Support column split in histogram builder (#8811)

src/tree/hist/histogram.h

@@ -29,6 +29,7 @@ class HistogramBuilder {
   size_t n_batches_{0};
   // Whether XGBoost is running in distributed environment.
   bool is_distributed_{false};
+  bool is_col_split_{false};
 
  public:
   /**
@@ -40,7 +41,7 @@ class HistogramBuilder {
    *                         of using global rabit variable.
    */
   void Reset(uint32_t total_bins, BatchParam p, int32_t n_threads, size_t n_batches,
-             bool is_distributed) {
+             bool is_distributed, bool is_col_split) {
     CHECK_GE(n_threads, 1);
     n_threads_ = n_threads;
     n_batches_ = n_batches;
@@ -50,6 +51,7 @@ class HistogramBuilder {
     buffer_.Init(total_bins);
     builder_ = common::GHistBuilder(total_bins);
     is_distributed_ = is_distributed;
+    is_col_split_ = is_col_split;
     // Workaround s390x gcc 7.5.0
     auto DMLC_ATTRIBUTE_UNUSED __force_instantiation = &GradientPairPrecise::Reduce;
   }
@@ -130,7 +132,7 @@ class HistogramBuilder {
       return;
     }
 
-    if (is_distributed_) {
+    if (is_distributed_ && !is_col_split_) {
       this->SyncHistogramDistributed(p_tree, nodes_for_explicit_hist_build,
                                      nodes_for_subtraction_trick,
                                      starting_index, sync_count);

src/tree/updater_approx.cc

@@ -76,7 +76,7 @@ class GloablApproxBuilder {
     }
 
     histogram_builder_.Reset(n_total_bins, BatchSpec(param_, hess), ctx_->Threads(), n_batches_,
-                             collective::IsDistributed());
+                             collective::IsDistributed(), p_fmat->IsColumnSplit());
     monitor_->Stop(__func__);
   }
 

src/tree/updater_quantile_hist.cc

@@ -281,7 +281,7 @@ void QuantileHistMaker::Builder::InitData(DMatrix *fmat, const RegTree &tree,
       ++page_id;
     }
     histogram_builder_->Reset(n_total_bins, HistBatch(param_), ctx_->Threads(), page_id,
-                              collective::IsDistributed());
+                              collective::IsDistributed(), fmat->IsColumnSplit());
 
     auto m_gpair =
         linalg::MakeTensorView(*gpair, {gpair->size(), static_cast<std::size_t>(1)}, ctx_->gpu_id);