def killall(self, wait = None):
        """
        Kill all active workers.
        @wait: Seconds to wait until last worker ends.
               If None it waits forever.
        """
        with self.tasks.mutex:
            self.tasks.queue.clear()
        self.count_lock.acquire()
        self.kill(self.workers)
        self.count_lock.release()
        self.kill_event.wait(wait)