diff --git a/Makefile b/Makefile
index c5d052b7c..da9bb22fb 100644
--- a/Makefile
+++ b/Makefile
@@ -2,7 +2,8 @@ export CC  = gcc
 export CXX = g++
 export MPICXX = mpicxx
 export LDFLAGS= -Llib
-export CFLAGS = -Wall -O3 -msse2  -Wno-unknown-pragmas -fPIC -Iinclude
+export WARNFLAGS= -Wall -Wextra -Wno-unused-parameter -Wno-unknown-pragmas -pedantic 
+export CFLAGS = -O3 -msse2 -fPIC -Iinclude $(WARNFLAGS) 
 
 # build path
 BPATH=.
@@ -15,7 +16,7 @@ ALIB= lib/librabit.a lib/librabit_mpi.a lib/librabit_empty.a lib/librabit_mock.a
 HEADERS=src/*.h include/*.h include/rabit/*.h
 .PHONY: clean all install mpi python
 
-all: lib/librabit.a lib/librabit_mock.a $(SLIB)
+all: lib/librabit.a lib/librabit_mock.a wrapper/librabit_wrapper.so wrapper/librabit_wrapper_mock.so
 mpi: lib/librabit_mpi.a wrapper/librabit_wrapper_mpi.so
 python: wrapper/librabit_wrapper.so wrapper/librabit_wrapper_mock.so
 
diff --git a/include/rabit.h b/include/rabit.h
index 17ef5e616..eb1f0a07f 100644
--- a/include/rabit.h
+++ b/include/rabit.h
@@ -203,6 +203,27 @@ inline int LoadCheckPoint(ISerializable *global_model,
    */
 inline void CheckPoint(const ISerializable *global_model,
                        const ISerializable *local_model = NULL);
+/*!
+ * \brief This function can be used to replace CheckPoint for global_model only,
+ *   when certain condition is met(see detailed expplaination).
+ * 
+ *   This is a "lazy" checkpoint such that only the pointer to global_model is
+ *   remembered and no memory copy is taken. To use this function, the user MUST ensure that:
+ *   The global_model must remain unchanged util last call of Allreduce/Broadcast in current version finishs.
+ *   In another words, global_model model can be changed only between last call of 
+ *   Allreduce/Broadcast and LazyCheckPoint in current version
+ *   
+ *   For example, suppose the calling sequence is:
+ *   LazyCheckPoint, code1, Allreduce, code2, Broadcast, code3, LazyCheckPoint
+ *   
+ *   If user can only changes global_model in code3, then LazyCheckPoint can be used to
+ *   improve efficiency of the program.
+ * \param global_model pointer to the globally shared model/state
+ *   when calling this function, the caller need to gauranttees that global_model
+ *   is the same in all nodes
+ * \sa LoadCheckPoint, CheckPoint, VersionNumber
+ */
+inline void LazyCheckPoint(const ISerializable *global_model);
 /*!
  * \return version number of current stored model,
  *         which means how many calls to CheckPoint we made so far
diff --git a/include/rabit/engine.h b/include/rabit/engine.h
index c06fbc6cc..dd85b508b 100644
--- a/include/rabit/engine.h
+++ b/include/rabit/engine.h
@@ -114,6 +114,27 @@ class IEngine {
    */
   virtual void CheckPoint(const ISerializable *global_model,
                           const ISerializable *local_model = NULL) = 0;
+  /*!
+   * \brief This function can be used to replace CheckPoint for global_model only,
+   *   when certain condition is met(see detailed expplaination.
+   * 
+   *   This is a "lazy" checkpoint such that only the pointer to global_model is
+   *   remembered and no memory copy is taken. To use this function, the user MUST ensure that:
+   *   The global_model must remain unchanged util last call of Allreduce/Broadcast in current version finishs.
+   *   In another words, global_model model can be changed only between last call of 
+   *   Allreduce/Broadcast and LazyCheckPoint in current version
+   *   
+   *   For example, suppose the calling sequence is:
+   *   LazyCheckPoint, code1, Allreduce, code2, Broadcast, code3, LazyCheckPoint
+   *   
+   *   If user can only changes global_model in code3, then LazyCheckPoint can be used to
+   *   improve efficiency of the program.
+   * \param global_model pointer to the globally shared model/state
+   *   when calling this function, the caller need to gauranttees that global_model
+   *   is the same in all nodes
+   * \sa LoadCheckPoint, CheckPoint, VersionNumber
+   */
+  virtual void LazyCheckPoint(const ISerializable *global_model) = 0;
   /*!
    * \return version number of current stored model,
    *         which means how many calls to CheckPoint we made so far
@@ -221,8 +242,10 @@ class ReduceHandle {
   static int TypeSize(const MPI::Datatype &dtype);
 
  protected:
-  // handle data field
+  // handle function field
   void *handle_;
+  // reduce function of the reducer
+  IEngine::ReduceFunction *redfunc_;
   // handle to the type field
   void *htype_;
   // the created type in 4 bytes
diff --git a/include/rabit/rabit-inl.h b/include/rabit/rabit-inl.h
index 3ba3ec95e..4ffd812e7 100644
--- a/include/rabit/rabit-inl.h
+++ b/include/rabit/rabit-inl.h
@@ -183,6 +183,10 @@ inline void CheckPoint(const ISerializable *global_model,
                        const ISerializable *local_model) {
   engine::GetEngine()->CheckPoint(global_model, local_model);
 }
+// lazy checkpoint the model, only remember the pointer to global_model
+inline void LazyCheckPoint(const ISerializable *global_model) {
+  engine::GetEngine()->LazyCheckPoint(global_model);
+}
 // return the version number of currently stored model
 inline int VersionNumber(void) {
   return engine::GetEngine()->VersionNumber();
@@ -197,7 +201,7 @@ inline void ReducerFunc_(const void *src_, void *dst_, int len_, const MPI::Data
   const char *psrc = reinterpret_cast<const char*>(src_);
   char *pdst = reinterpret_cast<char*>(dst_);
   DType tdst, tsrc;
-  for (size_t i = 0; i < len_; ++i) {
+  for (int i = 0; i < len_; ++i) {
     // use memcpy to avoid alignment issue
     std::memcpy(&tdst, pdst + i * kUnit, sizeof(tdst));
     std::memcpy(&tsrc, psrc + i * kUnit, sizeof(tsrc));
diff --git a/src/allreduce_base.cc b/src/allreduce_base.cc
index 671c53877..1736d8f6d 100644
--- a/src/allreduce_base.cc
+++ b/src/allreduce_base.cc
@@ -27,6 +27,7 @@ AllreduceBase::AllreduceBase(void) {
   hadoop_mode = 0;
   version_number = 0;
   task_id = "NULL";
+  err_link = NULL;
   this->SetParam("rabit_reduce_buffer", "256MB");
 }
 
@@ -213,7 +214,7 @@ void AllreduceBase::ReConnectLinks(const char *cmd) {
       } else {
         if (!all_links[i].sock.IsClosed()) all_links[i].sock.Close();
       }
-    }
+    }    
     int ngood = static_cast<int>(good_link.size());
     Assert(tracker.SendAll(&ngood, sizeof(ngood)) == sizeof(ngood),
            "ReConnectLink failure 5");
@@ -365,7 +366,7 @@ AllreduceBase::TryAllreduce(void *sendrecvbuf_,
           selecter.WatchException(links[i].sock);
           finished = false;
         }
-        if (size_up_out != total_size) {
+        if (size_up_out != total_size && size_up_out < size_up_reduce) {
           selecter.WatchWrite(links[i].sock);
         }
       } else {
@@ -373,8 +374,10 @@ AllreduceBase::TryAllreduce(void *sendrecvbuf_,
           selecter.WatchRead(links[i].sock);
         }
         // size_write <= size_read
-        if (links[i].size_write != total_size) {
-          selecter.WatchWrite(links[i].sock);
+        if (links[i].size_write != total_size){
+          if (links[i].size_write < size_down_in) {
+            selecter.WatchWrite(links[i].sock);
+          }
           // only watch for exception in live channels
           selecter.WatchException(links[i].sock);
           finished = false;
@@ -388,12 +391,17 @@ AllreduceBase::TryAllreduce(void *sendrecvbuf_,
     // exception handling
     for (int i = 0; i < nlink; ++i) {
       // recive OOB message from some link
-      if (selecter.CheckExcept(links[i].sock)) return kGetExcept;
+      if (selecter.CheckExcept(links[i].sock)) {
+        return ReportError(&links[i], kGetExcept);
+      }
     }
     // read data from childs
     for (int i = 0; i < nlink; ++i) {
       if (i != parent_index && selecter.CheckRead(links[i].sock)) {
-        if (!links[i].ReadToRingBuffer(size_up_out)) return kSockError;
+        ReturnType ret = links[i].ReadToRingBuffer(size_up_out);
+        if (ret != kSuccess) {
+          return ReportError(&links[i], ret);
+        }
       }
     }
     // this node have childs, peform reduce
@@ -433,13 +441,16 @@ AllreduceBase::TryAllreduce(void *sendrecvbuf_,
     }
     if (parent_index != -1) {
       // pass message up to parent, can pass data that are already been reduced
-      if (selecter.CheckWrite(links[parent_index].sock)) {
+      if (size_up_out < size_up_reduce) {
         ssize_t len = links[parent_index].sock.
             Send(sendrecvbuf + size_up_out, size_up_reduce - size_up_out);
         if (len != -1) {
           size_up_out += static_cast<size_t>(len);
         } else {
-          if (errno != EAGAIN && errno != EWOULDBLOCK) return kSockError;
+          ReturnType ret = Errno2Return(errno);
+          if (ret != kSuccess) {
+            return ReportError(&links[parent_index], ret);
+          }
         }
       }
       // read data from parent
@@ -448,14 +459,18 @@ AllreduceBase::TryAllreduce(void *sendrecvbuf_,
         ssize_t len = links[parent_index].sock.
             Recv(sendrecvbuf + size_down_in, total_size - size_down_in);
         if (len == 0) {
-          links[parent_index].sock.Close(); return kSockError;
+          links[parent_index].sock.Close(); 
+          return ReportError(&links[parent_index], kRecvZeroLen);
         }
         if (len != -1) {
           size_down_in += static_cast<size_t>(len);
           utils::Assert(size_down_in <= size_up_out,
                         "Allreduce: boundary error");
         } else {
-          if (errno != EAGAIN && errno != EWOULDBLOCK) return kSockError;
+          ReturnType ret = Errno2Return(errno);
+          if (ret != kSuccess) {
+            return ReportError(&links[parent_index], ret);
+          }
         }
       }
     } else {
@@ -464,9 +479,10 @@ AllreduceBase::TryAllreduce(void *sendrecvbuf_,
     }
     // can pass message down to childs
     for (int i = 0; i < nlink; ++i) {
-      if (i != parent_index && selecter.CheckWrite(links[i].sock)) {
-        if (!links[i].WriteFromArray(sendrecvbuf, size_down_in)) {
-          return kSockError;
+      if (i != parent_index && links[i].size_write < size_down_in) {
+        ReturnType ret = links[i].WriteFromArray(sendrecvbuf, size_down_in);
+        if (ret != kSuccess) {
+          return ReportError(&links[i], ret);
         }
       }
     }
@@ -516,7 +532,10 @@ AllreduceBase::TryBroadcast(void *sendrecvbuf_, size_t total_size, int root) {
         selecter.WatchRead(links[i].sock); finished = false;
       }
       if (in_link != -2 && i != in_link && links[i].size_write != total_size) {
-        selecter.WatchWrite(links[i].sock); finished = false;
+        if (links[i].size_write < size_in) {
+          selecter.WatchWrite(links[i].sock);
+        }
+        finished = false;
       }
       selecter.WatchException(links[i].sock);
     }
@@ -527,14 +546,17 @@ AllreduceBase::TryBroadcast(void *sendrecvbuf_, size_t total_size, int root) {
     // exception handling
     for (int i = 0; i < nlink; ++i) {
       // recive OOB message from some link
-      if (selecter.CheckExcept(links[i].sock)) return kGetExcept;
+      if (selecter.CheckExcept(links[i].sock)) {
+        return ReportError(&links[i], kGetExcept);
+      }
     }
     if (in_link == -2) {
       // probe in-link
       for (int i = 0; i < nlink; ++i) {
         if (selecter.CheckRead(links[i].sock)) {
-          if (!links[i].ReadToArray(sendrecvbuf_, total_size)) {
-            return kSockError;
+          ReturnType ret = links[i].ReadToArray(sendrecvbuf_, total_size);
+          if (ret != kSuccess) {
+            return ReportError(&links[i], ret);
           }
           size_in = links[i].size_read;
           if (size_in != 0) {
@@ -545,16 +567,20 @@ AllreduceBase::TryBroadcast(void *sendrecvbuf_, size_t total_size, int root) {
     } else {
       // read from in link
       if (in_link >= 0 && selecter.CheckRead(links[in_link].sock)) {
-        if (!links[in_link].ReadToArray(sendrecvbuf_, total_size)) {
-          return kSockError;
+        ReturnType ret = links[in_link].ReadToArray(sendrecvbuf_, total_size);
+        if (ret != kSuccess) {
+          return ReportError(&links[in_link], ret);
         }
         size_in = links[in_link].size_read;
       }
     }
     // send data to all out-link
     for (int i = 0; i < nlink; ++i) {
-      if (i != in_link && selecter.CheckWrite(links[i].sock)) {
-        if (!links[i].WriteFromArray(sendrecvbuf_, size_in)) return kSockError;
+      if (i != in_link && links[i].size_write < size_in) {
+        ReturnType ret = links[i].WriteFromArray(sendrecvbuf_, size_in);
+        if (ret != kSuccess) {
+          return ReportError(&links[i], ret);
+        }
       }
     }
   }
diff --git a/src/allreduce_base.h b/src/allreduce_base.h
index da57c34f6..b1cc71ef1 100644
--- a/src/allreduce_base.h
+++ b/src/allreduce_base.h
@@ -146,6 +146,29 @@ class AllreduceBase : public IEngine {
                           const ISerializable *local_model = NULL) {
     version_number += 1;
   }
+  /*!
+   * \brief This function can be used to replace CheckPoint for global_model only,
+   *   when certain condition is met(see detailed expplaination).
+   * 
+   *   This is a "lazy" checkpoint such that only the pointer to global_model is
+   *   remembered and no memory copy is taken. To use this function, the user MUST ensure that:
+   *   The global_model must remain unchanged util last call of Allreduce/Broadcast in current version finishs.
+   *   In another words, global_model model can be changed only between last call of 
+   *   Allreduce/Broadcast and LazyCheckPoint in current version
+   *   
+   *   For example, suppose the calling sequence is:
+   *   LazyCheckPoint, code1, Allreduce, code2, Broadcast, code3, LazyCheckPoint
+   *   
+   *   If user can only changes global_model in code3, then LazyCheckPoint can be used to
+   *   improve efficiency of the program.
+   * \param global_model pointer to the globally shared model/state
+   *   when calling this function, the caller need to gauranttees that global_model
+   *   is the same in all nodes
+   * \sa LoadCheckPoint, CheckPoint, VersionNumber
+   */
+  virtual void LazyCheckPoint(const ISerializable *global_model) {
+    version_number += 1;
+  }
   /*!
    * \return version number of current stored model,
    *         which means how many calls to CheckPoint we made so far
@@ -175,9 +198,13 @@ class AllreduceBase : public IEngine {
 
  protected:
   /*! \brief enumeration of possible returning results from Try functions */
-  enum ReturnType {
+  enum ReturnTypeEnum {
     /*! \brief execution is successful */
     kSuccess,
+    /*! \brief a link was reset by peer */
+    kConnReset,
+    /*! \brief received a zero length message */
+    kRecvZeroLen,
     /*! \brief a neighbor node go down, the connection is dropped */
     kSockError,
     /*! 
@@ -186,6 +213,26 @@ class AllreduceBase : public IEngine {
      */
     kGetExcept
   };
+  /*! \brief struct return type to avoid implicit conversion to int/bool */
+  struct ReturnType {
+    /*! \brief internal return type */
+    ReturnTypeEnum value;
+    // constructor
+    ReturnType() {}
+    ReturnType(ReturnTypeEnum value) : value(value){}
+    inline bool operator==(const ReturnTypeEnum &v) const {
+      return value == v;
+    }
+    inline bool operator!=(const ReturnTypeEnum &v) const {
+      return value != v;
+    }
+  };
+  /*! \brief translate errno to return type */
+  inline static ReturnType Errno2Return(int errsv) {
+    if (errsv == EAGAIN || errsv == EWOULDBLOCK) return kSuccess;
+    if (errsv == ECONNRESET) return kConnReset;
+    return kSockError;
+  }
   // link record to a neighbor
   struct LinkRecord {
    public:
@@ -202,7 +249,9 @@ class AllreduceBase : public IEngine {
     // buffer size, in bytes
     size_t buffer_size;
     // constructor
-    LinkRecord(void) {}
+    LinkRecord(void) 
+        : buffer_head(NULL), buffer_size(0) {
+    }
     // initialize buffer
     inline void InitBuffer(size_t type_nbytes, size_t count,
                            size_t reduce_buffer_size) {
@@ -226,22 +275,23 @@ class AllreduceBase : public IEngine {
      *  position after protect_start
      * \param protect_start all data start from protect_start is still needed in buffer
      *                      read shall not override this 
-     * \return true if it is an successful read, false if there is some error happens, check errno
+     * \return the type of reading
      */
-    inline bool ReadToRingBuffer(size_t protect_start) {
+    inline ReturnType ReadToRingBuffer(size_t protect_start) {
+      utils::Assert(buffer_head != NULL, "ReadToRingBuffer: buffer not allocated");
       size_t ngap = size_read - protect_start;
       utils::Assert(ngap <= buffer_size, "Allreduce: boundary check");
       size_t offset = size_read % buffer_size;
       size_t nmax = std::min(buffer_size - ngap, buffer_size - offset);
-      if (nmax == 0) return true;
+      if (nmax == 0) return kSuccess;
       ssize_t len = sock.Recv(buffer_head + offset, nmax);
       // length equals 0, remote disconnected
       if (len == 0) {
-        sock.Close(); return false;
+        sock.Close(); return kRecvZeroLen;
       }
-      if (len == -1) return errno == EAGAIN || errno == EWOULDBLOCK;
+      if (len == -1) return Errno2Return(errno);
       size_read += static_cast<size_t>(len);
-      return true;
+      return kSuccess;
     }
     /*!
      * \brief read data into array,
@@ -250,17 +300,17 @@ class AllreduceBase : public IEngine {
      * \param max_size maximum size of array
      * \return true if it is an successful read, false if there is some error happens, check errno
      */
-    inline bool ReadToArray(void *recvbuf_, size_t max_size) {
-      if (max_size == size_read) return true;
+    inline ReturnType ReadToArray(void *recvbuf_, size_t max_size) {
+      if (max_size == size_read) return kSuccess;
       char *p = static_cast<char*>(recvbuf_);
       ssize_t len = sock.Recv(p + size_read, max_size - size_read);
       // length equals 0, remote disconnected
       if (len == 0) {
-        sock.Close(); return false;
+        sock.Close(); return kRecvZeroLen;
       }
-      if (len == -1) return errno == EAGAIN || errno == EWOULDBLOCK;
+      if (len == -1) return Errno2Return(errno);
       size_read += static_cast<size_t>(len);
-      return true;
+      return kSuccess;
     }
     /*!
      * \brief write data in array to sock
@@ -268,12 +318,12 @@ class AllreduceBase : public IEngine {
      * \param max_size maximum size of array
      * \return true if it is an successful write, false if there is some error happens, check errno
      */
-    inline bool WriteFromArray(const void *sendbuf_, size_t max_size) {
+    inline ReturnType WriteFromArray(const void *sendbuf_, size_t max_size) {
       const char *p = static_cast<const char*>(sendbuf_);
       ssize_t len = sock.Send(p + size_write, max_size - size_write);
-      if (len == -1) return errno == EAGAIN || errno == EWOULDBLOCK;
+      if (len == -1) return Errno2Return(errno);
       size_write += static_cast<size_t>(len);
-      return true;
+      return kSuccess;
     }
 
    private:
@@ -333,6 +383,14 @@ class AllreduceBase : public IEngine {
    * \sa ReturnType
    */
   ReturnType TryBroadcast(void *sendrecvbuf_, size_t size, int root);
+  /*!
+   * \brief function used to report error when a link goes wrong 
+   * \param link the pointer to the link who causes the error
+   * \param err the error type
+   */
+  inline ReturnType ReportError(LinkRecord *link, ReturnType err) {
+    err_link = link; return err;
+  }
   //---- data structure related to model ----
   // call sequence counter, records how many calls we made so far
   // from last call to CheckPoint, LoadCheckPoint
@@ -348,6 +406,8 @@ class AllreduceBase : public IEngine {
   int parent_rank;
   // sockets of all links this connects to
   std::vector<LinkRecord> all_links;
+  // used to record the link where things goes wrong
+  LinkRecord *err_link;
   // all the links in the reduction tree connection
   RefLinkVector tree_links;
   // pointer to links in the ring
diff --git a/src/allreduce_robust-inl.h b/src/allreduce_robust-inl.h
index e0250e426..d8cc8dcdd 100644
--- a/src/allreduce_robust-inl.h
+++ b/src/allreduce_robust-inl.h
@@ -97,7 +97,9 @@ AllreduceRobust::MsgPassing(const NodeType &node_value,
     // exception handling
     for (int i = 0; i < nlink; ++i) {
       // recive OOB message from some link
-      if (selecter.CheckExcept(links[i].sock)) return kGetExcept;
+      if (selecter.CheckExcept(links[i].sock)) {
+        return ReportError(&links[i], kGetExcept);
+      }
     }
     if (stage == 0) {
       bool finished = true;
@@ -105,9 +107,8 @@ AllreduceRobust::MsgPassing(const NodeType &node_value,
       for (int i = 0; i < nlink; ++i) {
         if (i != parent_index) {
           if (selecter.CheckRead(links[i].sock)) {
-            if (!links[i].ReadToArray(&edge_in[i], sizeof(EdgeType))) {
-              return kSockError;
-            }
+            ReturnType ret = links[i].ReadToArray(&edge_in[i], sizeof(EdgeType));
+            if (ret != kSuccess) return ReportError(&links[i], ret);
           }
           if (links[i].size_read != sizeof(EdgeType)) finished = false;
         }
@@ -128,17 +129,15 @@ AllreduceRobust::MsgPassing(const NodeType &node_value,
     if (stage == 1) {
       const int pid = this->parent_index;
       utils::Assert(pid != -1, "MsgPassing invalid stage");
-      if (!links[pid].WriteFromArray(&edge_out[pid], sizeof(EdgeType))) {
-        return kSockError;
-      }
+      ReturnType ret = links[pid].WriteFromArray(&edge_out[pid], sizeof(EdgeType));
+      if (ret != kSuccess) return ReportError(&links[pid], ret);
       if (links[pid].size_write == sizeof(EdgeType)) stage = 2;
     }
     if (stage == 2) {
       const int pid = this->parent_index;
       utils::Assert(pid != -1, "MsgPassing invalid stage");
-      if (!links[pid].ReadToArray(&edge_in[pid], sizeof(EdgeType))) {
-        return kSockError;
-      }
+      ReturnType ret = links[pid].ReadToArray(&edge_in[pid], sizeof(EdgeType));
+      if (ret != kSuccess) return ReportError(&links[pid], ret);
       if (links[pid].size_read == sizeof(EdgeType)) {
         for (int i = 0; i < nlink; ++i) {
           if (i != pid) edge_out[i] = func(node_value, edge_in, i);
@@ -149,9 +148,8 @@ AllreduceRobust::MsgPassing(const NodeType &node_value,
     if (stage == 3) {
       for (int i = 0; i < nlink; ++i) {
         if (i != parent_index && links[i].size_write != sizeof(EdgeType)) {
-          if (!links[i].WriteFromArray(&edge_out[i], sizeof(EdgeType))) {
-            return kSockError;
-          }
+          ReturnType ret = links[i].WriteFromArray(&edge_out[i], sizeof(EdgeType));
+          if (ret != kSuccess) return ReportError(&links[i], ret);
         }
       }
     }
diff --git a/src/allreduce_robust.cc b/src/allreduce_robust.cc
index 7cb2b3611..33fdcd0f0 100644
--- a/src/allreduce_robust.cc
+++ b/src/allreduce_robust.cc
@@ -25,6 +25,9 @@ AllreduceRobust::AllreduceRobust(void) {
   seq_counter = 0;
   local_chkpt_version = 0;
   result_buffer_round = 1;
+  global_lazycheck = NULL;
+  use_local_model = -1;
+  recover_counter = 0;
 }
 void AllreduceRobust::Init(void) {
   AllreduceBase::Init();
@@ -154,9 +157,7 @@ int AllreduceRobust::LoadCheckPoint(ISerializable *global_model,
                                     ISerializable *local_model) {
   // skip action in single node
   if (world_size == 1) return 0;
-  if (local_model != NULL && num_local_replica == 0) {
-    num_local_replica = default_local_replica;
-  }
+  this->LocalModelCheck(local_model != NULL);
   if (num_local_replica == 0) {
     utils::Check(local_model == NULL,
                  "need to set rabit_local_replica larger than 1 to checkpoint local_model");
@@ -199,30 +200,50 @@ int AllreduceRobust::LoadCheckPoint(ISerializable *global_model,
   }
 }
 /*!
- * \brief checkpoint the model, meaning we finished a stage of execution
- *  every time we call check point, there is a version number which will increase by one
+ * \brief internal consistency check function,
+ *  use check to ensure user always call CheckPoint/LoadCheckPoint
+ *  with or without local but not both, this function will set the approperiate settings
+ *  in the first call of LoadCheckPoint/CheckPoint 
  *
+ * \param with_local whether the user calls CheckPoint with local model
+ */
+void AllreduceRobust::LocalModelCheck(bool with_local) {
+  if (use_local_model == -1) {
+    if (with_local) {
+      use_local_model = 1;
+      if (num_local_replica == 0) {
+        num_local_replica = default_local_replica;
+      }
+    } else {
+      use_local_model = 0;
+      num_local_replica = 0;
+    }
+  } else {
+    utils::Check(use_local_model == int(with_local),
+                 "Can only call Checkpoint/LoadCheckPoint always with"\
+                 "or without local_model, but not mixed case");
+  }
+}
+/*!
+ * \brief internal implementation of checkpoint, support both lazy and normal way
+ * 
  * \param global_model pointer to the globally shared model/state
  *   when calling this function, the caller need to gauranttees that global_model
  *   is the same in all nodes
  * \param local_model pointer to local model, that is specific to current node/rank
  *   this can be NULL when no local state is needed
+ * \param lazy_checkpt whether the action is lazy checkpoint
  *
- * NOTE: local_model requires explicit replication of the model for fault-tolerance, which will
- *       bring replication cost in CheckPoint function. global_model do not need explicit replication.
- *       So only CheckPoint with global_model if possible
- *
- * \sa LoadCheckPoint, VersionNumber
+ * \sa CheckPoint, LazyCheckPoint
  */
-void AllreduceRobust::CheckPoint(const ISerializable *global_model,
-                                 const ISerializable *local_model) {
+void AllreduceRobust::CheckPoint_(const ISerializable *global_model,
+                                  const ISerializable *local_model,
+                                  bool lazy_checkpt) {
   // never do check point in single machine mode
   if (world_size == 1) {
     version_number += 1; return;
   }
-  if (local_model != NULL && num_local_replica == 0) {
-    num_local_replica = default_local_replica;
-  }
+  this->LocalModelCheck(local_model != NULL);
   if (num_local_replica == 0) {
     utils::Check(local_model == NULL,
                  "need to set rabit_local_replica larger than 1 to checkpoint local_model");
@@ -255,10 +276,15 @@ void AllreduceRobust::CheckPoint(const ISerializable *global_model,
   // increase version number
   version_number += 1;
   // save model
-  global_checkpoint.resize(0);
-  utils::MemoryBufferStream fs(&global_checkpoint);
-  fs.Write(&version_number, sizeof(version_number));
-  global_model->Save(fs);
+  if (lazy_checkpt) {
+    global_lazycheck = global_model;
+  } else {
+    global_checkpoint.resize(0);
+    utils::MemoryBufferStream fs(&global_checkpoint);
+    fs.Write(&version_number, sizeof(version_number));
+    global_model->Save(fs);
+    global_lazycheck = NULL;
+  }
   // reset result buffer
   resbuf.Clear(); seq_counter = 0;
   // execute check ack step, load happens here
@@ -396,6 +422,8 @@ AllreduceRobust::ReturnType AllreduceRobust::TryResetLinks(void) {
  */
 bool AllreduceRobust::CheckAndRecover(ReturnType err_type) {
   if (err_type == kSuccess) return true;
+  utils::Assert(err_link != NULL, "must know the error source");
+  recover_counter += 1;
   {
     // simple way, shutdown all links
     for (size_t i = 0; i < all_links.size(); ++i) {
@@ -407,7 +435,7 @@ bool AllreduceRobust::CheckAndRecover(ReturnType err_type) {
   // this was old way
   // TryResetLinks still causes possible errors, so not use this one
   while (err_type != kSuccess) {
-    switch (err_type) {
+    switch (err_type.value) {
       case kGetExcept: err_type = TryResetLinks(); break;
       case kSockError: {
         TryResetLinks();
@@ -577,6 +605,9 @@ AllreduceRobust::TryRecoverData(RecoverType role,
     if (!req_data) return kSuccess;
   }
   utils::Assert(recv_link >= 0 || role == kHaveData, "recv_link must be active");
+  if (role == kPassData) {
+    links[recv_link].InitBuffer(1, size, reduce_buffer_size);
+  }
   for (int i = 0; i < nlink; ++i) {
     links[i].ResetSize();
   }
@@ -601,27 +632,33 @@ AllreduceRobust::TryRecoverData(RecoverType role,
     selecter.Select();
     // exception handling
     for (int i = 0; i < nlink; ++i) {
-      if (selecter.CheckExcept(links[i].sock)) return kGetExcept;
+      if (selecter.CheckExcept(links[i].sock)) {
+        return ReportError(&links[i], kGetExcept);
+      }
     }
     if (role == kRequestData) {
       const int pid = recv_link;
       if (selecter.CheckRead(links[pid].sock)) {
-        if (!links[pid].ReadToArray(sendrecvbuf_, size)) return kSockError;
+        ReturnType ret = links[pid].ReadToArray(sendrecvbuf_, size);        
+        if (ret != kSuccess) {
+          return ReportError(&links[pid], ret);
+        }
       }
       for (int i = 0; i < nlink; ++i) {
-        if (req_in[i] && links[i].size_write != links[pid].size_read &&
-            selecter.CheckWrite(links[i].sock)) {
-          if (!links[i].WriteFromArray(sendrecvbuf_, links[pid].size_read)) {
-            return kSockError;
+        if (req_in[i] && links[i].size_write != links[pid].size_read) {
+          ReturnType ret = links[i].WriteFromArray(sendrecvbuf_, links[pid].size_read);
+          if (ret != kSuccess) {
+            return ReportError(&links[i], ret);
           }
         }
       }
     }
     if (role == kHaveData) {
       for (int i = 0; i < nlink; ++i) {
-        if (req_in[i] && selecter.CheckWrite(links[i].sock)) {
-          if (!links[i].WriteFromArray(sendrecvbuf_, size)) {
-            return kSockError;
+        if (req_in[i] && links[i].size_write != size) {
+          ReturnType ret = links[i].WriteFromArray(sendrecvbuf_, size);
+          if (ret != kSuccess) {
+            return ReportError(&links[i], ret);
           }
         }
       }
@@ -635,11 +672,13 @@ AllreduceRobust::TryRecoverData(RecoverType role,
           if (req_in[i]) min_write = std::min(links[i].size_write, min_write);
         }
         utils::Assert(min_write <= links[pid].size_read, "boundary check");
-        if (!links[pid].ReadToRingBuffer(min_write)) return kSockError;
+        ReturnType ret = links[pid].ReadToRingBuffer(min_write);
+        if (ret != kSuccess) {
+          return ReportError(&links[pid], ret);
+        }
       }
       for (int i = 0; i < nlink; ++i) {
-        if (req_in[i] && selecter.CheckWrite(links[i].sock) &&
-            links[pid].size_read != links[i].size_write) {
+        if (req_in[i] && links[pid].size_read != links[i].size_write) {
           size_t start = links[i].size_write % buffer_size;
           // send out data from ring buffer
           size_t nwrite = std::min(buffer_size - start, links[pid].size_read - links[i].size_write);
@@ -647,7 +686,8 @@ AllreduceRobust::TryRecoverData(RecoverType role,
           if (len != -1) {
             links[i].size_write += len;
           } else {
-            if (errno != EAGAIN && errno != EWOULDBLOCK) return kSockError;
+            ReturnType ret = Errno2Return(errno);
+            if (ret != kSuccess) return ReportError(&links[i], ret);
           }
         }
       }
@@ -698,6 +738,14 @@ AllreduceRobust::ReturnType AllreduceRobust::TryLoadCheckPoint(bool requester) {
     utils::Check(state == 1 || state == 2,
                  "LoadCheckPoint: too many nodes fails, cannot recover local state");
   }
+  // do call save model if the checkpoint was lazy
+  if (role == kHaveData && global_lazycheck != NULL) {
+    global_checkpoint.resize(0);
+    utils::MemoryBufferStream fs(&global_checkpoint);
+    fs.Write(&version_number, sizeof(version_number));
+    global_lazycheck->Save(fs);
+    global_lazycheck = NULL;
+  }
   // recover global checkpoint
   size_t size = this->global_checkpoint.length();
   int recv_link;
@@ -1098,27 +1146,28 @@ AllreduceRobust::RingPassing(void *sendrecvbuf_,
     selecter.WatchException(next.sock);
     if (finished) break;
     selecter.Select();
-    if (selecter.CheckExcept(prev.sock)) return kGetExcept;
-    if (selecter.CheckExcept(next.sock)) return kGetExcept;
+    if (selecter.CheckExcept(prev.sock)) return ReportError(&prev, kGetExcept);
+    if (selecter.CheckExcept(next.sock)) return ReportError(&next, kGetExcept);
     if (read_ptr != read_end && selecter.CheckRead(prev.sock)) {
       ssize_t len = prev.sock.Recv(buf + read_ptr, read_end - read_ptr);
       if (len == 0) {
-        prev.sock.Close(); return kSockError;
+        prev.sock.Close(); return ReportError(&prev, kRecvZeroLen);
       }
       if (len != -1) {
         read_ptr += static_cast<size_t>(len);
       } else {
-        if (errno != EAGAIN && errno != EWOULDBLOCK) return kSockError;
+        ReturnType ret = Errno2Return(errno);
+        if (ret != kSuccess) return ReportError(&prev, ret);
       }
     }
-    if (write_ptr != write_end && write_ptr < read_ptr &&
-        selecter.CheckWrite(next.sock)) {
+    if (write_ptr != write_end && write_ptr < read_ptr) {
       size_t nsend = std::min(write_end - write_ptr, read_ptr - write_ptr);
       ssize_t len = next.sock.Send(buf + write_ptr, nsend);
       if (len != -1) {
         write_ptr += static_cast<size_t>(len);
       } else {
-        if (errno != EAGAIN && errno != EWOULDBLOCK) return kSockError;
+        ReturnType ret = Errno2Return(errno);
+        if (ret != kSuccess) return ReportError(&prev, ret);
       }
     }
   }
diff --git a/src/allreduce_robust.h b/src/allreduce_robust.h
index 921f18319..23a3964d8 100644
--- a/src/allreduce_robust.h
+++ b/src/allreduce_robust.h
@@ -99,7 +99,32 @@ class AllreduceRobust : public AllreduceBase {
    * \sa LoadCheckPoint, VersionNumber
    */
   virtual void CheckPoint(const ISerializable *global_model,
-                          const ISerializable *local_model = NULL);
+                          const ISerializable *local_model = NULL) {
+    this->CheckPoint_(global_model, local_model, false);
+  }
+  /*!
+   * \brief This function can be used to replace CheckPoint for global_model only,
+   *   when certain condition is met(see detailed expplaination).
+   * 
+   *   This is a "lazy" checkpoint such that only the pointer to global_model is
+   *   remembered and no memory copy is taken. To use this function, the user MUST ensure that:
+   *   The global_model must remain unchanged util last call of Allreduce/Broadcast in current version finishs.
+   *   In another words, global_model model can be changed only between last call of 
+   *   Allreduce/Broadcast and LazyCheckPoint in current version
+   *   
+   *   For example, suppose the calling sequence is:
+   *   LazyCheckPoint, code1, Allreduce, code2, Broadcast, code3, LazyCheckPoint
+   *   
+   *   If user can only changes global_model in code3, then LazyCheckPoint can be used to
+   *   improve efficiency of the program.
+   * \param global_model pointer to the globally shared model/state
+   *   when calling this function, the caller need to gauranttees that global_model
+   *   is the same in all nodes
+   * \sa LoadCheckPoint, CheckPoint, VersionNumber
+   */
+  virtual void LazyCheckPoint(const ISerializable *global_model) {
+    this->CheckPoint_(global_model, NULL, true);
+  }
   /*!
    * \brief explicitly re-init everything before calling LoadCheckPoint
    *    call this function when IEngine throw an exception out,
@@ -274,10 +299,38 @@ class AllreduceRobust : public AllreduceBase {
     std::vector<uint64_t> data_;
   };
   /*!
-   * \brief reset the all the existing links by sending Out-of-Band message marker
-   *  after this function finishes, all the messages received and sent before in all live links are discarded,
-   *  This allows us to get a fresh start after error has happened
+   * \brief internal consistency check function,
+   *  use check to ensure user always call CheckPoint/LoadCheckPoint
+   *  with or without local but not both, this function will set the approperiate settings
+   *  in the first call of LoadCheckPoint/CheckPoint 
    *
+   * \param with_local whether the user calls CheckPoint with local model
+   */
+  void LocalModelCheck(bool with_local);
+  /*!
+   * \brief internal implementation of checkpoint, support both lazy and normal way
+   * 
+   * \param global_model pointer to the globally shared model/state
+   *   when calling this function, the caller need to gauranttees that global_model
+   *   is the same in all nodes
+   * \param local_model pointer to local model, that is specific to current node/rank
+   *   this can be NULL when no local state is needed
+   * \param lazy_checkpt whether the action is lazy checkpoint
+   *
+   * \sa CheckPoint, LazyCheckPoint
+   */
+  void CheckPoint_(const ISerializable *global_model,
+                   const ISerializable *local_model,
+                   bool lazy_checkpt);
+  /*!
+   * \brief reset the all the existing links by sending Out-of-Band message marker
+   *  after this function finishes, all the messages received and sent
+   *  before in all live links are discarded,
+   *  This allows us to get a fresh start after error has happened
+   *    
+   *  TODO(tqchen): this function is not yet functioning was not used by engine,
+   *   simple resetlink and reconnect strategy is used
+   * 
    * \return this function can return kSuccess or kSockError
    *         when kSockError is returned, it simply means there are bad sockets in the links,
    *         and some link recovery proceduer is needed
@@ -468,12 +521,18 @@ o   *  the input state must exactly one saved state(local state of current node)
   ResultBuffer resbuf;
   // last check point global model
   std::string global_checkpoint;
+  // lazy checkpoint of global model
+  const ISerializable *global_lazycheck;
   // number of replica for local state/model
   int num_local_replica;
   // number of default local replica
   int default_local_replica;
+  // flag to decide whether local model is used, -1: unknown, 0: no, 1:yes
+  int use_local_model;
   // number of replica for global state/model
   int num_global_replica;
+  // number of times recovery happens
+  int recover_counter;
   // --- recovery data structure for local checkpoint
   // there is two version of the data structure,
   // at one time one version is valid and another is used as temp memory
diff --git a/src/engine.cc b/src/engine.cc
index 45bef329c..1aa220f0e 100644
--- a/src/engine.cc
+++ b/src/engine.cc
@@ -56,7 +56,8 @@ void Allreduce_(void *sendrecvbuf,
 }
 
 // code for reduce handle
-ReduceHandle::ReduceHandle(void) : handle_(NULL), htype_(NULL) {
+ReduceHandle::ReduceHandle(void) 
+  : handle_(NULL), redfunc_(NULL), htype_(NULL) {
 }
 ReduceHandle::~ReduceHandle(void) {}
 
@@ -64,17 +65,16 @@ int ReduceHandle::TypeSize(const MPI::Datatype &dtype) {
   return static_cast<int>(dtype.type_size);
 }
 void ReduceHandle::Init(IEngine::ReduceFunction redfunc, size_t type_nbytes) {
-  utils::Assert(handle_ == NULL, "cannot initialize reduce handle twice");
-  handle_ = reinterpret_cast<void*>(redfunc);
+  utils::Assert(redfunc_ == NULL, "cannot initialize reduce handle twice");
+  redfunc_ = redfunc;
 }
 void ReduceHandle::Allreduce(void *sendrecvbuf,
                              size_t type_nbytes, size_t count,
                              IEngine::PreprocFunction prepare_fun,
                              void *prepare_arg) {
-  utils::Assert(handle_ != NULL, "must intialize handle to call AllReduce");
+  utils::Assert(redfunc_ != NULL, "must intialize handle to call AllReduce");
   GetEngine()->Allreduce(sendrecvbuf, type_nbytes, count,
-                         reinterpret_cast<IEngine::ReduceFunction*>(handle_),
-                         prepare_fun, prepare_arg);
+                         redfunc_, prepare_fun, prepare_arg);
 }
 }  // namespace engine
 }  // namespace rabit
diff --git a/src/engine_empty.cc b/src/engine_empty.cc
index 0c7020914..3d14e3ef3 100644
--- a/src/engine_empty.cc
+++ b/src/engine_empty.cc
@@ -42,6 +42,9 @@ class EmptyEngine : public IEngine {
                           const ISerializable *local_model = NULL) {
     version_number += 1;
   }
+  virtual void LazyCheckPoint(const ISerializable *global_model) {
+    version_number += 1;
+  }
   virtual int VersionNumber(void) const {
     return version_number;
   }
diff --git a/src/engine_mpi.cc b/src/engine_mpi.cc
index c1b723572..006a91006 100644
--- a/src/engine_mpi.cc
+++ b/src/engine_mpi.cc
@@ -45,6 +45,9 @@ class MPIEngine : public IEngine {
                           const ISerializable *local_model = NULL) {
     version_number += 1;
   }
+  virtual void LazyCheckPoint(const ISerializable *global_model) {
+    version_number += 1;
+  }
   virtual int VersionNumber(void) const {
     return version_number;
   }
@@ -134,7 +137,8 @@ void Allreduce_(void *sendrecvbuf,
 }
 
 // code for reduce handle
-ReduceHandle::ReduceHandle(void) : handle_(NULL), htype_(NULL) {
+ReduceHandle::ReduceHandle(void) 
+    : handle_(NULL), redfunc_(NULL), htype_(NULL) {
 }
 ReduceHandle::~ReduceHandle(void) {
   if (handle_ != NULL) {
diff --git a/test/test.mk b/test/test.mk
index efc5b418c..99d146d08 100644
--- a/test/test.mk
+++ b/test/test.mk
@@ -1,13 +1,7 @@
-ifndef $(nslave)
-	nslave=2
-endif
-ifndef $(ndata)
-	ndata=10
-endif
-
 # this is a makefile used to show testcases of rabit
-.PHONY: model_recover local_recover speed
+.PHONY:all
 
+all:
 
 # this experiment test recovery with actually process exit, use keepalive to keep program alive
 model_recover_10_10k:
@@ -18,3 +12,7 @@ model_recover_10_10k_die_same:
 
 model_recover_10_10k_die_hard:
 	../tracker/rabit_demo.py -n 10 test_model_recover 10000 mock=0,0,1,0 mock=1,1,1,0 mock=1,1,1,1 mock=0,1,1,0 mock=4,1,1,0 mock=9,1,1,0 mock=8,1,2,0 mock=4,1,3,0
+
+
+local_recover_10_10k:
+	../tracker/rabit_demo.py -n 10 test_local_recover 10000 mock=0,0,1,0 mock=1,1,1,0 mock=0,1,1,0 mock=4,1,1,0 mock=9,1,1,0 mock=1,1,1,1
diff --git a/tracker/rabit_demo.py b/tracker/rabit_demo.py
index c7ee04b58..b99d03c6b 100755
--- a/tracker/rabit_demo.py
+++ b/tracker/rabit_demo.py
@@ -20,21 +20,41 @@ parser.add_argument('command', nargs='+',
                     help = 'command for rabit program')
 args = parser.parse_args()
 
+# bash script for keepalive
+# use it so that python do not need to communicate with subprocess
+echo="echo %s rabit_num_trial=$nrep;"
+keepalive = """
+nrep=0
+rc=254
+while [ $rc -eq 254 ]; 
+do
+    %s
+    %s %s rabit_num_trial=$nrep
+    rc=$?;
+    nrep=$((nrep+1));
+done
+"""
+
 def exec_cmd(cmd, taskid):
     if cmd[0].find('/') == -1 and os.path.exists(cmd[0]):
         cmd[0] = './' + cmd[0]
     cmd = ' '.join(cmd)
+    arg = ' rabit_task_id=%d' % (taskid) 
+    cmd = cmd + arg
     ntrial = 0
     while True:
         prep = 'PYTHONPATH=\"%s\" ' % WRAPPER_PATH
-        arg = ' rabit_task_id=%d rabit_num_trial=%d' % (taskid, ntrial)        
-        ret = subprocess.call(prep + cmd + arg, shell = True)
-        if ret == 254 or ret == -2:
-            ntrial += 1
-            continue
+        if args.verbose != 0:            
+            bash = keepalive % (echo % cmd, prep, cmd)
+        else:
+            bash = keepalive % ('', prep, cmd)            
+        ret = subprocess.call(bash, shell=True, executable='bash')
         if ret == 0:
+            if args.verbose != 0:        
+                print 'Thread %d exit with 0' % taskid
             return
-        raise Exception('Get nonzero return code=%d' % ret)
+        else:
+            raise Exception('Get nonzero return code=%d' % ret)
 #
 #  Note: this submit script is only used for demo purpose
 #  submission script using pyhton multi-threading
@@ -51,6 +71,7 @@ def mthread_submit(nslave, worker_args):
     procs = {}
     for i in range(nslave):
         procs[i] = Thread(target = exec_cmd, args = (args.command + worker_args, i))
+        procs[i].daemon = True
         procs[i].start()
     for i in range(nslave):
         procs[i].join()
diff --git a/tracker/rabit_tracker.py b/tracker/rabit_tracker.py
index 9823ef426..e260c19c3 100644
--- a/tracker/rabit_tracker.py
+++ b/tracker/rabit_tracker.py
@@ -257,6 +257,7 @@ class Tracker:
 def submit(nslave, args, fun_submit, verbose):
     master = Tracker(verbose = verbose)
     submit_thread = Thread(target = fun_submit, args = (nslave, args + master.slave_args()))
+    submit_thread.daemon = True
     submit_thread.start()
     master.accept_slaves(nslave)
     submit_thread.join()