diff --git a/python/xgboost_wrapper.cpp b/python/xgboost_wrapper.cpp
index 7f2365ba3..df05d9521 100644
--- a/python/xgboost_wrapper.cpp
+++ b/python/xgboost_wrapper.cpp
@@ -37,7 +37,7 @@ class Booster: public learner::BoostLearner<FMatrixS> {
     for (unsigned j = 0; j < ndata; ++j) {
       gpair_[j] = bst_gpair(grad[j], hess[j]);
     }
-    gbm_->DoBoost(gpair_, train.fmat, train.info.root_index);
+    gbm_->DoBoost(gpair_, train.fmat, train.info.info);
   }
   inline void CheckInitModel(void) {
     if (!init_model) {
@@ -151,8 +151,8 @@ extern "C"{
       if (src.info.weights.size() != 0) {
         ret.info.weights.push_back(src.info.weights[ridx]);
       }
-      if (src.info.root_index.size() != 0) {
-        ret.info.weights.push_back(src.info.root_index[ridx]);
+      if (src.info.info.root_index.size() != 0) {
+        ret.info.info.root_index.push_back(src.info.info.root_index[ridx]);
       }
     }
     return p_ret;
diff --git a/src/data.h b/src/data.h
index fea3f7a48..61d61e6a0 100644
--- a/src/data.h
+++ b/src/data.h
@@ -39,6 +39,24 @@ struct bst_gpair {
   bst_gpair(bst_float grad, bst_float hess) : grad(grad), hess(hess) {}
 };
 
+/*! 
+ * \brief extra information that might needed by gbm and tree module
+ * these information are not necessarily presented, and can be empty
+ */
+struct BoosterInfo {
+  /*!
+   * \brief specified root index of each instance,
+   *  can be used for multi task setting
+   */
+  std::vector<unsigned> root_index;
+  /*! \brief set fold indicator */
+  std::vector<unsigned> fold_index;
+  /*! \brief get root of ith instance */
+  inline unsigned GetRoot(size_t i) const {
+    return root_index.size() == 0 ? 0 : root_index[i];
+  }
+};
+
 /*! \brief read-only sparse instance batch in CSR format */
 struct SparseBatch {
   /*! \brief an entry of sparse vector */
diff --git a/src/gbm/gbm.h b/src/gbm/gbm.h
index dcc204868..f47adfdd2 100644
--- a/src/gbm/gbm.h
+++ b/src/gbm/gbm.h
@@ -43,12 +43,11 @@ class IGradBooster {
    * \brief peform update to the model(boosting)
    * \param gpair the gradient pair statistics of the data
    * \param fmat feature matrix that provide access to features
-   * \param root_index pre-partitioned root_index of each instance,
-   *   root_index.size() can be 0 which indicates that no pre-partition involved
+   * \param info meta information about training
    */
   virtual void DoBoost(const std::vector<bst_gpair> &gpair,
                        const FMatrix &fmat,
-                       const std::vector<unsigned> &root_index) = 0;
+                       const BoosterInfo &info) = 0;
   /*!
    * \brief generate predictions for given feature matrix
    * \param fmat feature matrix
@@ -56,13 +55,12 @@ class IGradBooster {
    *        this means we do not have buffer index allocated to the gbm
    *  a buffer index is assigned to each instance that requires repeative prediction
    *  the size of buffer is set by convention using IGradBooster.SetParam("num_pbuffer","size")
-   * \param root_index pre-partitioned root_index of each instance,
-   *   root_index.size() can be 0 which indicates that no pre-partition involved
+   * \param info extra side information that may be needed for prediction
    * \param out_preds output vector to hold the predictions
    */
   virtual void Predict(const FMatrix &fmat,
                        int64_t buffer_offset,
-                       const std::vector<unsigned> &root_index,
+                       const BoosterInfo &info,
                        std::vector<float> *out_preds) = 0;
   /*!
    * \brief dump the model in text format
diff --git a/src/gbm/gbtree-inl.hpp b/src/gbm/gbtree-inl.hpp
index 876e13c2d..3fa0f4dd7 100644
--- a/src/gbm/gbtree-inl.hpp
+++ b/src/gbm/gbtree-inl.hpp
@@ -84,9 +84,9 @@ class GBTree : public IGradBooster<FMatrix> {
   }
   virtual void DoBoost(const std::vector<bst_gpair> &gpair,
                        const FMatrix &fmat,
-                       const std::vector<unsigned> &root_index) {
+                       const BoosterInfo &info) {
     if (mparam.num_output_group == 1) {
-      this->BoostNewTrees(gpair, fmat, root_index, 0);
+      this->BoostNewTrees(gpair, fmat, info, 0);
     } else {
       const int ngroup = mparam.num_output_group;
       utils::Check(gpair.size() % ngroup == 0,
@@ -97,13 +97,13 @@ class GBTree : public IGradBooster<FMatrix> {
         for (size_t i = 0; i < tmp.size(); ++i) {
           tmp[i] = gpair[i * ngroup + gid];
         }
-        this->BoostNewTrees(tmp, fmat, root_index, gid);
+        this->BoostNewTrees(tmp, fmat, info, gid);
       }
     }
   }
   virtual void Predict(const FMatrix &fmat,
                        int64_t buffer_offset,
-                       const std::vector<unsigned> &root_index,
+                       const BoosterInfo &info,
                        std::vector<float> *out_preds) {
     int nthread;
     #pragma omp parallel
@@ -134,7 +134,7 @@ class GBTree : public IGradBooster<FMatrix> {
         const int tid = omp_get_thread_num();
         tree::RegTree::FVec &feats = thread_temp[tid];
         const size_t ridx = batch.base_rowid + i;
-        const unsigned root_idx = root_index.size() == 0 ? 0 : root_index[ridx];
+        const unsigned root_idx = info.GetRoot(i);
         // loop over output groups
         for (int gid = 0; gid < mparam.num_output_group; ++gid) {
           preds[ridx * mparam.num_output_group + gid] =
@@ -186,7 +186,7 @@ class GBTree : public IGradBooster<FMatrix> {
   // do group specific group
   inline void BoostNewTrees(const std::vector<bst_gpair> &gpair,
                             const FMatrix &fmat,
-                            const std::vector<unsigned> &root_index,
+                            const BoosterInfo &info,
                             int bst_group) {
     this->InitUpdater();
     // create the trees
@@ -200,7 +200,7 @@ class GBTree : public IGradBooster<FMatrix> {
     }
     // update the trees
     for (size_t i = 0; i < updaters.size(); ++i) {
-      updaters[i]->Update(gpair, fmat, root_index, new_trees);
+      updaters[i]->Update(gpair, fmat, info, new_trees);
     }
     // push back to model
     for (size_t i = 0; i < new_trees.size(); ++i) {
diff --git a/src/learner/dmatrix.h b/src/learner/dmatrix.h
index 5786fe6a1..b66cf86d0 100644
--- a/src/learner/dmatrix.h
+++ b/src/learner/dmatrix.h
@@ -28,11 +28,8 @@ struct MetaInfo {
   std::vector<bst_uint> group_ptr;
   /*! \brief weights of each instance, optional */
   std::vector<float> weights;
-  /*!
-   * \brief specified root index of each instance,
-   *  can be used for multi task setting
-   */
-  std::vector<unsigned> root_index;
+  /*! \brief information needed by booster */
+  BoosterInfo info;
   /*! 
    * \brief initialized margins,
    * if specified, xgboost will start from this init margin
@@ -48,7 +45,7 @@ struct MetaInfo {
     labels.clear();
     group_ptr.clear();
     weights.clear();
-    root_index.clear();
+    info.root_index.clear();
     base_margin.clear();
     num_row = num_col = 0;
   }
@@ -60,14 +57,6 @@ struct MetaInfo {
       return 1.0f;
     }
   }
-  /*! \brief get root index of i-th instance */
-  inline float GetRoot(size_t i) const {
-    if (root_index.size() != 0) {
-      return static_cast<float>(root_index[i]);
-    } else {
-      return 0;
-    }
-  }
   inline void SaveBinary(utils::IStream &fo) const {
     int version = kVersion;
     fo.Write(&version, sizeof(version));
@@ -76,7 +65,7 @@ struct MetaInfo {
     fo.Write(labels);
     fo.Write(group_ptr);
     fo.Write(weights);
-    fo.Write(root_index);
+    fo.Write(info.root_index);
     fo.Write(base_margin);
   }
   inline void LoadBinary(utils::IStream &fi) {
@@ -87,7 +76,7 @@ struct MetaInfo {
     utils::Check(fi.Read(&labels), "MetaInfo: invalid format");
     utils::Check(fi.Read(&group_ptr), "MetaInfo: invalid format");
     utils::Check(fi.Read(&weights), "MetaInfo: invalid format");
-    utils::Check(fi.Read(&root_index), "MetaInfo: invalid format");
+    utils::Check(fi.Read(&info.root_index), "MetaInfo: invalid format");
     utils::Check(fi.Read(&base_margin), "MetaInfo: invalid format");
   }
   // try to load group information from file, if exists
diff --git a/src/learner/learner-inl.hpp b/src/learner/learner-inl.hpp
index bd5cf6e3b..18611bea5 100644
--- a/src/learner/learner-inl.hpp
+++ b/src/learner/learner-inl.hpp
@@ -161,7 +161,7 @@ class BoostLearner {
   inline void UpdateOneIter(int iter, const DMatrix<FMatrix> &train) {
     this->PredictRaw(train, &preds_);
     obj_->GetGradient(preds_, train.info, iter, &gpair_);
-    gbm_->DoBoost(gpair_, train.fmat, train.info.root_index);
+    gbm_->DoBoost(gpair_, train.fmat, train.info.info);
   }
   /*!
    * \brief evaluate the model for specific iteration
@@ -242,7 +242,7 @@ class BoostLearner {
   inline void PredictRaw(const DMatrix<FMatrix> &data,
                          std::vector<float> *out_preds) const {
     gbm_->Predict(data.fmat, this->FindBufferOffset(data),
-                  data.info.root_index, out_preds);
+                  data.info.info, out_preds);
     // add base margin
     std::vector<float> &preds = *out_preds;
     const unsigned ndata = static_cast<unsigned>(preds.size());
diff --git a/src/tree/updater.h b/src/tree/updater.h
index 2664e2ebd..cdb625266 100644
--- a/src/tree/updater.h
+++ b/src/tree/updater.h
@@ -29,8 +29,7 @@ class IUpdater {
    * \brief peform update to the tree models
    * \param gpair the gradient pair statistics of the data
    * \param fmat feature matrix that provide access to features
-   * \param root_index pre-partitioned root_index of each instance,
-   *          root_index.size() can be 0 which indicates that no pre-partition involved
+   * \param info extra side information that may be need, such as root index
    * \param trees pointer to the trese to be updated, upater will change the content of the tree
    *   note: all the trees in the vector are updated, with the same statistics, 
    *         but maybe different random seeds, usually one tree is passed in at a time, 
@@ -38,7 +37,7 @@ class IUpdater {
    */
   virtual void Update(const std::vector<bst_gpair> &gpair,
                       const FMatrix &fmat,
-                      const std::vector<unsigned> &root_index,
+                      const BoosterInfo &info,
                       const std::vector<RegTree*> &trees) = 0;
   // destructor
   virtual ~IUpdater(void) {}
diff --git a/src/tree/updater_colmaker-inl.hpp b/src/tree/updater_colmaker-inl.hpp
index 35880b70e..919dfcc28 100644
--- a/src/tree/updater_colmaker-inl.hpp
+++ b/src/tree/updater_colmaker-inl.hpp
@@ -25,7 +25,7 @@ class ColMaker: public IUpdater<FMatrix> {
   }
   virtual void Update(const std::vector<bst_gpair> &gpair,
                       const FMatrix &fmat,
-                      const std::vector<unsigned> &root_index,
+                      const BoosterInfo &info,
                       const std::vector<RegTree*> &trees) {
     // rescale learning rate according to size of trees
     float lr = param.learning_rate;
@@ -33,7 +33,7 @@ class ColMaker: public IUpdater<FMatrix> {
     // build tree
     for (size_t i = 0; i < trees.size(); ++i) {
       Builder builder(param);
-      builder.Update(gpair, fmat, root_index, trees[i]);
+      builder.Update(gpair, fmat, info, trees[i]);
     }
     param.learning_rate = lr;
   }
@@ -77,9 +77,9 @@ class ColMaker: public IUpdater<FMatrix> {
     // update one tree, growing
     virtual void Update(const std::vector<bst_gpair> &gpair,
                         const FMatrix &fmat,
-                        const std::vector<unsigned> &root_index,
+                        const BoosterInfo &info,
                         RegTree *p_tree) {
-      this->InitData(gpair, fmat, root_index, *p_tree);
+      this->InitData(gpair, fmat, info.root_index, *p_tree);
       this->InitNewNode(qexpand, gpair, fmat, *p_tree);
       
       for (int depth = 0; depth < param.max_depth; ++depth) {
diff --git a/src/tree/updater_prune-inl.hpp b/src/tree/updater_prune-inl.hpp
index 363d6eec1..d92685ad7 100644
--- a/src/tree/updater_prune-inl.hpp
+++ b/src/tree/updater_prune-inl.hpp
@@ -24,7 +24,7 @@ class TreePruner: public IUpdater<FMatrix> {
   // update the tree, do pruning
   virtual void Update(const std::vector<bst_gpair> &gpair,
                       const FMatrix &fmat,
-                      const std::vector<unsigned> &root_index,
+                      const BoosterInfo &info,
                       const std::vector<RegTree*> &trees) {
     // rescale learning rate according to size of trees
     float lr = param.learning_rate;
diff --git a/src/tree/updater_refresh-inl.hpp b/src/tree/updater_refresh-inl.hpp
index 12bbcf864..e23174e51 100644
--- a/src/tree/updater_refresh-inl.hpp
+++ b/src/tree/updater_refresh-inl.hpp
@@ -24,7 +24,7 @@ class TreeRefresher: public IUpdater<FMatrix> {
   // update the tree, do pruning
   virtual void Update(const std::vector<bst_gpair> &gpair,
                       const FMatrix &fmat,
-                      const std::vector<unsigned> &root_index,
+                      const BoosterInfo &info,
                       const std::vector<RegTree*> &trees) {
     if (trees.size() == 0) return;
     // number of threads
@@ -66,7 +66,7 @@ class TreeRefresher: public IUpdater<FMatrix> {
         feats.Fill(inst);
         for (size_t j = 0; j < trees.size(); ++j) {
           AddStats(*trees[j], feats, gpair[ridx],
-                   root_index.size() == 0 ? 0 : root_index[ridx],
+                   info.GetRoot(j),
                    &stemp[tid * trees.size() + j]);
         }
         feats.Drop(inst);