Re-introduce double buffer in UpdatePosition, to fix perf regression in gpu_hist (#6757)

* Revert "gpu_hist performance tweaks (#5707)"

This reverts commit f779980f7ea7f6f07e86229b8e78144e8a74e6b3.

* Address reviewer's comment

* Fix build error

src/common/device_helpers.cuh

@@ -549,6 +549,36 @@ class TemporaryArray {
   size_t size_;
 };
 
+/**
+ * \brief A double buffer, useful for algorithms like sort.
+ */
+template <typename T>
+class DoubleBuffer {
+ public:
+  cub::DoubleBuffer<T> buff;
+  xgboost::common::Span<T> a, b;
+  DoubleBuffer() = default;
+  template <typename VectorT>
+  DoubleBuffer(VectorT *v1, VectorT *v2) {
+    a = xgboost::common::Span<T>(v1->data().get(), v1->size());
+    b = xgboost::common::Span<T>(v2->data().get(), v2->size());
+    buff = cub::DoubleBuffer<T>(a.data(), b.data());
+  }
+
+  size_t Size() const {
+    CHECK_EQ(a.size(), b.size());
+    return a.size();
+  }
+  cub::DoubleBuffer<T> &CubBuffer() { return buff; }
+
+  T *Current() { return buff.Current(); }
+  xgboost::common::Span<T> CurrentSpan() {
+    return xgboost::common::Span<T>{buff.Current(), Size()};
+  }
+
+  T *Other() { return buff.Alternate(); }
+};
+
 /**
  * \brief Copies device span to std::vector.
  *

src/tree/gpu_hist/row_partitioner.cu

@@ -103,13 +103,17 @@ void Reset(int device_idx, common::Span<RowPartitioner::RowIndexT> ridx,
 }
 
 RowPartitioner::RowPartitioner(int device_idx, size_t num_rows)
-    : device_idx_(device_idx), ridx_a_(num_rows), position_a_(num_rows) {
+    : device_idx_(device_idx), ridx_a_(num_rows), position_a_(num_rows),
+      ridx_b_(num_rows), position_b_(num_rows) {
   dh::safe_cuda(cudaSetDevice(device_idx_));
-  Reset(device_idx, dh::ToSpan(ridx_a_), dh::ToSpan(position_a_));
+  ridx_ = dh::DoubleBuffer<RowIndexT>{&ridx_a_, &ridx_b_};
+  position_ = dh::DoubleBuffer<bst_node_t>{&position_a_, &position_b_};
+  ridx_segments_.emplace_back(Segment(0, num_rows));
+
+  Reset(device_idx, ridx_.CurrentSpan(), position_.CurrentSpan());
   left_counts_.resize(256);
   thrust::fill(left_counts_.begin(), left_counts_.end(), 0);
   streams_.resize(2);
-  ridx_segments_.emplace_back(Segment(0, num_rows));
   for (auto& stream : streams_) {
     dh::safe_cuda(cudaStreamCreate(&stream));
   }
@@ -129,15 +133,15 @@ common::Span<const RowPartitioner::RowIndexT> RowPartitioner::GetRows(
   if (segment.Size() == 0) {
     return common::Span<const RowPartitioner::RowIndexT>();
   }
-  return dh::ToSpan(ridx_a_).subspan(segment.begin, segment.Size());
+  return ridx_.CurrentSpan().subspan(segment.begin, segment.Size());
 }
 
 common::Span<const RowPartitioner::RowIndexT> RowPartitioner::GetRows() {
-  return dh::ToSpan(ridx_a_);
+  return ridx_.CurrentSpan();
 }
 
 common::Span<const bst_node_t> RowPartitioner::GetPosition() {
-  return dh::ToSpan(position_a_);
+  return position_.CurrentSpan();
 }
 std::vector<RowPartitioner::RowIndexT> RowPartitioner::GetRowsHost(
     bst_node_t nidx) {
@@ -159,25 +163,23 @@ void RowPartitioner::SortPositionAndCopy(const Segment& segment,
                                          bst_node_t right_nidx,
                                          int64_t* d_left_count,
                                          cudaStream_t stream) {
-  dh::TemporaryArray<bst_node_t> position_temp(position_a_.size());
-  dh::TemporaryArray<RowIndexT> ridx_temp(ridx_a_.size());
   SortPosition(
       // position_in
-      common::Span<bst_node_t>(position_a_.data().get() + segment.begin,
+      common::Span<bst_node_t>(position_.Current() + segment.begin,
                                segment.Size()),
       // position_out
-      common::Span<bst_node_t>(position_temp.data().get() + segment.begin,
+      common::Span<bst_node_t>(position_.Other() + segment.begin,
                                segment.Size()),
       // row index in
-      common::Span<RowIndexT>(ridx_a_.data().get() + segment.begin, segment.Size()),
+      common::Span<RowIndexT>(ridx_.Current() + segment.begin, segment.Size()),
       // row index out
-      common::Span<RowIndexT>(ridx_temp.data().get() + segment.begin, segment.Size()),
+      common::Span<RowIndexT>(ridx_.Other() + segment.begin, segment.Size()),
       left_nidx, right_nidx, d_left_count, stream);
   // Copy back key/value
-  const auto d_position_current = position_a_.data().get() + segment.begin;
+  const auto d_position_current = position_.Current() + segment.begin;
-  const auto d_position_other = position_temp.data().get() + segment.begin;
+  const auto d_position_other = position_.Other() + segment.begin;
-  const auto d_ridx_current = ridx_a_.data().get() + segment.begin;
+  const auto d_ridx_current = ridx_.Current() + segment.begin;
-  const auto d_ridx_other = ridx_temp.data().get() + segment.begin;
+  const auto d_ridx_other = ridx_.Other() + segment.begin;
   dh::LaunchN(device_idx_, segment.Size(), stream, [=] __device__(size_t idx) {
     d_position_current[idx] = d_position_other[idx];
     d_ridx_current[idx] = d_ridx_other[idx];

src/tree/gpu_hist/row_partitioner.cuh

@@ -47,7 +47,17 @@ class RowPartitioner {
   /*! \brief Range of row index for each node, pointers into ridx below. */
   std::vector<Segment> ridx_segments_;
   dh::TemporaryArray<RowIndexT> ridx_a_;
+  dh::TemporaryArray<RowIndexT> ridx_b_;
   dh::TemporaryArray<bst_node_t> position_a_;
+  dh::TemporaryArray<bst_node_t> position_b_;
+  /*! \brief mapping for node id -> rows.
+   * This looks like:
+   * node id  |    1    |    2   |
+   * rows idx | 3, 5, 1 | 13, 31 |
+   */
+  dh::DoubleBuffer<RowIndexT> ridx_;
+  /*! \brief mapping for row -> node id. */
+  dh::DoubleBuffer<bst_node_t> position_;
   dh::caching_device_vector<int64_t>
       left_counts_;  // Useful to keep a bunch of zeroed memory for sort position
   std::vector<cudaStream_t> streams_;
@@ -100,8 +110,8 @@ class RowPartitioner {
   void UpdatePosition(bst_node_t nidx, bst_node_t left_nidx,
                       bst_node_t right_nidx, UpdatePositionOpT op) {
     Segment segment = ridx_segments_.at(nidx);  // rows belongs to node nidx
-    auto d_ridx = dh::ToSpan(ridx_a_);
+    auto d_ridx = ridx_.CurrentSpan();
-    auto d_position = dh::ToSpan(position_a_);
+    auto d_position = position_.CurrentSpan();
     if (left_counts_.size() <= nidx) {
       left_counts_.resize((nidx * 2) + 1);
       thrust::fill(left_counts_.begin(), left_counts_.end(), 0);
@@ -148,9 +158,9 @@ class RowPartitioner {
    */
   template <typename FinalisePositionOpT>
   void FinalisePosition(FinalisePositionOpT op) {
-    auto d_position = position_a_.data().get();
+    auto d_position = position_.Current();
-    const auto d_ridx = ridx_a_.data().get();
+    const auto d_ridx = ridx_.Current();
-    dh::LaunchN(device_idx_, position_a_.size(), [=] __device__(size_t idx) {
+    dh::LaunchN(device_idx_, position_.Size(), [=] __device__(size_t idx) {
       auto position = d_position[idx];
       RowIndexT ridx = d_ridx[idx];
       bst_node_t new_position = op(ridx, position);