Refactor radix cache helper functions to use iterative approaches

Replace recursive implementations with iterative ones using stacks in `_match_prefix_helper`, `_print_helper`, and `_total_size_helper` to prevent potential stack overflow issues. Simplify the return value handling in `_match_prefix_helper` by removing the list parameter and returning a tuple directly. Streamline the `_insert_helper` logic with a while loop for better readability and maintainability. These changes improve performance and reliability while maintaining the same functionality.

python/sglang/srt/mem_cache/radix_cache.py

@@ -111,14 +111,12 @@ def match_prefix(self, key: List[int], **kwargs) -> Tuple[torch.Tensor, int]:
         if self.disable:
             return [], self.root_node
 
-        value = []
-        last_node = [self.root_node]
-        self._match_prefix_helper(self.root_node, key, value, last_node)
+        value, last_node = self._match_prefix_helper(self.root_node, key)
         if value:
             value = torch.concat(value)
         else:
             value = torch.tensor([], dtype=torch.int32)
-        return value, last_node[0]
+        return value, last_node
 
     def insert(self, key: List, value=None):
         if self.disable:
@@ -191,7 +189,7 @@ def pretty_print(self):
         print(f"#tokens: {self.total_size()}")
 
     def total_size(self):
-        return self._total_size_helper(self.root_node)
+        return self._total_size_helper()
 
     def evict(self, num_tokens: int, evict_callback: Callable):
         if self.disable:
@@ -253,24 +251,23 @@ def protected_size(self):
 
     ##### Internal Helper Functions #####
 
-    def _match_prefix_helper(
-        self, node: TreeNode, key: List, value, last_node: TreeNode
-    ):
+    def _match_prefix_helper(self, node: TreeNode, key: List):
         node.last_access_time = time.time()
-        if len(key) == 0:
-            return
-
-        if key[0] in node.children.keys():
+        value = []
+        while len(key) > 0 and key[0] in node.children.keys():
             child = node.children[key[0]]
+            child.last_access_time = time.time()
             prefix_len = _key_match(child.key, key)
             if prefix_len < len(child.key):
                 new_node = self._split_node(child.key, child, prefix_len)
                 value.append(new_node.value)
-                last_node[0] = new_node
+                node = new_node
+                break
             else:
                 value.append(child.value)
-                last_node[0] = child
-                self._match_prefix_helper(child, key[prefix_len:], value, last_node)
+                node = child
+                key = key[prefix_len:]
+        return value, node
 
     def _split_node(self, key, child: TreeNode, split_len: int):
         # new_node -> child
@@ -291,22 +288,25 @@ def _insert_helper(self, node: TreeNode, key: List, value):
         if len(key) == 0:
             return 0
 
-        if key[0] in node.children.keys():
+        total_prefix_length = 0
+        while len(key) > 0 and key[0] in node.children.keys():
             child = node.children[key[0]]
+            child.last_access_time = time.time()
             prefix_len = _key_match(child.key, key)
-
+            total_prefix_length += prefix_len
             if prefix_len == len(child.key):
                 if prefix_len == len(key):
-                    return prefix_len
+                    break
                 else:
                     key = key[prefix_len:]
                     value = value[prefix_len:]
-                    return prefix_len + self._insert_helper(child, key, value)
-
-            new_node = self._split_node(child.key, child, prefix_len)
-            return prefix_len + self._insert_helper(
-                new_node, key[prefix_len:], value[prefix_len:]
-            )
+                    node = child
+            else:
+                new_node = self._split_node(child.key, child, prefix_len)
+                key = key[prefix_len:]
+                value = value[prefix_len:]
+                node = new_node
+                break
 
         if len(key):
             new_node = TreeNode()
@@ -315,12 +315,21 @@ def _insert_helper(self, node: TreeNode, key: List, value):
             new_node.value = value
             node.children[key[0]] = new_node
             self.evictable_size_ += len(value)
-        return 0
+        return total_prefix_length
 
     def _print_helper(self, node: TreeNode, indent: int):
-        for _, child in node.children.items():
-            print(" " * indent, len(child.key), child.key[:10], f"r={child.lock_ref}")
-            self._print_helper(child, indent=indent + 2)
+        """Prints the radix tree in a human-readable format."""
+        stack = [(node, indent)]
+        while stack:
+            current_node, current_indent = stack.pop()
+            print(
+                " " * current_indent,
+                len(current_node.key),
+                current_node.key[:10],
+                f"r={current_node.lock_ref}",
+            )
+            for _, child in current_node.children.items():
+                stack.append((child, current_indent + 2))
 
     def _delete_leaf(self, node):
         for k, v in node.parent.children.items():
@@ -329,13 +338,14 @@ def _delete_leaf(self, node):
         del node.parent.children[k]
         self.evictable_size_ -= len(node.key)
 
-    def _total_size_helper(self, node: TreeNode):
-        if node.evicted:
-            return 0
-        x = len(node.value)
-        for child in node.children.values():
-            x += self._total_size_helper(child)
-        return x
+    def _total_size_helper(self):
+        total_size = 0
+        stack = [self.root_node]
+        while stack:
+            current_node = stack.pop()
+            total_size += len(current_node.value)
+            stack.extend(current_node.children.values())
+        return total_size
 
     def _collect_leaves(self):
         ret_list = []