memcached never expired in rails?

It's very strange that the session will never expire 开发者_如何学Cif I use memcached store even I set

config.action_controller.session :session_expires => 1.seconds.from_now.

And I use extended_fragment_cache to cache fragment, I meet the same problem

<% Cache "my_page", {:expires => 1.minutes} do ... %>

never expired!

Anyone know where's the problem?

I change the codes to

ActionController::Base.session_options[:expire_after] = 1.seconds

and everything now.