diff --git a/oneflow/core/graph/chain_graph.cpp b/oneflow/core/graph/chain_graph.cpp
index 9262ffbb63db879d083beced8287ced98e0c322b..1f387c06c6be7bab885af9708d1cecfa31c09179 100644
--- a/oneflow/core/graph/chain_graph.cpp
+++ b/oneflow/core/graph/chain_graph.cpp
@@ -21,7 +21,6 @@ class ChainMerger final {
   void InitTaskNode2UId();
   void InitChains();
   bool DoMerge(std::list<ChainIt>& chains, ChainIt rhs);
-  bool TryMerge();
   void MergeChains();
   int64_t get_task_uid(TaskNode* task_node) const {
     auto uid_it = task_node2uid_.find(task_node);
@@ -79,26 +78,19 @@ bool ChainMerger::DoMerge(std::list<ChainIt>& chains, ChainIt rhs) {
   return false;
 }
 
-bool ChainMerger::TryMerge() {
+void ChainMerger::MergeChains() {
   HashMap<std::pair<int64_t, int64_t>, std::list<ChainIt>> stream_area2chains;
-  bool merge_happened = false;
   for (auto cur_chain_it = chain_list_.begin(); cur_chain_it != chain_list_.end();) {
     std::pair<int64_t, int64_t> stream_area_id = {cur_chain_it->stream_id, cur_chain_it->area_id};
     auto stream_area_it = stream_area2chains.find(stream_area_id);
     if (stream_area_it != stream_area2chains.end()
         && DoMerge(stream_area_it->second, cur_chain_it)) {
       cur_chain_it = chain_list_.erase(cur_chain_it);
-      merge_happened = true;
     } else {
       stream_area2chains[stream_area_id].push_back(cur_chain_it);
       ++cur_chain_it;
     }
   }
-  return merge_happened;
-}
-
-void ChainMerger::MergeChains() {
-  while (TryMerge()) {}
 }
 
 }  // namespace
diff --git a/oneflow/core/graph/task_graph.cpp b/oneflow/core/graph/task_graph.cpp
index dc75e2c313b0ae7f5266c3adf358262d9645191a..2d48617303f5c993115c34276f47bbc85bf7391a 100644
--- a/oneflow/core/graph/task_graph.cpp
+++ b/oneflow/core/graph/task_graph.cpp
@@ -484,19 +484,14 @@ void TaskGraph::AddOrderCtrlEdgeBetweenCopyAndMdUpdt() {
 }
 
 void TaskGraph::CollectAncestorsForEachNode() {
-  std::vector<TaskNode*> ordered_nodes;
-  AcyclicTopoForEachNode([&](TaskNode* node) { ordered_nodes.emplace_back(node); });
-  for (auto it = ordered_nodes.begin(); it != ordered_nodes.end(); ++it) {
-    TaskNode* task_node = *it;
+  AcyclicTopoForEachNode([&](TaskNode* task_node) {
     task_node->mut_ancestors().clear();
-    task_node->ForEachNodeOnInEdge([&](TaskNode* node_on_in_edge) {
-      if (IsBackEdge(node_on_in_edge, task_node)) return;
-      task_node->mut_ancestors().insert(node_on_in_edge->ancestors().begin(),
-                                        node_on_in_edge->ancestors().end());
-      task_node->mut_ancestors().insert(node_on_in_edge);
-
+    task_node->ForEachNodeOnInEdge([&](TaskNode* in_node) {
+      if (IsBackEdge(in_node, task_node)) return;
+      task_node->mut_ancestors().insert(in_node->ancestors().begin(), in_node->ancestors().end());
+      task_node->mut_ancestors().insert(in_node);
     });
-  }
+  });
 }
 
 void TaskGraph::AcyclicTopoForEachNode(std::function<void(TaskNode* node)> handler) const {