diff --git a/shadowsocks/lru_cache.py b/shadowsocks/lru_cache.py
index 2a161df..9017f81 100644
--- a/shadowsocks/lru_cache.py
+++ b/shadowsocks/lru_cache.py
@@ -107,10 +107,10 @@ class LRUCache(collections.MutableMapping):
             logging.debug('%d keys swept' % c)
         return c < SWEEP_MAX_ITEMS
 
-    def clear(self):
+    def clear(self, keep):
         now = time.time()
         c = 0
-        while c < SWEEP_MAX_ITEMS:
+        while len(self._keys_to_last_time) > keep:
             if len(self._keys_to_last_time) == 0:
                 break
             for key in self._keys_to_last_time:
diff --git a/shadowsocks/udprelay.py b/shadowsocks/udprelay.py
index e4d515a..a4587b8 100644
--- a/shadowsocks/udprelay.py
+++ b/shadowsocks/udprelay.py
@@ -1138,6 +1138,8 @@ class UDPRelay(object):
                         (common.to_str(server_addr), server_port,
                             r_addr[0], r_addr[1]))
 
+        self._cache.clear(256)
+
         if self._is_local:
             ref_iv = [encrypt.encrypt_new_iv(self._method)]
             self._protocol.obfs.server_info.iv = ref_iv[0]
@@ -1344,7 +1346,7 @@ class UDPRelay(object):
         logging.debug('UDP close')
         self._closed = True
         if not next_tick:
-            self._cache.clear()
+            self._cache.clear(0)
             if self._eventloop:
                 self._eventloop.remove_periodic(self.handle_periodic)
                 self._eventloop.remove(self._server_socket)