问题描述
我希望将 multiprocessing.Queue 转储到列表中.对于该任务,我编写了以下函数:
I wish to dump a multiprocessing.Queue into a list. For that task I've written the following function:
import Queue def dump_queue(queue): """ Empties all pending items in a queue and returns them in a list. """ result = [] # START DEBUG CODE initial_size = queue.qsize() print("Queue has %s items initially." % initial_size) # END DEBUG CODE while True: try: thing = queue.get(block=False) result.append(thing) except Queue.Empty: # START DEBUG CODE current_size = queue.qsize() total_size = current_size + len(result) print("Dumping complete:") if current_size == initial_size: print("No items were added to the queue.") else: print("%s items were added to the queue." % (total_size - initial_size)) print("Extracted %s items from the queue, queue has %s items left" % (len(result), current_size)) # END DEBUG CODE return result
但由于某种原因它不起作用.
But for some reason it doesn't work.
观察以下 shell 会话:
Observe the following shell session:
>>> import multiprocessing >>> q = multiprocessing.Queue() >>> for i in range(100): ... q.put([range(200) for j in range(100)]) ... >>> q.qsize() 100 >>> l=dump_queue(q) Queue has 100 items initially. Dumping complete: 0 items were added to the queue. Extracted 1 items from the queue, queue has 99 items left >>> l=dump_queue(q) Queue has 99 items initially. Dumping complete: 0 items were added to the queue. Extracted 3 items from the queue, queue has 96 items left >>> l=dump_queue(q) Queue has 96 items initially. Dumping complete: 0 items were added to the queue. Extracted 1 items from the queue, queue has 95 items left >>>
这里发生了什么?为什么不是所有的物品都被倾倒了?
What's happening here? Why aren't all the items being dumped?
推荐答案
试试这个:
import Queue import time def dump_queue(queue): """ Empties all pending items in a queue and returns them in a list. """ result = [] for i in iter(queue.get, 'STOP'): result.append(i) time.sleep(.1) return result import multiprocessing q = multiprocessing.Queue() for i in range(100): q.put([range(200) for j in range(100)]) q.put('STOP') l=dump_queue(q) print len(l)
多处理队列有一个内部缓冲区,该缓冲区有一个馈线线程,该线程从缓冲区中提取工作并将其刷新到管道中.如果不是所有的对象都被刷新,我可以看到 Empty 过早引发的情况.使用哨兵来指示队列的结束是安全的(可靠的).此外,使用 iter(get, sentinel) 习惯用法比依赖 Empty 更好.
Multiprocessing queues have an internal buffer which has a feeder thread which pulls work off a buffer and flushes it to the pipe. If not all of the objects have been flushed, I could see a case where Empty is raised prematurely. Using a sentinel to indicate the end of the queue is safe (and reliable). Also, using the iter(get, sentinel) idiom is just better than relying on Empty.
我不喜欢它可能由于刷新时间而升空(我添加了 time.sleep(.1) 以允许上下文切换到馈线线程,您可能不需要它,没有它它也可以工作 - 它是释放 GIL 的习惯).
I don't like that it could raise empty due to flushing timing (I added the time.sleep(.1) to allow a context switch to the feeder thread, you may not need it, it works without it - it's a habit to release the GIL).