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

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,
-                 GHistRowT hist);
-  // construct a histogram via subtraction trick
-  void SubtractionTrick(GHistRowT self,
-                        GHistRowT sibling,
-                        GHistRowT parent);
-
+                 const GHistIndexMatrix &gmat, GHistRowT hist) const;
   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 };
 };