Support external memory in CPU histogram building. (#7372)

src/common/hist_util.cc

@@ -133,148 +133,161 @@ struct Prefetch {
 
 constexpr size_t Prefetch::kNoPrefetchSize;
 
-
+template <typename FPType, bool do_prefetch, typename BinIdxType,
-template<typename FPType, bool do_prefetch, typename BinIdxType, bool any_missing = true>
+          bool first_page, bool any_missing = true>
-void BuildHistKernel(const std::vector<GradientPair>& gpair,
+void BuildHistKernel(const std::vector<GradientPair> &gpair,
                      const RowSetCollection::Elem row_indices,
-                     const GHistIndexMatrix& gmat,
+                     const GHistIndexMatrix &gmat, GHistRow<FPType> hist) {
-                     GHistRow<FPType> hist) {
   const size_t size = row_indices.Size();
-  const size_t* rid = row_indices.begin;
+  const size_t *rid = row_indices.begin;
-  const float* pgh = reinterpret_cast<const float*>(gpair.data());
+  auto const *pgh = reinterpret_cast<const float *>(gpair.data());
-  const BinIdxType* gradient_index = gmat.index.data<BinIdxType>();
+  const BinIdxType *gradient_index = gmat.index.data<BinIdxType>();
-  const size_t* row_ptr =  gmat.row_ptr.data();
+
-  const uint32_t* offsets = gmat.index.Offset();
+  auto const &row_ptr = gmat.row_ptr.data();
-  const size_t n_features = row_ptr[row_indices.begin[0]+1] - row_ptr[row_indices.begin[0]];
+  auto base_rowid = gmat.base_rowid;
-  FPType* hist_data = reinterpret_cast<FPType*>(hist.data());
+  const uint32_t *offsets = gmat.index.Offset();
-  const uint32_t two {2};  // Each element from 'gpair' and 'hist' contains
+  auto get_row_ptr = [&](size_t ridx) {
+    return first_page ? row_ptr[ridx] : row_ptr[ridx - base_rowid];
+  };
+  auto get_rid = [&](size_t ridx) {
+    return first_page ? ridx : (ridx - base_rowid);
+  };
+
+  const size_t n_features =
+      get_row_ptr(row_indices.begin[0] + 1) - get_row_ptr(row_indices.begin[0]);
+  auto hist_data = reinterpret_cast<FPType *>(hist.data());
+  const uint32_t two{2};  // Each element from 'gpair' and 'hist' contains
                           // 2 FP values: gradient and hessian.
                           // So we need to multiply each row-index/bin-index by 2
                           // to work with gradient pairs as a singe row FP array
 
   for (size_t i = 0; i < size; ++i) {
-    const size_t icol_start = any_missing ? row_ptr[rid[i]] : rid[i] * n_features;
+    const size_t icol_start =
-    const size_t icol_end =  any_missing ? row_ptr[rid[i]+1] : icol_start + n_features;
+        any_missing ? get_row_ptr(rid[i]) : get_rid(rid[i]) * n_features;
+    const size_t icol_end =
+        any_missing ? get_row_ptr(rid[i] + 1) : icol_start + n_features;
+
     const size_t row_size = icol_end - icol_start;
     const size_t idx_gh = two * rid[i];
 
     if (do_prefetch) {
-      const size_t icol_start_prefetch = any_missing ? row_ptr[rid[i+Prefetch::kPrefetchOffset]] :
+      const size_t icol_start_prefetch =
-                                       rid[i + Prefetch::kPrefetchOffset] * n_features;
+          any_missing
-      const size_t icol_end_prefetch = any_missing ?  row_ptr[rid[i+Prefetch::kPrefetchOffset]+1] :
+              ? get_row_ptr(rid[i + Prefetch::kPrefetchOffset])
-                                      icol_start_prefetch + n_features;
+              : get_rid(rid[i + Prefetch::kPrefetchOffset]) * n_features;
+      const size_t icol_end_prefetch =
+          any_missing ? get_row_ptr(rid[i + Prefetch::kPrefetchOffset] + 1)
+                      : icol_start_prefetch + n_features;
 
       PREFETCH_READ_T0(pgh + two * rid[i + Prefetch::kPrefetchOffset]);
       for (size_t j = icol_start_prefetch; j < icol_end_prefetch;
-        j+=Prefetch::GetPrefetchStep<uint32_t>()) {
+           j += Prefetch::GetPrefetchStep<uint32_t>()) {
         PREFETCH_READ_T0(gradient_index + j);
       }
     }
-    const BinIdxType* gr_index_local = gradient_index + icol_start;
+    const BinIdxType *gr_index_local = gradient_index + icol_start;
 
     for (size_t j = 0; j < row_size; ++j) {
-      const uint32_t idx_bin = two * (static_cast<uint32_t>(gr_index_local[j]) + (
+      const uint32_t idx_bin = two * (static_cast<uint32_t>(gr_index_local[j]) +
-                                      any_missing ? 0 : offsets[j]));
+                                      (any_missing ? 0 : offsets[j]));
-
       hist_data[idx_bin] += pgh[idx_gh];
-      hist_data[idx_bin+1] += pgh[idx_gh+1];
+      hist_data[idx_bin + 1] += pgh[idx_gh + 1];
     }
   }
 }
 
-template<typename FPType, bool do_prefetch, bool any_missing>
+template <typename FPType, bool do_prefetch, bool any_missing>
-void BuildHistDispatch(const std::vector<GradientPair>& gpair,
+void BuildHistDispatch(const std::vector<GradientPair> &gpair,
                        const RowSetCollection::Elem row_indices,
-                       const GHistIndexMatrix& gmat, GHistRow<FPType> hist) {
+                       const GHistIndexMatrix &gmat, GHistRow<FPType> hist) {
+  auto first_page = gmat.base_rowid == 0;
+  if (first_page) {
     switch (gmat.index.GetBinTypeSize()) {
     case kUint8BinsTypeSize:
-      BuildHistKernel<FPType, do_prefetch, uint8_t, any_missing>(gpair, row_indices,
+      BuildHistKernel<FPType, do_prefetch, uint8_t, true, any_missing>(
-                                                                      gmat, hist);
+          gpair, row_indices, gmat, hist);
       break;
     case kUint16BinsTypeSize:
-      BuildHistKernel<FPType, do_prefetch, uint16_t, any_missing>(gpair, row_indices,
+      BuildHistKernel<FPType, do_prefetch, uint16_t, true, any_missing>(
-                                                                       gmat, hist);
+          gpair, row_indices, gmat, hist);
       break;
     case kUint32BinsTypeSize:
-      BuildHistKernel<FPType, do_prefetch, uint32_t, any_missing>(gpair, row_indices,
+      BuildHistKernel<FPType, do_prefetch, uint32_t, true, any_missing>(
-                                                                       gmat, hist);
+          gpair, row_indices, gmat, hist);
       break;
     default:
       CHECK(false);  // no default behavior
     }
+  } else {
+    switch (gmat.index.GetBinTypeSize()) {
+    case kUint8BinsTypeSize:
+      BuildHistKernel<FPType, do_prefetch, uint8_t, false, any_missing>(
+          gpair, row_indices, gmat, hist);
+      break;
+    case kUint16BinsTypeSize:
+      BuildHistKernel<FPType, do_prefetch, uint16_t, false, any_missing>(
+          gpair, row_indices, gmat, hist);
+      break;
+    case kUint32BinsTypeSize:
+      BuildHistKernel<FPType, do_prefetch, uint32_t, false, any_missing>(
+          gpair, row_indices, gmat, hist);
+      break;
+    default:
+      CHECK(false);  // no default behavior
+    }
+  }
 }
 
 template <typename GradientSumT>
 template <bool any_missing>
 void GHistBuilder<GradientSumT>::BuildHist(
     const std::vector<GradientPair> &gpair,
-    const RowSetCollection::Elem row_indices,
+    const RowSetCollection::Elem row_indices, const GHistIndexMatrix &gmat,
-    const GHistIndexMatrix &gmat,
+    GHistRowT hist) const {
-    GHistRowT hist) {
   const size_t nrows = row_indices.Size();
   const size_t no_prefetch_size = Prefetch::NoPrefetchSize(nrows);
 
   // if need to work with all rows from bin-matrix (e.g. root node)
-  const bool contiguousBlock = (row_indices.begin[nrows - 1] - row_indices.begin[0]) == (nrows - 1);
+  const bool contiguousBlock =
+      (row_indices.begin[nrows - 1] - row_indices.begin[0]) == (nrows - 1);
 
   if (contiguousBlock) {
     // contiguous memory access, built-in HW prefetching is enough
-    BuildHistDispatch<GradientSumT, false, any_missing>(gpair, row_indices, gmat, hist);
+    BuildHistDispatch<GradientSumT, false, any_missing>(gpair, row_indices,
+                                                        gmat, hist);
   } else {
-    const RowSetCollection::Elem span1(row_indices.begin, row_indices.end - no_prefetch_size);
+    const RowSetCollection::Elem span1(row_indices.begin,
-    const RowSetCollection::Elem span2(row_indices.end - no_prefetch_size, row_indices.end);
+                                       row_indices.end - no_prefetch_size);
+    const RowSetCollection::Elem span2(row_indices.end - no_prefetch_size,
+                                       row_indices.end);
 
-    BuildHistDispatch<GradientSumT, true, any_missing>(gpair, span1, gmat, hist);
+    BuildHistDispatch<GradientSumT, true, any_missing>(gpair, span1, gmat,
+                                                       hist);
     // no prefetching to avoid loading extra memory
-    BuildHistDispatch<GradientSumT, false, any_missing>(gpair, span2, gmat, hist);
+    BuildHistDispatch<GradientSumT, false, any_missing>(gpair, span2, gmat,
+                                                        hist);
   }
 }
+
 template void
 GHistBuilder<float>::BuildHist<true>(const std::vector<GradientPair> &gpair,
                                      const RowSetCollection::Elem row_indices,
                                      const GHistIndexMatrix &gmat,
-                                     GHistRow<float> hist);
+                                     GHistRow<float> hist) const;
 template void
 GHistBuilder<float>::BuildHist<false>(const std::vector<GradientPair> &gpair,
                                       const RowSetCollection::Elem row_indices,
                                       const GHistIndexMatrix &gmat,
-                                      GHistRow<float> hist);
+                                      GHistRow<float> hist) const;
 template void
 GHistBuilder<double>::BuildHist<true>(const std::vector<GradientPair> &gpair,
                                       const RowSetCollection::Elem row_indices,
                                       const GHistIndexMatrix &gmat,
-                                      GHistRow<double> hist);
+                                      GHistRow<double> hist) const;
 template void
 GHistBuilder<double>::BuildHist<false>(const std::vector<GradientPair> &gpair,
                                        const RowSetCollection::Elem row_indices,
                                        const GHistIndexMatrix &gmat,
-                                       GHistRow<double> hist);
+                                       GHistRow<double> hist) const;
-
-template<typename GradientSumT>
-void GHistBuilder<GradientSumT>::SubtractionTrick(GHistRowT self,
-                                                  GHistRowT sibling,
-                                                  GHistRowT parent) {
-  const size_t size = self.size();
-  CHECK_EQ(sibling.size(), size);
-  CHECK_EQ(parent.size(), size);
-
-  const size_t block_size = 1024;  // aproximatly 1024 values per block
-  size_t n_blocks = size/block_size + !!(size%block_size);
-
-  ParallelFor(omp_ulong(n_blocks), [&](omp_ulong iblock) {
-    const size_t ibegin = iblock*block_size;
-    const size_t iend = (((iblock+1)*block_size > size) ? size : ibegin + block_size);
-    SubtractionHist(self, parent, sibling, ibegin, iend);
-  });
-}
-template
-void GHistBuilder<float>::SubtractionTrick(GHistRow<float> self,
-                                           GHistRow<float> sibling,
-                                           GHistRow<float> parent);
-template
-void GHistBuilder<double>::SubtractionTrick(GHistRow<double> self,
-                                            GHistRow<double> sibling,
-                                            GHistRow<double> parent);
-
 }  // namespace common
 }  // namespace xgboost

src/common/hist_util.h

@@ -460,7 +460,7 @@ class ParallelGHistBuilder {
   }
 
   // Reduce following bins (begin, end] for nid-node in dst across threads
-  void ReduceHist(size_t nid, size_t begin, size_t end) {
+  void ReduceHist(size_t nid, size_t begin, size_t end) const {
     CHECK_GT(end, begin);
     CHECK_LT(nid, nodes_);
 
@@ -486,7 +486,6 @@ class ParallelGHistBuilder {
     }
   }
 
- protected:
   void MatchThreadsToNodes(const BlockedSpace2d& space) {
     const size_t space_size = space.Size();
     const size_t chunck_size = space_size / nthreads_ + !!(space_size % nthreads_);
@@ -533,6 +532,7 @@ class ParallelGHistBuilder {
     }
   }
 
+ private:
   void MatchNodeNidPairToHist() {
     size_t hist_allocated_additionally = 0;
 
@@ -586,26 +586,18 @@ class GHistBuilder {
   using GHistRowT = GHistRow<GradientSumT>;
 
   GHistBuilder() = default;
-  GHistBuilder(size_t nthread, uint32_t nbins) : nthread_{nthread}, nbins_{nbins} {}
+  explicit GHistBuilder(uint32_t nbins): nbins_{nbins} {}
 
   // construct a histogram via histogram aggregation
   template <bool any_missing>
-  void BuildHist(const std::vector<GradientPair>& gpair,
+  void BuildHist(const std::vector<GradientPair> &gpair,
                  const RowSetCollection::Elem row_indices,
-                 const GHistIndexMatrix& gmat,
+                 const GHistIndexMatrix &gmat, GHistRowT hist) const;
-                 GHistRowT hist);
-  // construct a histogram via subtraction trick
-  void SubtractionTrick(GHistRowT self,
-                        GHistRowT sibling,
-                        GHistRowT parent);
-
   uint32_t GetNumBins() const {
       return nbins_;
   }
 
  private:
-  /*! \brief number of threads for parallel computation */
-  size_t nthread_ { 0 };
   /*! \brief number of all bins over all features */
   uint32_t nbins_ { 0 };
 };

src/tree/hist/histogram.h

@@ -11,6 +11,7 @@
 #include "rabit/rabit.h"
 #include "xgboost/tree_model.h"
 #include "../../common/hist_util.h"
+#include "../../data/gradient_index.h"
 
 namespace xgboost {
 namespace tree {
@@ -25,10 +26,11 @@ template <typename GradientSumT, typename ExpandEntry> class HistogramBuilder {
   common::GHistBuilder<GradientSumT> builder_;
   common::ParallelGHistBuilder<GradientSumT> buffer_;
   rabit::Reducer<GradientPairT, GradientPairT::Reduce> reducer_;
-  int32_t max_bin_ {-1};
+  BatchParam param_;
-  int32_t n_threads_ {-1};
+  int32_t n_threads_{-1};
+  size_t n_batches_{0};
   // Whether XGBoost is running in distributed environment.
-  bool is_distributed_ {false};
+  bool is_distributed_{false};
 
  public:
   /**
@@ -39,59 +41,54 @@ template <typename GradientSumT, typename ExpandEntry> class HistogramBuilder {
    * \param is_distributed   Mostly used for testing to allow injecting parameters instead
    *                         of using global rabit variable.
    */
-  void Reset(uint32_t total_bins, int32_t max_bin_per_feat, int32_t n_threads,
+  void Reset(uint32_t total_bins, BatchParam p, int32_t n_threads, size_t n_batches,
-             bool is_distributed = rabit::IsDistributed()) {
+             bool is_distributed) {
     CHECK_GE(n_threads, 1);
     n_threads_ = n_threads;
-    CHECK_GE(max_bin_per_feat, 2);
+    n_batches_ = n_batches;
-    max_bin_ = max_bin_per_feat;
+    param_ = p;
     hist_.Init(total_bins);
     hist_local_worker_.Init(total_bins);
     buffer_.Init(total_bins);
-    builder_ = common::GHistBuilder<GradientSumT>(n_threads, total_bins);
+    builder_ = common::GHistBuilder<GradientSumT>(total_bins);
     is_distributed_ = is_distributed;
   }
 
   template <bool any_missing>
-  void
+  void BuildLocalHistograms(size_t page_idx, common::BlockedSpace2d space,
-  BuildLocalHistograms(DMatrix *p_fmat,
+                            GHistIndexMatrix const &gidx,
-                       std::vector<ExpandEntry> nodes_for_explicit_hist_build,
+                            std::vector<ExpandEntry> const &nodes_for_explicit_hist_build,
                             common::RowSetCollection const &row_set_collection,
                             const std::vector<GradientPair> &gpair_h) {
     const size_t n_nodes = nodes_for_explicit_hist_build.size();
-
+    CHECK_GT(n_nodes, 0);
-    // create space of size (# rows in each node)
-    common::BlockedSpace2d space(
-        n_nodes,
-        [&](size_t node) {
-          const int32_t nid = nodes_for_explicit_hist_build[node].nid;
-          return row_set_collection[nid].Size();
-        },
-        256);
 
     std::vector<GHistRowT> target_hists(n_nodes);
     for (size_t i = 0; i < n_nodes; ++i) {
       const int32_t nid = nodes_for_explicit_hist_build[i].nid;
       target_hists[i] = hist_[nid];
     }
+    if (page_idx == 0) {
+      // FIXME(jiamingy): Handle different size of space.  Right now we use the maximum
+      // partition size for the buffer, which might not be efficient if partition sizes
+      // has significant variance.
       buffer_.Reset(this->n_threads_, n_nodes, space, target_hists);
+    }
 
     // Parallel processing by nodes and data in each node
-    for (auto const &gmat : p_fmat->GetBatches<GHistIndexMatrix>(
+    common::ParallelFor2d(space, this->n_threads_, [&](size_t nid_in_set, common::Range1d r) {
-             BatchParam{GenericParameter::kCpuId, max_bin_})) {
-      common::ParallelFor2d(
-          space, this->n_threads_, [&](size_t nid_in_set, common::Range1d r) {
       const auto tid = static_cast<unsigned>(omp_get_thread_num());
       const int32_t nid = nodes_for_explicit_hist_build[nid_in_set].nid;
-
+      auto elem = row_set_collection[nid];
-            auto start_of_row_set = row_set_collection[nid].begin;
+      auto start_of_row_set = std::min(r.begin(), elem.Size());
-            auto rid_set = common::RowSetCollection::Elem(
+      auto end_of_row_set = std::min(r.end(), elem.Size());
-                start_of_row_set + r.begin(), start_of_row_set + r.end(), nid);
+      auto rid_set = common::RowSetCollection::Elem(elem.begin + start_of_row_set,
-            builder_.template BuildHist<any_missing>(
+                                                    elem.begin + end_of_row_set, nid);
-                gpair_h, rid_set, gmat,
+      auto hist = buffer_.GetInitializedHist(tid, nid_in_set);
-                buffer_.GetInitializedHist(tid, nid_in_set));
+      if (rid_set.Size() != 0) {
-          });
+        builder_.template BuildHist<any_missing>(gpair_h, rid_set, gidx, hist);
       }
+    });
   }
 
   void
@@ -110,24 +107,34 @@ template <typename GradientSumT, typename ExpandEntry> class HistogramBuilder {
     }
   }
 
-  /* Main entry point of this class, build histogram for tree nodes. */
+  /** Main entry point of this class, build histogram for tree nodes. */
-  void BuildHist(DMatrix *p_fmat, RegTree *p_tree,
+  void BuildHist(size_t page_id, common::BlockedSpace2d space, GHistIndexMatrix const &gidx,
-                 common::RowSetCollection const &row_set_collection,
+                 RegTree *p_tree, common::RowSetCollection const &row_set_collection,
                  std::vector<ExpandEntry> const &nodes_for_explicit_hist_build,
                  std::vector<ExpandEntry> const &nodes_for_subtraction_trick,
                  std::vector<GradientPair> const &gpair) {
     int starting_index = std::numeric_limits<int>::max();
     int sync_count = 0;
+    if (page_id == 0) {
       this->AddHistRows(&starting_index, &sync_count,
                         nodes_for_explicit_hist_build,
                         nodes_for_subtraction_trick, p_tree);
-    if (p_fmat->IsDense()) {
+    }
-      BuildLocalHistograms<false>(p_fmat, nodes_for_explicit_hist_build,
+    if (gidx.IsDense()) {
+      this->BuildLocalHistograms<false>(page_id, space, gidx,
+                                        nodes_for_explicit_hist_build,
                                         row_set_collection, gpair);
     } else {
-      BuildLocalHistograms<true>(p_fmat, nodes_for_explicit_hist_build,
+      this->BuildLocalHistograms<true>(page_id, space, gidx,
+                                       nodes_for_explicit_hist_build,
                                        row_set_collection, gpair);
     }
+
+    CHECK_GE(n_batches_, 1);
+    if (page_id != n_batches_ - 1) {
+      return;
+    }
+
     if (is_distributed_) {
       this->SyncHistogramDistributed(p_tree, nodes_for_explicit_hist_build,
                                      nodes_for_subtraction_trick,
@@ -138,6 +145,25 @@ template <typename GradientSumT, typename ExpandEntry> class HistogramBuilder {
                                sync_count);
     }
   }
+  /** same as the other build hist but handles only single batch data (in-core) */
+  void BuildHist(size_t page_id, GHistIndexMatrix const &gidx, RegTree *p_tree,
+                 common::RowSetCollection const &row_set_collection,
+                 std::vector<ExpandEntry> const &nodes_for_explicit_hist_build,
+                 std::vector<ExpandEntry> const &nodes_for_subtraction_trick,
+                 std::vector<GradientPair> const &gpair) {
+    const size_t n_nodes = nodes_for_explicit_hist_build.size();
+    // create space of size (# rows in each node)
+    common::BlockedSpace2d space(
+        n_nodes,
+        [&](size_t nidx_in_set) {
+          const int32_t nidx = nodes_for_explicit_hist_build[nidx_in_set].nid;
+          return row_set_collection[nidx].Size();
+        },
+        256);
+    this->BuildHist(page_id, space, gidx, p_tree, row_set_collection,
+                    nodes_for_explicit_hist_build, nodes_for_subtraction_trick,
+                    gpair);
+  }
 
   void SyncHistogramDistributed(
       RegTree *p_tree,

src/tree/updater_quantile_hist.cc

@@ -127,9 +127,14 @@ void QuantileHistMaker::Builder<GradientSumT>::InitRoot(
   nodes_for_subtraction_trick_.clear();
   nodes_for_explicit_hist_build_.push_back(node);
 
-  this->histogram_builder_->BuildHist(p_fmat, p_tree, row_set_collection_,
+  size_t page_id = 0;
-                                      nodes_for_explicit_hist_build_,
+  for (auto const &gidx : p_fmat->GetBatches<GHistIndexMatrix>(
-                                      nodes_for_subtraction_trick_, gpair_h);
+           {GenericParameter::kCpuId, param_.max_bin})) {
+    this->histogram_builder_->BuildHist(
+        page_id, gidx, p_tree, row_set_collection_,
+        nodes_for_explicit_hist_build_, nodes_for_subtraction_trick_, gpair_h);
+    ++page_id;
+  }
 
   {
     auto nid = RegTree::kRoot;
@@ -259,9 +264,15 @@ void QuantileHistMaker::Builder<GradientSumT>::ExpandTree(
       SplitSiblings(nodes_for_apply_split, &nodes_to_evaluate, p_tree);
 
       if (depth < param_.max_depth) {
+        size_t i = 0;
+        for (auto const &gidx : p_fmat->GetBatches<GHistIndexMatrix>(
+                 {GenericParameter::kCpuId, param_.max_bin})) {
           this->histogram_builder_->BuildHist(
-            p_fmat, p_tree, row_set_collection_, nodes_for_explicit_hist_build_,
+              i, gidx, p_tree, row_set_collection_,
-            nodes_for_subtraction_trick_, gpair_h);
+              nodes_for_explicit_hist_build_, nodes_for_subtraction_trick_,
+              gpair_h);
+          ++i;
+        }
       } else {
         int starting_index = std::numeric_limits<int>::max();
         int sync_count = 0;
@@ -432,7 +443,9 @@ void QuantileHistMaker::Builder<GradientSumT>::InitData(
       });
     }
     exc.Rethrow();
-    this->histogram_builder_->Reset(nbins, param_.max_bin, this->nthread_);
+    this->histogram_builder_->Reset(
+        nbins, BatchParam{GenericParameter::kCpuId, param_.max_bin},
+        this->nthread_, 1, rabit::IsDistributed());
 
     std::vector<size_t>& row_indices = *row_set_collection_.Data();
     row_indices.resize(info.num_row_);

tests/cpp/tree/hist/test_evaluate_splits.cc

@@ -36,7 +36,7 @@ template <typename GradientSumT> void TestEvaluateSplits() {
   std::iota(row_indices.begin(), row_indices.end(), 0);
   row_set_collection.Init();
 
-  auto hist_builder = GHistBuilder<GradientSumT>(omp_get_max_threads(), gmat.cut.Ptrs().back());
+  auto hist_builder = GHistBuilder<GradientSumT>(gmat.cut.Ptrs().back());
   hist.Init(gmat.cut.Ptrs().back());
   hist.AddHistRow(0);
   hist.AllocateAllData();

tests/cpp/tree/hist/test_histogram.cc

@@ -8,6 +8,16 @@
 
 namespace xgboost {
 namespace tree {
+namespace {
+void InitRowPartitionForTest(RowSetCollection *row_set, size_t n_samples,
+                             size_t base_rowid = 0) {
+  auto &row_indices = *row_set->Data();
+  row_indices.resize(n_samples);
+  std::iota(row_indices.begin(), row_indices.end(), base_rowid);
+  row_set->Init();
+}
+}  // anonymous namespace
+
 template <typename GradientSumT>
 void TestAddHistRows(bool is_distributed) {
   std::vector<CPUExpandEntry> nodes_for_explicit_hist_build_;
@@ -35,8 +45,9 @@ void TestAddHistRows(bool is_distributed) {
   nodes_for_subtraction_trick_.emplace_back(6, tree.GetDepth(6), 0.0f);
 
   HistogramBuilder<GradientSumT, CPUExpandEntry> histogram_builder;
-  histogram_builder.Reset(gmat.cut.TotalBins(), kMaxBins, omp_get_max_threads(),
+  histogram_builder.Reset(gmat.cut.TotalBins(),
-                          is_distributed);
+                          {GenericParameter::kCpuId, kMaxBins},
+                          omp_get_max_threads(), 1, is_distributed);
   histogram_builder.AddHistRows(&starting_index, &sync_count,
                                 nodes_for_explicit_hist_build_,
                                 nodes_for_subtraction_trick_, &tree);
@@ -81,7 +92,8 @@ void TestSyncHist(bool is_distributed) {
 
   HistogramBuilder<GradientSumT, CPUExpandEntry> histogram;
   uint32_t total_bins = gmat.cut.Ptrs().back();
-  histogram.Reset(total_bins, kMaxBins, omp_get_max_threads(), is_distributed);
+  histogram.Reset(total_bins, {GenericParameter::kCpuId, kMaxBins},
+                  omp_get_max_threads(), 1, is_distributed);
 
   RowSetCollection row_set_collection_;
   {
@@ -247,22 +259,26 @@ void TestBuildHistogram(bool is_distributed) {
 
   bst_node_t nid = 0;
   HistogramBuilder<GradientSumT, CPUExpandEntry> histogram;
-  histogram.Reset(total_bins, kMaxBins, omp_get_max_threads(), is_distributed);
+  histogram.Reset(total_bins, {GenericParameter::kCpuId, kMaxBins},
+                  omp_get_max_threads(), 1, is_distributed);
 
   RegTree tree;
 
-  RowSetCollection row_set_collection_;
+  RowSetCollection row_set_collection;
-  row_set_collection_.Clear();
+  row_set_collection.Clear();
-  std::vector<size_t> &row_indices = *row_set_collection_.Data();
+  std::vector<size_t> &row_indices = *row_set_collection.Data();
   row_indices.resize(kNRows);
   std::iota(row_indices.begin(), row_indices.end(), 0);
-  row_set_collection_.Init();
+  row_set_collection.Init();
 
   CPUExpandEntry node(RegTree::kRoot, tree.GetDepth(0), 0.0f);
-  std::vector<CPUExpandEntry> nodes_for_explicit_hist_build_;
+  std::vector<CPUExpandEntry> nodes_for_explicit_hist_build;
-  nodes_for_explicit_hist_build_.push_back(node);
+  nodes_for_explicit_hist_build.push_back(node);
-  histogram.BuildHist(p_fmat.get(), &tree, row_set_collection_,
+  for (auto const &gidx : p_fmat->GetBatches<GHistIndexMatrix>(
-                      nodes_for_explicit_hist_build_, {}, gpair);
+           {GenericParameter::kCpuId, kMaxBins})) {
+    histogram.BuildHist(0, gidx, &tree, row_set_collection,
+                        nodes_for_explicit_hist_build, {}, gpair);
+  }
 
   // Check if number of histogram bins is correct
   ASSERT_EQ(histogram.Histogram()[nid].size(), gmat.cut.Ptrs().back());
@@ -294,5 +310,88 @@ TEST(CPUHistogram, BuildHist) {
   TestBuildHistogram<float>(false);
   TestBuildHistogram<double>(false);
 }
+
+TEST(CPUHistogram, ExternalMemory) {
+  size_t constexpr kEntries = 1 << 16;
+
+  int32_t constexpr kBins = 32;
+  auto m = CreateSparsePageDMatrix(kEntries, "cache");
+  std::vector<size_t> partition_size(1, 0);
+  size_t total_bins{0};
+  size_t n_samples{0};
+
+  auto gpair = GenerateRandomGradients(m->Info().num_row_, 0.0, 1.0);
+  auto const &h_gpair = gpair.HostVector();
+
+  RegTree tree;
+  std::vector<CPUExpandEntry> nodes;
+  nodes.emplace_back(0, tree.GetDepth(0), 0.0f);
+
+  GHistRow<double> multi_page;
+  HistogramBuilder<double, CPUExpandEntry> multi_build;
+  {
+    /**
+     * Multi page
+     */
+    std::vector<RowSetCollection> rows_set;
+    std::vector<float> hess(m->Info().num_row_, 1.0);
+    for (auto const &page : m->GetBatches<GHistIndexMatrix>(
+             {GenericParameter::kCpuId, kBins, hess})) {
+      CHECK_LT(page.base_rowid, m->Info().num_row_);
+      auto n_rows_in_node = page.Size();
+      partition_size[0] = std::max(partition_size[0], n_rows_in_node);
+      total_bins = page.cut.TotalBins();
+      n_samples += n_rows_in_node;
+
+      rows_set.emplace_back();
+      InitRowPartitionForTest(&rows_set.back(), n_rows_in_node, page.base_rowid);
+    }
+    ASSERT_EQ(n_samples, m->Info().num_row_);
+
+    common::BlockedSpace2d space{
+        1, [&](size_t nidx_in_set) { return partition_size.at(nidx_in_set); },
+        256};
+
+    multi_build.Reset(total_bins, {GenericParameter::kCpuId, kBins},
+                      omp_get_max_threads(), rows_set.size(), false);
+
+    size_t page_idx{0};
+    for (auto const &page : m->GetBatches<GHistIndexMatrix>(
+             {GenericParameter::kCpuId, kBins, hess})) {
+      multi_build.BuildHist(page_idx, space, page, &tree,
+                                  rows_set.at(page_idx), nodes, {}, h_gpair);
+      ++page_idx;
+    }
+    ASSERT_EQ(page_idx, 2);
+    multi_page = multi_build.Histogram()[0];
+  }
+
+  HistogramBuilder<double, CPUExpandEntry> single_build;
+  GHistRow<double> single_page;
+  {
+    /**
+     * Single page
+     */
+    RowSetCollection row_set_collection;
+    InitRowPartitionForTest(&row_set_collection, n_samples);
+
+    single_build.Reset(total_bins, {GenericParameter::kCpuId, kBins},
+                       omp_get_max_threads(), 1, false);
+    size_t n_batches{0};
+    for (auto const &page :
+         m->GetBatches<GHistIndexMatrix>({GenericParameter::kCpuId, kBins})) {
+      single_build.BuildHist(0, page, &tree, row_set_collection, nodes, {},
+                             h_gpair);
+      n_batches ++;
+    }
+    ASSERT_EQ(n_batches, 1);
+    single_page = single_build.Histogram()[0];
+  }
+
+  for (size_t i = 0; i < single_page.size(); ++i) {
+    ASSERT_NEAR(single_page[i].GetGrad(), multi_page[i].GetGrad(), kRtEps);
+    ASSERT_NEAR(single_page[i].GetHess(), multi_page[i].GetHess(), kRtEps);
+  }
+}
 }  // namespace tree
 }  // namespace xgboost