Predict on Ellpack. (#5327)

* Unify GPU prediction node.
* Add `PageExists`.
* Dispatch prediction on input data for GPU Predictor.

src/data/ellpack_page.cu

@@ -31,8 +31,8 @@ __global__ void CompressBinEllpackKernel(
     common::CompressedByteT* __restrict__ buffer,  // gidx_buffer
     const size_t* __restrict__ row_ptrs,           // row offset of input data
     const Entry* __restrict__ entries,      // One batch of input data
-    const float* __restrict__ cuts,         // HistogramCuts::cut
-    const uint32_t* __restrict__ cut_rows,  // HistogramCuts::row_ptrs
+    const float* __restrict__ cuts,         // HistogramCuts::cut_values_
+    const uint32_t* __restrict__ cut_rows,  // HistogramCuts::cut_ptrs_
     size_t base_row,                        // batch_row_begin
     size_t n_rows,
     size_t row_stride,

src/data/ellpack_page.cuh

@@ -76,6 +76,9 @@ struct EllpackInfo {
   size_t NumSymbols() const {
     return n_bins + 1;
   }
+  size_t NumFeatures() const {
+    return min_fvalue.size();
+  }
 };
 
 /** \brief Struct for accessing and manipulating an ellpack matrix on the
@@ -89,7 +92,7 @@ struct EllpackMatrix {
 
   // 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
-  __device__ bst_float GetElement(size_t ridx, size_t fidx) const {
+  __device__ int32_t GetBinIndex(size_t ridx, size_t fidx) const {
     ridx -= base_rowid;
     auto row_begin = info.row_stride * ridx;
     auto row_end = row_begin + info.row_stride;
@@ -103,6 +106,10 @@ struct EllpackMatrix {
                              info.feature_segments[fidx],
                              info.feature_segments[fidx + 1]);
     }
+    return gidx;
+  }
+  __device__ bst_float GetFvalue(size_t ridx, size_t fidx) const {
+    auto gidx = GetBinIndex(ridx, fidx);
     if (gidx == -1) {
       return nan("");
     }

src/data/simple_dmatrix.cc

@@ -61,11 +61,15 @@ BatchSet<SortedCSCPage> SimpleDMatrix::GetSortedColumnBatches() {
 }
 
 BatchSet<EllpackPage> SimpleDMatrix::GetEllpackBatches(const BatchParam& param) {
-  CHECK_GE(param.gpu_id, 0);
-  CHECK_GE(param.max_bin, 2);
   // ELLPACK page doesn't exist, generate it
-  if (!ellpack_page_) {
+  if (!(batch_param_ != BatchParam{})) {
+    CHECK(param != BatchParam{}) << "Batch parameter is not initialized.";
+  }
+  if (!ellpack_page_  || (batch_param_ != param && param != BatchParam{})) {
+    CHECK_GE(param.gpu_id, 0);
+    CHECK_GE(param.max_bin, 2);
     ellpack_page_.reset(new EllpackPage(this, param));
+    batch_param_ = param;
   }
   auto begin_iter =
       BatchIterator<EllpackPage>(new SimpleBatchIteratorImpl<EllpackPage>(ellpack_page_.get()));

src/data/simple_dmatrix.h

@@ -48,6 +48,14 @@ class SimpleDMatrix : public DMatrix {
   std::unique_ptr<CSCPage> column_page_;
   std::unique_ptr<SortedCSCPage> sorted_column_page_;
   std::unique_ptr<EllpackPage> ellpack_page_;
+  BatchParam batch_param_;
+
+  bool EllpackExists() const override {
+    return static_cast<bool>(ellpack_page_);
+  }
+  bool SparsePageExists() const override {
+    return true;
+  }
 };
 }  // namespace data
 }  // namespace xgboost

src/data/sparse_page_dmatrix.cc

@@ -1,5 +1,5 @@
 /*!
- * Copyright 2014 by Contributors
+ * Copyright 2014-2020 by Contributors
  * \file sparse_page_dmatrix.cc
  * \brief The external memory version of Page Iterator.
  * \author Tianqi Chen
@@ -47,7 +47,7 @@ BatchSet<EllpackPage> SparsePageDMatrix::GetEllpackBatches(const BatchParam& par
   CHECK_GE(param.gpu_id, 0);
   CHECK_GE(param.max_bin, 2);
   // Lazily instantiate
-  if (!ellpack_source_ || batch_param_ != param) {
+  if (!ellpack_source_ || (batch_param_ != param && param != BatchParam{})) {
     ellpack_source_.reset(new EllpackPageSource(this, cache_info_, param));
     batch_param_ = param;
   }

src/data/sparse_page_dmatrix.h

@@ -58,6 +58,13 @@ class SparsePageDMatrix : public DMatrix {
   std::string cache_info_;
   // Store column densities to avoid recalculating
   std::vector<float> col_density_;
+
+  bool EllpackExists() const override {
+    return static_cast<bool>(ellpack_source_);
+  }
+  bool SparsePageExists() const override {
+    return static_cast<bool>(row_source_);
+  }
 };
 }  // namespace data
 }  // namespace xgboost