Copy data from Ellpack to GHist.

src/common/algorithm.cuh

@@ -0,0 +1,27 @@
+/*!
+ * Copyright 2022 by XGBoost Contributors
+ */
+#pragma once
+
+#include <thrust/binary_search.h>     // thrust::upper_bound
+#include <thrust/execution_policy.h>  // thrust::seq
+
+#include "xgboost/base.h"
+#include "xgboost/span.h"
+
+namespace xgboost {
+namespace common {
+namespace cuda {
+template <typename It>
+size_t XGBOOST_DEVICE SegmentId(It first, It last, size_t idx) {
+  size_t segment_id = thrust::upper_bound(thrust::seq, first, last, idx) - 1 - first;
+  return segment_id;
+}
+
+template <typename T>
+size_t XGBOOST_DEVICE SegmentId(Span<T> segments_ptr, size_t idx) {
+  return SegmentId(segments_ptr.cbegin(), segments_ptr.cend(), idx);
+}
+}  // namespace cuda
+}  // namespace common
+}  // namespace xgboost

src/common/algorithm.h

@@ -0,0 +1,16 @@
+/*!
+ * Copyright 2022 by XGBoost Contributors
+ */
+#pragma once
+#include <algorithm>  // std::upper_bound
+#include <cinttypes>  // std::size_t
+
+namespace xgboost {
+namespace common {
+template <typename It, typename Idx>
+auto SegmentId(It first, It last, Idx idx) {
+  std::size_t segment_id = std::upper_bound(first, last, idx) - 1 - first;
+  return segment_id;
+}
+}  // namespace common
+}  // namespace xgboost

src/common/column_matrix.h

@@ -18,6 +18,7 @@
 
 #include "../data/adapter.h"
 #include "../data/gradient_index.h"
+#include "algorithm.h"
 #include "hist_util.h"
 
 namespace xgboost {
@@ -135,6 +136,22 @@ class DenseColumnIter : public Column<BinIdxT> {
 class ColumnMatrix {
   void InitStorage(GHistIndexMatrix const& gmat, double sparse_threshold);
 
+  template <typename ColumnBinT, typename BinT, typename RIdx>
+  void SetBinSparse(BinT bin_id, RIdx rid, bst_feature_t fid, ColumnBinT* local_index) {
+    if (type_[fid] == kDenseColumn) {
+      ColumnBinT* begin = &local_index[feature_offsets_[fid]];
+      begin[rid] = bin_id - index_base_[fid];
+      // not thread-safe with bool vector.  FIXME(jiamingy): We can directly assign
+      // kMissingId to the index to avoid missing flags.
+      missing_flags_[feature_offsets_[fid] + rid] = false;
+    } else {
+      ColumnBinT* begin = &local_index[feature_offsets_[fid]];
+      begin[num_nonzeros_[fid]] = bin_id - index_base_[fid];
+      row_ind_[feature_offsets_[fid] + num_nonzeros_[fid]] = rid;
+      ++num_nonzeros_[fid];
+    }
+  }
+
  public:
   // get number of features
   bst_feature_t GetNumFeature() const { return static_cast<bst_feature_t>(type_.size()); }
@@ -144,34 +161,66 @@ class ColumnMatrix {
     this->InitStorage(gmat, sparse_threshold);
   }
 
+  /**
+   * \brief Initialize ColumnMatrix from GHistIndexMatrix with reference to the original
+   *        SparsePage.
+   */
+  void InitFromSparse(SparsePage const& page, const GHistIndexMatrix& gmat, double sparse_threshold,
+                      int32_t n_threads) {
+    auto batch = data::SparsePageAdapterBatch{page.GetView()};
+    this->InitStorage(gmat, sparse_threshold);
+    // ignore base row id here as we always has one column matrix for each sparse page.
+    this->PushBatch(n_threads, batch, std::numeric_limits<float>::quiet_NaN(), gmat, 0);
+  }
+
+  /**
+   * \brief Initialize ColumnMatrix from GHistIndexMatrix without reference to actual
+   * data.
+   *
+   *    This function requires a binary search for each bin to get back the feature index
+   *    for those bins.
+   */
+  void InitFromGHist(Context const* ctx, GHistIndexMatrix const& gmat) {
+    auto n_threads = ctx->Threads();
+    if (!any_missing_) {
+      // row index is compressed, we need to dispatch it.
+      DispatchBinType(gmat.index.GetBinTypeSize(), [&, size = gmat.Size(), n_threads = n_threads,
+                                                    n_features = gmat.Features()](auto t) {
+        using RowBinIdxT = decltype(t);
+        SetIndexNoMissing(gmat.base_rowid, gmat.index.data<RowBinIdxT>(), size, n_features,
+                          n_threads);
+      });
+    } else {
+      SetIndexMixedColumns(gmat);
+    }
+  }
+
+  /**
+   * \brief Push batch of data for Quantile DMatrix support.
+   *
+   * \param batch      Input data wrapped inside a adapter batch.
+   * \param gmat       The row-major histogram index that contains index for ALL data.
+   * \param base_rowid The beginning row index for current batch.
+   */
   template <typename Batch>
   void PushBatch(int32_t n_threads, Batch const& batch, float missing, GHistIndexMatrix const& gmat,
                  size_t base_rowid) {
     // pre-fill index_ for dense columns
-    auto n_features = gmat.Features();
     if (!any_missing_) {
-      missing_flags_.resize(feature_offsets_[n_features], false);
       // row index is compressed, we need to dispatch it.
-      DispatchBinType(gmat.index.GetBinTypeSize(), [&, size = batch.Size(), n_features = n_features,
-                                                    n_threads = n_threads](auto t) {
+
+      // use base_rowid from input parameter as gmat is a single matrix that contains all
+      // the histogram index instead of being only a batch.
+      DispatchBinType(gmat.index.GetBinTypeSize(), [&, size = batch.Size(), n_threads = n_threads,
+                                                    n_features = gmat.Features()](auto t) {
         using RowBinIdxT = decltype(t);
         SetIndexNoMissing(base_rowid, gmat.index.data<RowBinIdxT>(), size, n_features, n_threads);
       });
     } else {
-      missing_flags_.resize(feature_offsets_[n_features], true);
-      SetIndexMixedColumns(base_rowid, batch, gmat, n_features, missing);
+      SetIndexMixedColumns(base_rowid, batch, gmat, missing);
     }
   }
 
-  // construct column matrix from GHistIndexMatrix
-  void Init(SparsePage const& page, const GHistIndexMatrix& gmat, double sparse_threshold,
-            int32_t n_threads) {
-    auto batch = data::SparsePageAdapterBatch{page.GetView()};
-    this->InitStorage(gmat, sparse_threshold);
-    // ignore base row id here as we always has one column matrix for each sparse page.
-    this->PushBatch(n_threads, batch, std::numeric_limits<float>::quiet_NaN(), gmat, 0);
-  }
-
   /* Set the number of bytes based on numeric limit of maximum number of bins provided by user */
   void SetTypeSize(size_t max_bin_per_feat) {
     if ((max_bin_per_feat - 1) <= static_cast<int>(std::numeric_limits<uint8_t>::max())) {
@@ -210,6 +259,7 @@ class ColumnMatrix {
   template <typename RowBinIdxT>
   void SetIndexNoMissing(bst_row_t base_rowid, RowBinIdxT const* row_index, const size_t n_samples,
                          const size_t n_features, int32_t n_threads) {
+    missing_flags_.resize(feature_offsets_[n_features], false);
     DispatchBinType(bins_type_size_, [&](auto t) {
       using ColumnBinT = decltype(t);
       auto column_index = Span<ColumnBinT>{reinterpret_cast<ColumnBinT*>(index_.data()),
@@ -232,29 +282,16 @@ class ColumnMatrix {
    */
   template <typename Batch>
   void SetIndexMixedColumns(size_t base_rowid, Batch const& batch, const GHistIndexMatrix& gmat,
-                            size_t n_features, float missing) {
+                            float missing) {
+    auto n_features = gmat.Features();
+    missing_flags_.resize(feature_offsets_[n_features], true);
     auto const* row_index = gmat.index.data<uint32_t>() + gmat.row_ptr[base_rowid];
-    auto is_valid = data::IsValidFunctor {missing};
+    num_nonzeros_.resize(n_features, 0);
+    auto is_valid = data::IsValidFunctor{missing};
 
     DispatchBinType(bins_type_size_, [&](auto t) {
       using ColumnBinT = decltype(t);
       ColumnBinT* local_index = reinterpret_cast<ColumnBinT*>(index_.data());
-      num_nonzeros_.resize(n_features, 0);
-      auto get_bin_idx = [&](auto bin_id, auto rid, bst_feature_t fid) {
-        if (type_[fid] == kDenseColumn) {
-          ColumnBinT* begin = reinterpret_cast<ColumnBinT*>(&local_index[feature_offsets_[fid]]);
-          begin[rid] = bin_id - index_base_[fid];
-          // not thread-safe with bool vector.  FIXME(jiamingy): We can directly assign
-          // kMissingId to the index to avoid missing flags.
-          missing_flags_[feature_offsets_[fid] + rid] = false;
-        } else {
-          ColumnBinT* begin = reinterpret_cast<ColumnBinT*>(&local_index[feature_offsets_[fid]]);
-          begin[num_nonzeros_[fid]] = bin_id - index_base_[fid];
-          row_ind_[feature_offsets_[fid] + num_nonzeros_[fid]] = rid;
-          ++num_nonzeros_[fid];
-        }
-      };
-
       size_t const batch_size = batch.Size();
       size_t k{0};
       for (size_t rid = 0; rid < batch_size; ++rid) {
@@ -264,14 +301,48 @@ class ColumnMatrix {
           if (is_valid(coo)) {
             auto fid = coo.column_idx;
             const uint32_t bin_id = row_index[k];
-            get_bin_idx(bin_id, rid + base_rowid, fid);
+            SetBinSparse(bin_id, rid + base_rowid, fid, local_index);
             ++k;
           }
         }
       }
     });
   }
 
+  /**
+   * \brief Set column index for both dense and sparse columns, but with only GHistMatrix
+   *        available and requires a search for each bin.
+   */
+  void SetIndexMixedColumns(const GHistIndexMatrix& gmat) {
+    auto n_features = gmat.Features();
+    missing_flags_.resize(feature_offsets_[n_features], true);
+    auto const* row_index = gmat.index.data<uint32_t>() + gmat.row_ptr[gmat.base_rowid];
+    num_nonzeros_.resize(n_features, 0);
+    auto const& ptrs = gmat.cut.Ptrs();
+
+    DispatchBinType(bins_type_size_, [&](auto t) {
+      using ColumnBinT = decltype(t);
+      ColumnBinT* local_index = reinterpret_cast<ColumnBinT*>(index_.data());
+      auto const batch_size = gmat.Size();
+      size_t k{0};
+
+      for (size_t ridx = 0; ridx < batch_size; ++ridx) {
+        auto r_beg = gmat.row_ptr[ridx];
+        auto r_end = gmat.row_ptr[ridx + 1];
+        bst_feature_t fidx{0};
+        for (size_t j = r_beg; j < r_end; ++j) {
+          const uint32_t bin_idx = row_index[k];
+          // find the feature index for current bin.
+          while (bin_idx >= ptrs[fidx + 1]) {
+            fidx++;
+          }
+          SetBinSparse(bin_idx, ridx, fidx, local_index);
+          ++k;
+        }
+      }
+    });
+  }
+
   BinTypeSize GetTypeSize() const { return bins_type_size_; }
   auto GetColumnType(bst_feature_t fidx) const { return type_[fidx]; }
 

src/common/device_helpers.cuh

@@ -35,6 +35,7 @@
 #include "xgboost/global_config.h"
 
 #include "common.h"
+#include "algorithm.cuh"
 
 #ifdef XGBOOST_USE_NCCL
 #include "nccl.h"
@@ -1556,17 +1557,7 @@ XGBOOST_DEVICE thrust::transform_iterator<FuncT, IterT, ReturnT> MakeTransformIt
   return thrust::transform_iterator<FuncT, IterT, ReturnT>(iter, func);
 }
 
-template <typename It>
-size_t XGBOOST_DEVICE SegmentId(It first, It last, size_t idx) {
-  size_t segment_id = thrust::upper_bound(thrust::seq, first, last, idx) -
-                      1 - first;
-  return segment_id;
-}
-
-template <typename T>
-size_t XGBOOST_DEVICE SegmentId(xgboost::common::Span<T> segments_ptr, size_t idx) {
-  return SegmentId(segments_ptr.cbegin(), segments_ptr.cend(), idx);
-}
+using xgboost::common::cuda::SegmentId;  // import it for compatibility
 
 namespace detail {
 template <typename Key, typename KeyOutIt>

src/common/hist_util.h

@@ -22,6 +22,7 @@
 #include "row_set.h"
 #include "threading_utils.h"
 #include "timer.h"
+#include "algorithm.h"  // SegmentId
 
 namespace xgboost {
 class GHistIndexMatrix;
@@ -130,19 +131,23 @@ class HistogramCuts {
   /**
    * \brief Search the bin index for categorical feature.
    */
-  bst_bin_t SearchCatBin(float value, bst_feature_t fidx) const {
-    auto const &ptrs = this->Ptrs();
-    auto const &vals = this->Values();
+  bst_bin_t SearchCatBin(float value, bst_feature_t fidx, std::vector<uint32_t> const& ptrs,
+                         std::vector<float> const& vals) const {
     auto end = ptrs.at(fidx + 1) + vals.cbegin();
     auto beg = ptrs[fidx] + vals.cbegin();
     // Truncates the value in case it's not perfectly rounded.
-    auto v  = static_cast<float>(common::AsCat(value));
+    auto v = static_cast<float>(common::AsCat(value));
     auto bin_idx = std::lower_bound(beg, end, v) - vals.cbegin();
     if (bin_idx == ptrs.at(fidx + 1)) {
       bin_idx -= 1;
     }
     return bin_idx;
   }
+  bst_bin_t SearchCatBin(float value, bst_feature_t fidx) const {
+    auto const& ptrs = this->Ptrs();
+    auto const& vals = this->Values();
+    return this->SearchCatBin(value, fidx, ptrs, vals);
+  }
   bst_bin_t SearchCatBin(Entry const& e) const { return SearchCatBin(e.fvalue, e.index); }
 };
 
@@ -189,6 +194,28 @@ auto DispatchBinType(BinTypeSize type, Fn&& fn) {
  *   storage class.
  */
 struct Index {
+  // Inside the compressor, bin_idx is the index for cut value across all features. By
+  // subtracting it with starting pointer of each feature, we can reduce it to smaller
+  // value and store it with smaller types. Usable only with dense data.
+  //
+  // For sparse input we have to store an addition feature index (similar to sparse matrix
+  // formats like CSR) for each bin in index field to choose the right offset.
+  template <typename T>
+  struct CompressBin {
+    uint32_t const* offsets;
+
+    template <typename Bin, typename Feat>
+    auto operator()(Bin bin_idx, Feat fidx) const {
+      return static_cast<T>(bin_idx - offsets[fidx]);
+    }
+  };
+
+  template <typename T>
+  CompressBin<T> MakeCompressor() const {
+    uint32_t const* offsets = this->Offset();
+    return CompressBin<T>{offsets};
+  }
+
   Index() { SetBinTypeSize(binTypeSize_); }
   Index(const Index& i) = delete;
   Index& operator=(Index i) = delete;

src/data/ellpack_page.cu

@@ -547,4 +547,15 @@ EllpackDeviceAccessor EllpackPageImpl::GetDeviceAccessor(
                                                NumSymbols()),
           feature_types};
 }
+EllpackDeviceAccessor EllpackPageImpl::GetHostAccessor(
+    common::Span<FeatureType const> feature_types) const {
+  return {Context::kCpuId,
+          cuts_,
+          is_dense,
+          row_stride,
+          base_rowid,
+          n_rows,
+          common::CompressedIterator<uint32_t>(gidx_buffer.ConstHostPointer(), NumSymbols()),
+          feature_types};
+}
 }  // namespace xgboost

src/data/ellpack_page.cuh

@@ -43,12 +43,18 @@ struct EllpackDeviceAccessor {
         base_rowid(base_rowid),
         n_rows(n_rows) ,gidx_iter(gidx_iter),
         feature_types{feature_types} {
-    cuts.cut_values_.SetDevice(device);
-    cuts.cut_ptrs_.SetDevice(device);
-    cuts.min_vals_.SetDevice(device);
-    gidx_fvalue_map = cuts.cut_values_.ConstDeviceSpan();
-    feature_segments = cuts.cut_ptrs_.ConstDeviceSpan();
-    min_fvalue = cuts.min_vals_.ConstDeviceSpan();
+    if (device == Context::kCpuId) {
+      gidx_fvalue_map = cuts.cut_values_.ConstHostSpan();
+      feature_segments = cuts.cut_ptrs_.ConstHostSpan();
+      min_fvalue = cuts.min_vals_.ConstHostSpan();
+    } else {
+      cuts.cut_values_.SetDevice(device);
+      cuts.cut_ptrs_.SetDevice(device);
+      cuts.min_vals_.SetDevice(device);
+      gidx_fvalue_map = cuts.cut_values_.ConstDeviceSpan();
+      feature_segments = cuts.cut_ptrs_.ConstDeviceSpan();
+      min_fvalue = cuts.min_vals_.ConstDeviceSpan();
+    }
   }
   // Get a matrix element, uses binary search for look up Return NaN if missing
   // Given a row index and a feature index, returns the corresponding cut value
@@ -202,6 +208,7 @@ class EllpackPageImpl {
   EllpackDeviceAccessor
   GetDeviceAccessor(int device,
                     common::Span<FeatureType const> feature_types = {}) const;
+  EllpackDeviceAccessor GetHostAccessor(common::Span<FeatureType const> feature_types = {}) const;
 
  private:
   /*!