
+}
+
+TEST(GpuHist, DriverLossGuided) {
+  DeviceSplitCandidate high_gain;
+  high_gain.loss_chg = 5.0f;
+  DeviceSplitCandidate low_gain;
+  low_gain.loss_chg = 1.0f;
+
+  Driver driver(TrainParam::kLossGuide);
+  EXPECT_TRUE(driver.Pop().empty());
+  ExpandEntry root(0, 0, high_gain);
+  driver.Push({root});
+  EXPECT_EQ(driver.Pop().front().nid, 0);
+  // Select high gain first
+  driver.Push({ExpandEntry{1, 1, low_gain}});
+  driver.Push({ExpandEntry{2, 2, high_gain}});
+  auto res = driver.Pop();
+  EXPECT_EQ(res.size(), 1);
+  EXPECT_EQ(res[0].nid, 2);
+  res = driver.Pop();
+  EXPECT_EQ(res.size(), 1);
+  EXPECT_EQ(res[0].nid, 1);
+
+  // If equal gain, use nid
+  driver.Push({ExpandEntry{2, 1, low_gain}});
+  driver.Push({ExpandEntry{1, 1, low_gain}});
+  res = driver.Pop();
+  EXPECT_EQ(res[0].nid, 1);
+  res = driver.Pop();
+  EXPECT_EQ(res[0].nid, 2);
+}
+}  // namespace tree
+}  // namespace xgboost