我意识到，在Google Colab环境中缓存PySpark Dataframe 后，使用任何方法（如show（）或任何其他方法）都会出现以下错误：
例如：

df.show(5)

---------------------------------------------------------------------------
ConnectionRefusedError                    Traceback (most recent call last)
/tmp/ipykernel_26/1842469281.py in <module>
----> 1 df.show(5)

/opt/conda/lib/python3.7/site-packages/pyspark/sql/dataframe.py in show(self, n, truncate, vertical)
    604 
    605         if isinstance(truncate, bool) and truncate:
--> 606             print(self._jdf.showString(n, 20, vertical))
    607         else:
    608             try:

/opt/conda/lib/python3.7/site-packages/py4j/java_gateway.py in __call__(self, *args)
   1318             proto.END_COMMAND_PART
   1319 
-> 1320         answer = self.gateway_client.send_command(command)
   1321         return_value = get_return_value(
   1322             answer, self.gateway_client, self.target_id, self.name)

/opt/conda/lib/python3.7/site-packages/py4j/java_gateway.py in send_command(self, command, retry, binary)
   1034          if `binary` is `True`.
   1035         """
-> 1036         connection = self._get_connection()
   1037         try:
   1038             response = connection.send_command(command)

/opt/conda/lib/python3.7/site-packages/py4j/clientserver.py in _get_connection(self)
    282 
    283         if connection is None or connection.socket is None:
--> 284             connection = self._create_new_connection()
    285         return connection
    286 

/opt/conda/lib/python3.7/site-packages/py4j/clientserver.py in _create_new_connection(self)
    289             self.java_parameters, self.python_parameters,
    290             self.gateway_property, self)
--> 291         connection.connect_to_java_server()
    292         self.set_thread_connection(connection)
    293         return connection

/opt/conda/lib/python3.7/site-packages/py4j/clientserver.py in connect_to_java_server(self)
    436                 self.socket = self.ssl_context.wrap_socket(
    437                     self.socket, server_hostname=self.java_address)
--> 438             self.socket.connect((self.java_address, self.java_port))
    439             self.stream = self.socket.makefile("rb")
    440             self.is_connected = True

ConnectionRefusedError: [Errno 111] Connection refused

我是Spark/PySpark的新手，不明白为什么会发生这种情况。是因为我没有使用合适的集群吗？