diff --git a/Makefile b/Makefile
index b952dbf5d..bdc4fb583 100644
--- a/Makefile
+++ b/Makefile
@@ -14,7 +14,7 @@ endif
 BIN = 
 OBJ = updater.o gbm.o io.o main.o
 MPIOBJ = sync.o
-MPIBIN = test/test xgboost
+MPIBIN = xgboost
 SLIB = #wrapper/libxgboostwrapper.so 
 
 .PHONY: clean all python Rpack
diff --git a/src/tree/updater_colmaker-inl.hpp b/src/tree/updater_colmaker-inl.hpp
index 347326fe7..6db19732e 100644
--- a/src/tree/updater_colmaker-inl.hpp
+++ b/src/tree/updater_colmaker-inl.hpp
@@ -132,17 +132,17 @@ class ColMaker: public IUpdater {
         // mark delete for the deleted datas
         for (size_t i = 0; i < rowset.size(); ++i) {
           const bst_uint ridx = rowset[i];
-          if (gpair[ridx].hess < 0.0f) position[ridx] = -1;
+          if (gpair[ridx].hess < 0.0f) position[ridx] = ~position[ridx];
         }
         // mark subsample
         if (param.subsample < 1.0f) {
           for (size_t i = 0; i < rowset.size(); ++i) {
             const bst_uint ridx = rowset[i];
             if (gpair[ridx].hess < 0.0f) continue;
-            if (random::SampleBinary(param.subsample) == 0) position[ridx] = -1;
+            if (random::SampleBinary(param.subsample) == 0) position[ridx] = ~position[ridx];
           }
         }
-      }    
+      }
       {
         // initialize feature index
         unsigned ncol = static_cast<unsigned>(fmat.NumCol());
@@ -473,6 +473,9 @@ class ColMaker: public IUpdater {
         if (e.best.loss_chg > rt_eps) {
           p_tree->AddChilds(nid);
           (*p_tree)[nid].set_split(e.best.split_index(), e.best.split_value, e.best.default_left());
+          // mark right child as 0, to indicate fresh leaf
+          (*p_tree)[(*p_tree)[nid].cleft()].set_leaf(0.0f, 0);
+          (*p_tree)[(*p_tree)[nid].cright()].set_leaf(0.0f, 0);
         } else {
           (*p_tree)[nid].set_leaf(e.weight * param.learning_rate);
         }
@@ -480,28 +483,33 @@ class ColMaker: public IUpdater {
     }
     // reset position of each data points after split is created in the tree
     inline void ResetPosition(const std::vector<int> &qexpand, IFMatrix *p_fmat, const RegTree &tree) {
+      // set the positions in the nondefault
+      this->SetNonDefaultPosition(qexpand, p_fmat, tree);      
+      // set rest of instances to default position
       const std::vector<bst_uint> &rowset = p_fmat->buffered_rowset();
-      // step 1, set default direct nodes to default, and leaf nodes to -1
+      // set default direct nodes to default
+      // for leaf nodes that are not fresh, mark then to ~nid, 
+      // so that they are ignored in future statistics collection
       const bst_omp_uint ndata = static_cast<bst_omp_uint>(rowset.size());
+      
       #pragma omp parallel for schedule(static)
       for (bst_omp_uint i = 0; i < ndata; ++i) {
         const bst_uint ridx = rowset[i];
-        int nid = position[ridx];
-        if (nid < 0)  nid = ~nid;        
+        const int nid = this->DecodePosition(ridx);
         if (tree[nid].is_leaf()) {
-          position[ridx] = ~nid;          
+          // mark finish when it is not a fresh leaf
+          if (tree[nid].cright() == -1) {
+            position[ridx] = ~nid;
+          }
         } else {
-          // push to default branch, correct latter
-          int pid = tree[nid].default_left() ? tree[nid].cleft(): tree[nid].cright();
-          if (position[ridx] < 0) {
-            position[ridx] = ~pid;
+          // push to default branch
+          if (tree[nid].default_left()) {
+            this->SetEncodePosition(ridx, tree[nid].cleft());
           } else {
-            position[ridx] = pid;
+            this->SetEncodePosition(ridx, tree[nid].cright());
           }
         }
       }
-      // set the positions in the nondefault places
-      this->SetNonDefaultPosition(qexpand, p_fmat, tree);
     }
     // customization part
     // synchronize the best solution of each node
@@ -516,7 +524,7 @@ class ColMaker: public IUpdater {
     }
     virtual void SetNonDefaultPosition(const std::vector<int> &qexpand,
                                        IFMatrix *p_fmat, const RegTree &tree) {
-      // step 2, classify the non-default data into right places
+      // step 1, classify the non-default data into right places
       std::vector<unsigned> fsplits;
       for (size_t i = 0; i < qexpand.size(); ++i) {
         const int nid = qexpand[i];
@@ -538,22 +546,33 @@ class ColMaker: public IUpdater {
           for (bst_omp_uint j = 0; j < ndata; ++j) {
             const bst_uint ridx = col[j].index;
             const float fvalue = col[j].fvalue;
-            int nid = position[ridx];
-            if (nid < 0)  nid = ~nid;            
-
+            const int nid = this->DecodePosition(ridx);
             // go back to parent, correct those who are not default
-            nid = tree[nid].parent();
-            if (tree[nid].split_index() == fid) {
-              if (fvalue < tree[nid].split_cond()) {
-                position[ridx] = tree[nid].cleft();
+            if (!tree[nid].is_leaf() && tree[nid].split_index() == fid) {
+              if(fvalue < tree[nid].split_cond()) {
+                this->SetEncodePosition(ridx, tree[nid].cleft());
               } else {
-                position[ridx] = tree[nid].cright();
+                this->SetEncodePosition(ridx, tree[nid].cright());
               }
             }
           }
         }
       }
     }
+    // utils to get/set position, with encoded format
+    // return decoded position
+    inline int DecodePosition(bst_uint ridx) const{
+      const int pid = position[ridx];
+      return pid < 0 ? ~pid : pid;
+    }
+    // encode the encoded position value for ridx
+    inline void SetEncodePosition(bst_uint ridx, int nid) {
+      if (position[ridx] < 0) {
+        position[ridx] = ~nid;
+      } else {
+        position[ridx] = nid;
+      }
+    }
     //--data fields--
     const TrainParam &param;
     // number of omp thread used during training
diff --git a/src/tree/updater_distcol-inl.hpp b/src/tree/updater_distcol-inl.hpp
index aae084561..e5d1450a5 100644
--- a/src/tree/updater_distcol-inl.hpp
+++ b/src/tree/updater_distcol-inl.hpp
@@ -100,11 +100,8 @@ class DistColMaker : public ColMaker<TStats> {
           for (bst_omp_uint j = 0; j < ndata; ++j) {
             const bst_uint ridx = col[j].index;
             const float fvalue = col[j].fvalue;
-            int nid = this->position[ridx];
-            if (nid < 0) continue;
-            // go back to parent, correct those who are not default
-            nid = tree[nid].parent();
-            if (tree[nid].split_index() == fid) {
+            const int nid = this->DecodePosition(ridx);
+            if (!tree[nid].is_leaf() && tree[nid].split_index() == fid) {
               if (fvalue < tree[nid].split_cond()) {
                 if (!tree[nid].default_left()) bitmap.SetTrue(ridx);
               } else {
@@ -122,13 +119,13 @@ class DistColMaker : public ColMaker<TStats> {
       #pragma omp parallel for schedule(static)
       for (bst_omp_uint i = 0; i < ndata; ++i) {
         const bst_uint ridx = rowset[i];
-        int nid = this->position[ridx];
-        if (nid >= 0 && bitmap.Get(ridx)) {
-          nid = tree[nid].parent();
+        const int nid = this->DecodePosition(ridx);
+        if (bitmap.Get(ridx)) {
+          utils::Assert(!tree[nid].is_leaf(), "inconsistent reduce information");
           if (tree[nid].default_left()) {
-            this->position[ridx] = tree[nid].cright();
+            this->SetEncodePosition(ridx, tree[nid].cright());
           } else {
-            this->position[ridx] = tree[nid].cleft();
+            this->SetEncodePosition(ridx, tree[nid].cright());
           }
         }
       }