def put(self, taskid, priority=0, exetime=0):
        """
        Put a task into task queue
        
        when use heap sort, if we put tasks(with the same priority and exetime=0) into queue,
        the queue is not a strict FIFO queue, but more like a FILO stack.
        It is very possible that when there are continuous big flow, the speed of select is 
        slower than request, resulting in priority-queue accumulation in short time.
        In this scenario, the tasks more earlier entering the priority-queue will not get 
        processed until the request flow becomes small. 
        
        Thus, we store a global atom self increasing value into task.sequence which represent 
        the task enqueue sequence. When the comparison of exetime and priority have no 
        difference, we compare task.sequence to ensure that the entire queue is ordered.
        """
        now = time.time()

        task = InQueueTask(taskid, priority, exetime)

        self.mutex.acquire()
        if taskid in self.priority_queue:
            self.priority_queue.put(task)
        elif taskid in self.time_queue:
            self.time_queue.put(task)
        elif taskid in self.processing and self.processing[taskid].taskid:
            # force update a processing task is not allowed as there are so many
            # problems may happen
            pass
        else:
            if exetime and exetime > now:
                self.time_queue.put(task)
            else:
                task.exetime = 0
                self.priority_queue.put(task)

        self.mutex.release()