diff --git a/chunk_codegen.py b/chunk_codegen.py
index 18d9a0c8d..5e2130ee7 100644
--- a/chunk_codegen.py
+++ b/chunk_codegen.py
@@ -958,6 +958,8 @@ class MemoryEstimator(object):
 
     def _add_active_node(self, n, active_list):
         new_active = self._get_output_node(n)[1]
+        if n.op == 'placeholder':
+            new_active.append(n.name)
         for i in new_active:
             if i not in active_list:
                 active_list.append(i)
@@ -965,7 +967,7 @@ class MemoryEstimator(object):
     def _get_delete_node(self, user, user_to_last_uses, to_keep=None):
         delete_size = 0
         delete_node = []
-        if user.op not in ("placeholder", "output"):
+        if user.op not in ("output",):
             nodes_to_delete = user_to_last_uses.get(user, [])
             if to_keep is not None:
                 keep_list = []
@@ -1258,24 +1260,30 @@ class ChunkRegionSearch(object):
 
     def _search_max_chunk_region(self, active_node, peak_node, chunk_regions):
         free_vars = self._get_free_var()
-        min_var = self._get_min_free_var(active_node, free_vars)
-
+        free_var_num = len(free_vars)
+        active_node_num = [len(i) for i in active_node]
+        min_active_node_num = min(active_node_num[free_var_num:])
+        threshold = max(free_var_num, min_active_node_num)
+        
         # from peak_node to free_var
-        chunk_region_start = len(free_vars)
+        inside_flag = False
+        chunk_region_start = free_var_num
         for i in range(peak_node, -1, -1):
-            if len(active_node[i]) == min_var:
+            if active_node_num[i] <= threshold:
+                inside_flag = True
+            if inside_flag and active_node_num[i] > threshold:
                 chunk_region_start = i + 1
                 break
-            if i in free_vars or i == 0:
-                raise RuntimeError()
+
         # from peak_node to len-2
+        inside_flag = False
         chunk_region_end = len(active_node) - 1
         for i in range(peak_node, len(active_node)):
-            if len(active_node[i]) == min_var:
+            if active_node_num[i] <= threshold:
+                inside_flag = True
+            if inside_flag and active_node_num[i] > threshold:
                 chunk_region_end = i
                 break
-            if i in free_vars or i == 0:
-                raise RuntimeError()
 
         for i in chunk_regions:
             region = i["region"]
@@ -1374,15 +1382,34 @@ class ChunkRegionSearch(object):
                     possible_chunk_region.extend(chunk_info)
         return possible_chunk_region
 
-    def _search_best_chunk_region(self, possible_chunk_regions):
+    def _search_best_chunk_region(self, possible_chunk_regions, chunk_infos):
         max_region_range = 0
-        best_regions = None
-        for i in possible_chunk_regions:
-            if i["region"][1] - i["region"][0] > max_region_range:
-                best_regions = i
-                max_region_range = i["region"][1] - i["region"][0]
-        return best_regions
-
+        best_region = None
+        while len(possible_chunk_regions) > 0:
+            for i in possible_chunk_regions:
+                if i["region"][1] - i["region"][0] > max_region_range:
+                    best_region = i
+                    max_region_range = i["region"][1] - i["region"][0]
+            if self._is_legal_region(best_region, chunk_infos):
+                break
+            possible_chunk_regions.remove(i)
+            max_region_range = 0
+            best_region = None
+        return best_region
+    
+    def _is_legal_region(self, cur_chunk_info, chunk_infos):
+        (chunk_region_start, chunk_region_end) = cur_chunk_info["region"]
+        if cur_chunk_info in chunk_infos:
+            return False
+        if chunk_region_end < chunk_region_start:
+            return False
+        for i in chunk_infos:
+            region = i["region"]
+            if not ((chunk_region_start > region[1] and chunk_region_end > region[1]) 
+                    or (chunk_region_start < region[0] and chunk_region_end < region[0])):
+                return False
+        return True
+    
     def _step_search(self, mem_peak, active_node, chunk_regions):
         peak_node = self._find_peak_node(mem_peak)
         max_chunk_region = self._search_max_chunk_region(
@@ -1393,7 +1420,7 @@ class ChunkRegionSearch(object):
         possible_chunk_regions = self._search_possible_chunk_regions(
             max_chunk_region, peak_node
         )
-        best_chunk_region = self._search_best_chunk_region(possible_chunk_regions)
+        best_chunk_region = self._search_best_chunk_region(possible_chunk_regions, chunk_regions)
         return best_chunk_region
 
     def _stop_search(self, init_mem_peak, mem_peak):
@@ -1919,5 +1946,5 @@ if CODEGEN_AVAILABLE:
 
 {prologue}
 {code}"""
-            print(fn_code)
+            # print(fn_code)
             return PythonCode(fn_code, globals_)