Memcache Keys Not Expiring

        > //设置memcache session缓存
        > $di->set('smssession', function(){
        >     $memcache = new Phalcon\Session\Adapter\Memcache(array(
        >         'uniqueId'   => 'msms',
        >         'host'       => '127.0.0.1',
        >         'port'       => 11211,
        >         // 'lifetime'   => 1,            // 300 5分钟
        >         'prefix'     => 'xxf',
        >         'persistent' => false
        >     ));
        >     $memcache->start();
        >     return $memcache;
        > });

as above: but my smssession never end after 300 second, why