diff --git a/src/utils/quantile.h b/src/utils/quantile.h
index 0fadd9c35..e51dc16ba 100644
--- a/src/utils/quantile.h
+++ b/src/utils/quantile.h
@@ -9,6 +9,7 @@
 #include <vector>
 #include <cstring>
 #include <algorithm>
+#include <iostream>
 #include "./utils.h"
 
 namespace xgboost {
@@ -100,7 +101,6 @@ struct WQSummary {
       }
     }
   };
-
   /*! \brief data field */
   Entry *data;
   /*! \brief number of elements in the summary */
@@ -123,19 +123,15 @@ struct WQSummary {
   inline RType MaxRank(void) const {
     return data[size - 1].rmax;
   }
-  /*! \brief set size to 0 */
-  inline void Clear(void) {
-    size = 0;
-  }
   /*!
    * \brief copy content from src
    * \param src source sketch
    */
   inline void CopyFrom(const WQSummary &src) {
     size = src.size;
-    std::memcpy(data, src.data, sizeof(Entry) * size);
-  
-}  /*! 
+    std::memcpy(data, src.data, sizeof(Entry) * size);    
+  }  
+  /*! 
    * \brief debug function, validate whether the summary 
    *  run consistency check to check if it is a valid summary
    * \param eps the tolerate error level, used when RType is floating point and 
@@ -153,9 +149,9 @@ struct WQSummary {
   /*! \brief used for debug purpose, print the summary */
   inline void Print(void) const {
     for (size_t i = 0; i < size; ++i) {
-      printf("x=%f\t[%f, %f] wmin=%f\n", 
-             data[i].value, data[i].rmin,
-             data[i].rmax, data[i].wmin);
+      std::cout << "x=" << data[i].value << "\t"
+                << "[" << data[i].rmin << "," << data[i].rmax << "]"
+                << " wmin=" << data[i].wmin << std::endl;
     }
   }
   /*!
@@ -250,6 +246,147 @@ struct WQSummary {
   }
 };
 
+/*! 
+ * \brief traditional GK summary
+ */
+template<typename DType, typename RType>
+struct GKSummary {
+  /*! \brief an entry in the sketch summary */
+  struct Entry {
+    /*! \brief minimum rank */
+    RType rmin;
+    /*! \brief maximum rank */
+    RType rmax;
+    /*! \brief the value of data */
+    DType value;
+    // constructor
+    Entry(void) {}
+    // constructor
+    Entry(RType rmin, RType rmax, DType value)
+        : rmin(rmin), rmax(rmax), value(value) {}
+  };
+  /*! \brief input data queue before entering the summary */
+  struct Queue {
+    // the input queue
+    std::vector<DType> queue;
+    // end of the queue
+    size_t qtail;
+    // push data to the queue
+    inline void Push(DType x, RType w) {
+      queue[qtail++] = x;
+    }   
+    inline void MakeSummary(GKSummary *out) {
+      std::sort(queue.begin(), queue.begin() + qtail);
+      out->size = qtail;
+      for (size_t i = 0; i < qtail; ++i) {
+        out->data[i] = Entry(i + 1, i + 1, queue[i]);
+      }
+    }
+  };
+  /*! \brief data field */
+  Entry *data;
+  /*! \brief number of elements in the summary */
+  size_t size;
+  GKSummary(Entry *data, size_t size)
+      : data(data), size(size) {} 
+  /*! \brief the maximum error of the summary */
+  inline RType MaxError(void) const {
+    RType res = 0;
+    for (size_t i = 1; i < size; ++i) {
+      res = std::max(data[i].rmax - data[i-1].rmin, res);
+    }
+    return res;
+  }
+  /*! \return maximum rank in the summary */
+  inline RType MaxRank(void) const {
+    return data[size - 1].rmax;
+  }
+  /*! 
+   * \brief copy content from src
+   * \param src source sketch
+   */
+  inline void CopyFrom(const GKSummary &src) {
+    size = src.size;
+    std::memcpy(data, src.data, sizeof(Entry) * size);
+  }
+  inline void CheckValid(RType eps) const {
+    // assume always valid
+  }
+  /*! \brief used for debug purpose, print the summary */
+  inline void Print(void) const {
+    for (size_t i = 0; i < size; ++i) {
+      std::cout << "x=" << data[i].value << "\t"
+                << "[" << data[i].rmin << "," << data[i].rmax << "]"
+                << std::endl;
+    }
+  }  
+  /*! 
+   * \brief set current summary to be pruned summary of src
+   *        assume data field is already allocated to be at least maxsize
+   * \param src source summary
+   * \param maxsize size we can afford in the pruned sketch
+   */
+  inline void SetPrune(const GKSummary &src, RType maxsize) {
+    if (src.size <= maxsize) {
+      this->CopyFrom(src); return;
+    }
+    const RType max_rank = src.MaxRank();
+    this->size = maxsize;
+    data[0] = src.data[0];
+    size_t n = maxsize - 1;
+    RType top = 1;
+    for (size_t i = 1; i < n; ++i) {
+      RType k = (i * max_rank) / n;
+      while (k > src.data[top + 1].rmax) ++top;
+      // assert src.data[top].rmin <= k
+      // because k > src.data[top].rmax >= src.data[top].rmin
+      if ((k - src.data[top].rmin) < (src.data[top+1].rmax - k)) {
+        data[i] = src.data[top];
+      } else {
+        data[i] = src.data[top + 1];
+      }
+    }
+    data[n] = src.data[src.size - 1];
+  }
+  inline void SetCombine(const GKSummary &sa,
+                         const GKSummary &sb) {
+    utils::Assert(sa.size > 0 && sb.size > 0, "invalid input for merge"); 
+    const Entry *a = sa.data, *a_end = sa.data + sa.size;
+    const Entry *b = sb.data, *b_end = sb.data + sb.size;
+    this->size = sa.size + sb.size;
+    RType aprev_rmin = 0, bprev_rmin = 0;
+    Entry *dst = this->data;
+    while (a != a_end && b != b_end) {
+      if (a->value < b->value) {
+        *dst = Entry(bprev_rmin + a->rmin,
+                     a->rmax + b->rmax - 1, a->value);
+        aprev_rmin = a->rmin;
+        ++dst; ++a;
+      } else {
+        *dst = Entry(aprev_rmin + b->rmin, 
+                     b->rmax + a->rmax - 1, b->value);
+        bprev_rmin = b->rmin;
+        ++dst; ++b;
+      }
+    }
+    if (a != a_end) {
+      RType bprev_rmax = (b_end - 1)->rmax;
+      do {
+        *dst = Entry(bprev_rmin + a->rmin, bprev_rmax + a->rmax, a->value);
+        ++dst; ++a;
+      } while (a != a_end);
+    }
+    if (b != b_end) {
+      RType aprev_rmax = (a_end - 1)->rmax;
+      do {
+        *dst = Entry(aprev_rmin + b->rmin, aprev_rmax + b->rmax, b->value);
+        ++dst; ++b;
+      } while (b != b_end);
+    }
+    utils::Assert(dst == data + size, "bug in combine");
+  }
+};
+
 /*!
  * \brief template for all quantle sketch algorithm
  *        that uses merge/prune scheme
@@ -418,7 +555,7 @@ class QuantileSketchTemplate {
 };
 
 /*!
- * \brief Quantiel sketch use WQSummary
+ * \brief Quantile sketch use WQSummary
  * \tparam DType type of data content
  * \tparam RType type of rank
  */
@@ -426,262 +563,16 @@ template<typename DType, typename RType=unsigned>
 class WQuantileSketch : 
       public QuantileSketchTemplate<DType, RType, WQSummary<DType, RType> >{
 };
-
 /*!
- * \brief a helper class to compute streaming quantile
+ * \brief Quantile sketch use WQSummary
  * \tparam DType type of data content
  * \tparam RType type of rank
  */
 template<typename DType, typename RType=unsigned>
-class GKQuantileSketch {
- public:
-  /*! \brief an entry in the sketch summary */
-  struct Entry {
-    /*! \brief minimum rank */
-    RType rmin;
-    /*! \brief maximum rank */
-    RType rmax;
-    /*! \brief the value of data */
-    DType value;
-    // constructor
-    Entry(void) {}
-    // constructor
-    Entry(RType rmin, RType rmax, DType value)
-        : rmin(rmin), rmax(rmax), value(value) {}
-  };
-  /*! 
-   * \brief this is data structure presenting one summary
-   */
-  struct Summary {
-    /*! \brief data field */
-    Entry *data;
-    /*! \brief number of elements in the summary */
-    RType size;
-    /*! \brief the maximum error of the summary */
-    inline RType MaxError(void) const {
-      RType res = 0;
-      for (size_t i = 1; i < size; ++i) {
-        res = std::max(data[i].rmax - data[i-1].rmin, res);
-      }
-      return res;
-    }
-    /*! \return maximum rank in the summary */
-    inline RType MaxRank(void) const {
-      return data[size - 1].rmax;
-    }
-    /*! \brief set size to 0 */
-    inline void Clear(void) {
-      size = 0;
-    }
-    /*! 
-     * \brief copy content from src
-     * \param src source sketch
-     */
-    inline void CopyFrom(const Summary &src) {
-      size = src.size;
-      std::memcpy(data, src.data, sizeof(Entry) * size);
-    }
-    /*! 
-     * \brief set current summary to be pruned summary of src
-     *        assume data field is already allocated to be at least maxsize
-     * \param src source summary
-     * \param maxsize size we can afford in the pruned sketch
-     */
-    inline void SetPrune(const Summary &src, RType maxsize) {
-      const RType max_rank = src.MaxRank();
-      this->size = maxsize;
-      data[0] = src.data[0];
-      RType n = maxsize - 1;
-      RType top = 1;
-      for (size_t i = 1; i < n; ++i) {
-        RType k = (i * max_rank) / n;
-        while (k > src.data[top + 1].rmax) ++top;
-        // assert src.data[top].rmin <= k
-        // because k > src.data[top].rmax >= src.data[top].rmin
-        if ((k - src.data[top].rmin) < (src.data[top+1].rmax - k)) {
-          data[i] = src.data[top];
-        } else {
-          data[i] = src.data[top + 1];
-        }
-      }
-      data[n] = src.data[src.size - 1];
-    }
-    inline void SetCombine(const Summary &sa,
-                           const Summary &sb) {
-      utils::Assert(sa.size > 0 && sb.size > 0, "invalid input for merge"); 
-      const Entry *a = sa.data, *a_end = sa.data + sa.size;
-      const Entry *b = sb.data, *b_end = sb.data + sb.size;
-      this->size = sa.size + sb.size;
-      RType aprev_rmin = 0, bprev_rmin = 0;
-      Entry *dst = this->data;
-      while (a != a_end && b != b_end) {
-        if (a->value < b->value) {
-          *dst = Entry(bprev_rmin + a->rmin,
-                       a->rmax + b->rmax - 1, a->value);
-          aprev_rmin = a->rmin;
-          ++dst; ++a;
-        } else {
-          *dst = Entry(aprev_rmin + b->rmin, 
-                       b->rmax + a->rmax - 1, b->value);
-          bprev_rmin = b->rmin;
-          ++dst; ++b;
-        }
-      }
-      if (a != a_end) {
-        RType bprev_rmax = (b_end - 1)->rmax;
-        do {
-          *dst = Entry(bprev_rmin + a->rmin, bprev_rmax + a->rmax, a->value);
-          ++dst; ++a;
-        } while (a != a_end);
-      }
-      if (b != b_end) {
-        RType aprev_rmax = (a_end - 1)->rmax;
-        do {
-          *dst = Entry(aprev_rmin + b->rmin, aprev_rmax + b->rmax, b->value);
-          ++dst; ++b;
-        } while (b != b_end);
-      }
-      utils::Assert(dst == data + size, "bug in combine");
-    }
-  };
-  // same as summary, but use STL to backup the space
-  struct SummaryContainer : public Summary {
-    std::vector<Entry> space;
-    /*! \brief reserve space for summary */
-    inline void Reserve(size_t size) {
-      space.resize(size);
-      this->data = BeginPtr(space);
-    }
-    /*! 
-     * \brief set the space to be merge of all Summary arrays
-     * \param begin begining position in th summary array
-     * \param end ending position in the Summary array
-     */
-    inline void SetMerge(const Summary *begin,
-                         const Summary *end) {
-      utils::Assert(begin < end, "can not set combine to empty instance");
-      size_t len = end - begin;
-      if (len == 1) {
-        this->Reserve(begin[0].size);
-        this->CopyFrom(begin[0]);
-      } else if (len == 2) {
-        this->Reserve(begin[0].size + begin[1].size);
-        this->SetMerge(begin[0], begin[1]);
-      } else {
-        // recursive merge
-        SummaryContainer lhs, rhs;        
-        lhs.SetCombine(begin, begin + len / 2);
-        rhs.SetCombine(begin + len / 2, end);
-        this->Reserve(lhs.size + rhs.size);
-        this->SetCombine(lhs, rhs);
-      }
-    }
-  };
-  /*! 
-   * \brief intialize the quantile sketch, given the performance specification
-   * \param maxn maximum number of data points can be encountered
-   * \param eps accuracy level of summary
-   */
-  inline void Init(RType maxn, double eps) {
-    eps  = eps * 0.5;
-    size_t L = 0;
-    size_t b = std::max(floor(log2(eps * maxn) / eps), 8.0);
-    // check for lower 
-    while (b < maxn) {
-      L = ceil(log2(maxn / b)) + 1;
-      if (L < eps * b) break;
-      ++b;
-    }
-    L += 1;
-    inqueue.resize(b);
-    limit_size = (b + 1) / 2 + 1;
-    temp.Reserve(limit_size * 2);
-    data.resize(limit_size * L);
-    for (size_t l = 0; l < L; ++l) {
-      Summary s; s.size = 0;
-      s.data = BeginPtr(data) + l * limit_size;
-      level.push_back(s);
-    }
-    qtail = 0;
-  }
-  /*! 
-   * \brief add an element to a sketch 
-   * \param x the elemented added to the sketch
-   */
-  inline void Push(DType x) {
-    inqueue[qtail++] = x;
-    if (qtail == inqueue.size()) {
-      // start update sketch
-      std::sort(inqueue.begin(), inqueue.end());
-      for (size_t i = 0; i < qtail; ++i) {
-        temp.data[i] = Entry(i + 1, i + 1, inqueue[i]);
-      }
-      temp.size = static_cast<RType>(qtail);
-      // clean up queue
-      qtail = 0;
-      for (size_t l = 1; l < level.size(); ++l) {
-        // check if level l is empty
-        if (level[l].size == 0) {
-          level[l].SetPrune(temp, limit_size);
-          return;
-        } else {
-          // level 0 is actually temp space
-          level[0].SetPrune(temp, limit_size);
-          temp.SetCombine(level[0], level[l]);
-          level[l].size = 0;
-        }
-      }
-      utils::Error("adding more element than allowed");
-    }
-  }
-  /*! 
-   * \brief finalize the result after all data has been passed 
-   *        copy the final result to level 0
-   *        this can only be called once
-   */
-  inline void Finalize(void) {
-    // start update sketch
-    std::sort(inqueue.begin(), inqueue.begin() + qtail);
-    for (size_t i = 0; i < qtail; ++i) {
-      temp.data[i] = Entry(i + 1, i + 1, inqueue[i]);
-    }
-    temp.size = static_cast<RType>(qtail);
-    if (temp.size < limit_size) {
-      level[0].CopyFrom(temp);
-    } else {
-      level[0].SetPrune(temp, limit_size);
-    }
-    // start adding other things in
-    for (size_t l = 1; l < level.size(); ++l) {
-      if (level[l].size == 0) continue;
-      if (level[0].size == 0) {
-        level[0].CopyFrom(level[l]);
-      } else {
-        temp.SetCombine(level[0], level[l]);
-        level[0].SetPrune(temp, limit_size);        
-      }
-      level[l].size = 0;
-    }
-  }
-  /*! \brief get the summary after finalize */
-  inline Summary GetSummary(void) const {
-    return level[0];
-  }  
-  
- private:  
-  // the input queue
-  std::vector<DType> inqueue;
-  // end of the queue
-  size_t qtail;
-  // size of summary in each level
-  size_t limit_size;
-  // content of the summary
-  std::vector<Entry> data;
-  // different level of summary
-  std::vector<Summary> level;  
-  // temporal summary, used for temp-merge
-  SummaryContainer temp;  
+class GKQuantileSketch : 
+      public QuantileSketchTemplate<DType, RType, GKSummary<DType, RType> >{
 };
+
 }  // utils
 }  // xgboost
 #endif
diff --git a/test/test_quantile.cpp b/test/test_quantile.cpp
index 89e976897..e6af5b1ec 100644
--- a/test/test_quantile.cpp
+++ b/test/test_quantile.cpp
@@ -2,8 +2,10 @@
 #include <utils/quantile.h>
 using namespace xgboost;
 
-int main(int argc, char *argv[]) {  
-  utils::WQuantileSketch<float, float> sketch;
+
+template<typename Sketch, typename RType>
+inline void test(void) {
+  Sketch sketch;
   size_t n;
   double wsum = 0.0;
   float eps, x, w;
@@ -11,13 +13,25 @@ int main(int argc, char *argv[]) {
   sketch.Init(n, eps);
   printf("nlevel = %lu, limit_size=%lu\n", sketch.nlevel, sketch.limit_size);
   while (scanf("%f%f", &x, &w) == 2) {
-    sketch.Push(x, w);
+    sketch.Push(x, static_cast<RType>(w));
     wsum += w;
   }
-  sketch.CheckValid(0.1);
-  utils::WQuantileSketch<float, float>::SummaryContainer out;
+  sketch.CheckValid(static_cast<RType>(0.1));
+  typename Sketch::SummaryContainer out;
   sketch.GetSummary(&out);
-  printf("MaxError=%f/%f = %g\n", out.MaxError(), wsum, out.MaxError() / wsum);
+  double maxerr = static_cast<double>(out.MaxError());
+  printf("MaxError=%g/%g = %g\n", maxerr, wsum, maxerr / wsum);
   out.Print();
+}
+
+int main(int argc, char *argv[]) {
+  const char *method = "wq";
+  if (argc > 1) method = argv[1];
+  if (!strcmp(method, "wq")) {
+    test<utils::WQuantileSketch<float, float>, float>();
+  }
+  if (!strcmp(method, "gk")) {
+    test<utils::GKQuantileSketch<float, unsigned>, unsigned>();
+  }
   return 0;
 }