public function gc($lifetime = 0)
    {
        $timeout = $this->getConfig('timeout');

        //find all sessions that are older than $timeout
        $olderThan = time() - $timeout;

        //no ack required
        $this->sessions->remove(
            array('last_accessed' => array('$lt' => new MongoDate($olderThan))),
            ( (class_exists('MongoClient')) ? (array('w'=>0)) : (array('safe'=>false)) )
        );

        return true;
    }