add lazy check, need test, find a race condition

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

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();