change hist update to lazy

Makefile

@@ -10,6 +10,12 @@ else
 	CFLAGS += -fopenmp
 endif
 
+# by default use c++11
+ifeq ($(no_cxx11),1)
+else 
+	CFLAGS += -std=c++11
+endif
+
 # specify tensor path
 BIN = xgboost
 OBJ = updater.o gbm.o io.o main.o 

src/tree/updater_histmaker-inl.hpp

@@ -306,6 +306,12 @@ class CQHistMaker: public HistMaker<TStats> {
     } 
     // start to work
     this->wspace.Init(this->param, 1);
+    // if it is C++11, use lazy evaluation for Allreduce,
+    // to gain speedup in recovery
+#if __cplusplus >= 201103L
+    auto lazy_get_hist = [&]()
+#endif
+    {
       thread_hist.resize(this->get_nthread());
       // start accumulating statistics
       utils::IIterator<ColBatch> *iter = p_fmat->ColIterator(fset);
@@ -330,8 +336,15 @@ class CQHistMaker: public HistMaker<TStats> {
         this->wspace.hset[0][fset.size() + wid * (fset.size()+1)]
             .data[0] = node_stats[nid];
       }
+    };
     // sync the histogram
+    // if it is C++11, use lazy evaluation for Allreduce
+#if __cplusplus >= 201103L
+    this->histred.Allreduce(BeginPtr(this->wspace.hset[0].data), 
+                            this->wspace.hset[0].data.size(), lazy_get_hist);
+#else
     this->histred.Allreduce(BeginPtr(this->wspace.hset[0].data), this->wspace.hset[0].data.size());   
+#endif    
   }
   virtual void ResetPositionAfterSplit(IFMatrix *p_fmat,
                                        const RegTree &tree) {
@@ -354,13 +367,24 @@ class CQHistMaker: public HistMaker<TStats> {
         feat2workindex[fset[i]] = -2;  
       }
     }      
-        
     this->GetNodeStats(gpair, *p_fmat, tree, info,
                        &thread_stats, &node_stats);       
     sketchs.resize(this->qexpand.size() * freal_set.size());
     for (size_t i = 0; i < sketchs.size(); ++i) {
       sketchs[i].Init(info.num_row, this->param.sketch_eps);
     }
+    // intitialize the summary array
+    summary_array.resize(sketchs.size());
+    // setup maximum size
+    unsigned max_size = this->param.max_sketch_size();
+    for (size_t i = 0; i < sketchs.size(); ++i) {
+      summary_array[i].Reserve(max_size);
+    }
+    // if it is C++11, use lazy evaluation for Allreduce
+#if __cplusplus >= 201103L
+    auto lazy_get_summary = [&]()
+#endif
+    {// get smmary
       thread_sketch.resize(this->get_nthread());
       // number of rows in
       const size_t nrows = p_fmat->buffered_rowset().size();
@@ -383,19 +407,20 @@ class CQHistMaker: public HistMaker<TStats> {
           }
         }
       }
-    // setup maximum size
-    unsigned max_size = this->param.max_sketch_size();
-    // synchronize sketch
-    summary_array.resize(sketchs.size());
       for (size_t i = 0; i < sketchs.size(); ++i) {
         utils::WXQuantileSketch<bst_float, bst_float>::SummaryContainer out;
         sketchs[i].GetSummary(&out);
-      summary_array[i].Reserve(max_size);
         summary_array[i].SetPrune(out, max_size);
       }
+      utils::Assert(summary_array.size() == sketchs.size(), "shape mismatch");
+    };
     if (summary_array.size() != 0) {
       size_t nbytes = WXQSketch::SummaryContainer::CalcMemCost(max_size);
+#if __cplusplus >= 201103L
+      sreducer.Allreduce(BeginPtr(summary_array), nbytes, summary_array.size(), lazy_get_summary);
+#else
       sreducer.Allreduce(BeginPtr(summary_array), nbytes, summary_array.size());
+#endif
     }
     // now we get the final result of sketch, setup the cut
     this->wspace.cut.clear();
@@ -623,6 +648,7 @@ class QuantileHistMaker: public HistMaker<TStats> {
       summary_array[i].Reserve(max_size);
       summary_array[i].SetPrune(out, max_size);
     }
+    
     size_t nbytes = WXQSketch::SummaryContainer::CalcMemCost(max_size);    
     sreducer.Allreduce(BeginPtr(summary_array), nbytes, summary_array.size());
     // now we get the final result of sketch, setup the cut

src/tree/updater_refresh-inl.hpp

@@ -52,6 +52,12 @@ class TreeRefresher: public IUpdater {
       std::fill(stemp[tid].begin(), stemp[tid].end(), TStats(param));
       fvec_temp[tid].Init(trees[0]->param.num_feature);
     }
+    // if it is C++11, use lazy evaluation for Allreduce,
+    // to gain speedup in recovery
+#if __cplusplus >= 201103L
+    auto lazy_get_stats = [&]()
+#endif
+    {
       // start accumulating statistics
       utils::IIterator<RowBatch> *iter = p_fmat->RowIterator();
       iter->BeforeFirst();
@@ -84,8 +90,12 @@ class TreeRefresher: public IUpdater {
           stemp[0][nid].Add(stemp[tid][nid]);
         }
       }
-    // AllReduce, add statistics up
+    };
+#if __cplusplus >= 201103L
+    reducer.Allreduce(BeginPtr(stemp[0]), stemp[0].size(), lazy_get_stats);
+#else
     reducer.Allreduce(BeginPtr(stemp[0]), stemp[0].size());
+#endif
     // rescale learning rate according to size of trees
     float lr = param.learning_rate;
     param.learning_rate = lr / trees.size();