make clear seperation

src/sync/sync_empty.cpp

@@ -0,0 +1,27 @@
+#include "./sync.h"
+#include "../utils/utils.h"
+// no synchronization module, single thread mode does not need it anyway
+namespace xgboost {
+namespace sync {
+int GetRank(void) {
+  return 0;
+}
+void Init(int argc, char *argv[]) {
+}
+void Finalize(void) {
+}
+template<>
+void AllReduce<uint32_t>(uint32_t *sendrecvbuf, int count, ReduceOp op) {
+}
+template<>
+void AllReduce<float>(float *sendrecvbuf, int count, ReduceOp op) {
+}
+void Bcast(std::string *sendrecv_data, int root) {
+}
+ReduceHandle::ReduceHandle(void) : handle(NULL) {}
+ReduceHandle::~ReduceHandle(void) {}
+void ReduceHandle::Init(ReduceFunction redfunc, bool commute) {}
+void ReduceHandle::AllReduce(void *sendrecvbuf, size_t n4byte) {}
+}  // namespace sync
+}  // namespace xgboost
+

src/sync/sync_mpi.cpp

@@ -1,10 +1,9 @@
 #include "./sync.h"
 #include "../utils/utils.h"
 #include "mpi.h"
-
+// use MPI to implement sync
 namespace xgboost {
 namespace sync {
-
 int GetRank(void) {
   return MPI::COMM_WORLD.Get_rank();
 }