很抱歉这个问题很长，但我找不到更好的方法来总结它。
我有一个程序，它使用Python的multiprocessing在并行运行一些计算。流程之间的通信使用两个Queue对象，work_queue和result_queue完成。
主进程使用用于计算的数据填充work_queue，然后启动多个子进程，这些子进程将使用此队列并将结果存储到result_queue。
一切似乎都运行良好，但随着我对样本数据量（即进入work_queue的数据量）和子进程数量的影响，我开始得到这个错误令我困惑了几个小时。
以下代码说明了问题：
# -- queue_bug.py --
import sys
import time
import random
import datetime
import traceback
# Need this to catch the Queue.Empty exception
import Queue
import multiprocessing
from multiprocessing import Process
from multiprocessing import Queue as MultiprocessingQueue
# -------------------------------------------------------------------
# do_calculation
# -------------------------------------------------------------------
def do_calculation(p_name, work_queue, result_queue):
def log(msg):
print '%s [%s] %s' % (datetime.datetime.now(), p_name, msg)
log('Starting up...')
while True:
# Get work from queue
try:
work = work_queue.get(timeout = 0.1)
test_id = work[0]
test_data = work[1]
except Queue.Empty:
break
# this is just a dummy loop
for i in range(100):
test_result = [x * random.random() for x in test_data]
result_queue.put((test_id, test_data, test_result))
log('Finished')
# -------------------------------------------------------------------
# main
# -------------------------------------------------------------------
def main():
def log(msg):
print '%s [main ] %s' % (datetime.datetime.now(), msg)
try:
num_tests = int(sys.argv[1])
num_procs = int(sys.argv[2])
except Exception:
print 'usage: <prog> number-of-tests number-of-subprocesses'
sys.exit()
log('Initializing queues...')
work_queue = MultiprocessingQueue()
result_queue = MultiprocessingQueue()
log('Creating subprocesses...')
process_list = []
for i in range(num_procs):
p_name = 'PROC_%02d' % (i+1)
log(' Initializing %s' % p_name)
p = Process(
target = do_calculation,
args = (p_name, work_queue, result_queue),
name = p_name)
p.daemon = True
process_list.append(p)
log('Populating the work_queue...')
for test_id in range(num_tests):
work_queue.put((test_id, [test_id]*20))
log('Work_queue size is %d' % work_queue.qsize())
log('Starting the subprocesses...')
for p in process_list:
p.start()
log('Waiting until the work_queue is empty...')
while True:
log(' Work_queue size is %d' % work_queue.qsize())
if work_queue.qsize() > 0:
time.sleep(0.5)
else:
break
log('Waiting until the result_queue is completely filled...')
while True:
log(' Result_queue size is %d' % result_queue.qsize())
if result_queue.qsize() < num_tests:
time.sleep(0.5)
else:
break
log('Getting results...')
result_dict = {}
while True:
try:
queue_data = result_queue.get_nowait()
test_id = queue_data[0]
test_data = queue_data[1]
test_result = queue_data[2]
result_dict[test_id] = test_result
except Queue.Empty:
log(' All results loaded from result_queue')
break
log('Storing test results in result_summary...')
result_summary = []
for test_id in range(num_tests):
try:
test_result = result_dict[test_id]
result_summary.append((test_id, test_result))
except KeyError:
ex = traceback.format_exc()
log('ERROR: Exception found: %s' % ex)
sys.exit()
log('Success.')
return result_summary
if __name__ == '__main__':
main()
现在，当我尝试运行它时：
$ python queue_bug.py 10000 10
2012-12-04 19:24:25.430667 [main ] Initializing queues...
2012-12-04 19:24:25.440521 [main ] Creating subprocesses...
2012-12-04 19:24:25.440550 [main ] Initializing PROC_01
2012-12-04 19:24:25.440576 [main ] Initializing PROC_02
2012-12-04 19:24:25.440597 [main ] Initializing PROC_03
2012-12-04 19:24:25.440617 [main ] Initializing PROC_04
2012-12-04 19:24:25.440637 [main ] Initializing PROC_05
2012-12-04 19:24:25.440656 [main ] Initializing PROC_06
2012-12-04 19:24:25.440679 [main ] Initializing PROC_07
2012-12-04 19:24:25.440699 [main ] Initializing PROC_08
2012-12-04 19:24:25.440721 [main ] Initializing PROC_09
2012-12-04 19:24:25.440741 [main ] Initializing PROC_10
2012-12-04 19:24:25.440759 [main ] Populating the work_queue...
2012-12-04 19:24:25.494263 [main ] Work_queue size is 10000
2012-12-04 19:24:25.494301 [main ] Starting the subprocesses...
2012-12-04 19:24:25.495515 [PROC_01] Starting up...
2012-12-04 19:24:25.495802 [PROC_02] Starting up...
2012-12-04 19:24:25.496212 [PROC_03] Starting up...
2012-12-04 19:24:25.496557 [PROC_04] Starting up...
2012-12-04 19:24:25.496896 [PROC_05] Starting up...
2012-12-04 19:24:25.497300 [PROC_06] Starting up...
2012-12-04 19:24:25.497705 [PROC_07] Starting up...
2012-12-04 19:24:25.498074 [PROC_08] Starting up...
2012-12-04 19:24:25.498258 [main ] Waiting until the work_queue is empty...
2012-12-04 19:24:25.498349 [main ] Work_queue size is 9974
2012-12-04 19:24:25.498661 [PROC_09] Starting up...
2012-12-04 19:24:25.499765 [PROC_10] Starting up...
2012-12-04 19:24:25.998914 [main ] Work_queue size is 0
2012-12-04 19:24:25.998954 [main ] Waiting until the result_queue is completely filled...
2012-12-04 19:24:25.998976 [main ] Result_queue size is 10000
2012-12-04 19:24:25.998993 [main ] Getting results...
2012-12-04 19:24:26.029774 [PROC_06] Finished
2012-12-04 19:24:26.029798 [PROC_03] Finished
2012-12-04 19:24:26.029824 [PROC_08] Finished
2012-12-04 19:24:26.029853 [PROC_02] Finished
2012-12-04 19:24:26.029868 [PROC_01] Finished
2012-12-04 19:24:26.029898 [PROC_07] Finished
2012-12-04 19:24:26.029921 [PROC_09] Finished
2012-12-04 19:24:26.029942 [PROC_10] Finished
2012-12-04 19:24:26.031040 [PROC_04] Finished
2012-12-04 19:24:26.031057 [PROC_05] Finished
2012-12-04 19:24:26.087804 [main ] All results loaded from result_queue
2012-12-04 19:24:26.087844 [main ] Storing test results in result_summary...
2012-12-04 19:24:26.092477 [main ] Success.
$ python queue_bug.py 70000 10
2012-12-04 19:25:01.083092 [main ] Initializing queues...
2012-12-04 19:25:01.093483 [main ] Creating subprocesses...
2012-12-04 19:25:01.093520 [main ] Initializing PROC_01
2012-12-04 19:25:01.093548 [main ] Initializing PROC_02
2012-12-04 19:25:01.093570 [main ] Initializing PROC_03
2012-12-04 19:25:01.093591 [main ] Initializing PROC_04
2012-12-04 19:25:01.093612 [main ] Initializing PROC_05
2012-12-04 19:25:01.093632 [main ] Initializing PROC_06
2012-12-04 19:25:01.093656 [main ] Initializing PROC_07
2012-12-04 19:25:01.093676 [main ] Initializing PROC_08
2012-12-04 19:25:01.093699 [main ] Initializing PROC_09
2012-12-04 19:25:01.093720 [main ] Initializing PROC_10
2012-12-04 19:25:01.093738 [main ] Populating the work_queue...
2012-12-04 19:25:01.395974 [main ] Work_queue size is 70000
2012-12-04 19:25:01.396012 [main ] Starting the subprocesses...
2012-12-04 19:25:01.397601 [PROC_01] Starting up...
2012-12-04 19:25:01.398183 [PROC_02] Starting up...
2012-12-04 19:25:01.398545 [PROC_03] Starting up...
2012-12-04 19:25:01.399021 [PROC_04] Starting up...
2012-12-04 19:25:01.399621 [PROC_05] Starting up...
2012-12-04 19:25:01.400137 [PROC_06] Starting up...
2012-12-04 19:25:01.400675 [PROC_07] Starting up...
2012-12-04 19:25:01.401200 [PROC_08] Starting up...
2012-12-04 19:25:01.401645 [main ] Waiting until the work_queue is empty...
2012-12-04 19:25:01.401691 [PROC_09] Starting up...
2012-12-04 19:25:01.401738 [main ] Work_queue size is 69959
2012-12-04 19:25:01.402387 [PROC_10] Starting up...
2012-12-04 19:25:01.902063 [main ] Work_queue size is 58415
2012-12-04 19:25:02.402640 [main ] Work_queue size is 47302
2012-12-04 19:25:02.903067 [main ] Work_queue size is 36145
2012-12-04 19:25:03.403650 [main ] Work_queue size is 24992
2012-12-04 19:25:03.904065 [main ] Work_queue size is 13481
2012-12-04 19:25:04.404643 [main ] Work_queue size is 1951
2012-12-04 19:25:04.588562 [PROC_02] Finished
2012-12-04 19:25:04.588580 [PROC_06] Finished
2012-12-04 19:25:04.588611 [PROC_10] Finished
2012-12-04 19:25:04.588631 [PROC_03] Finished
2012-12-04 19:25:04.589705 [PROC_04] Finished
2012-12-04 19:25:04.589741 [PROC_09] Finished
2012-12-04 19:25:04.589764 [PROC_05] Finished
2012-12-04 19:25:04.589791 [PROC_08] Finished
2012-12-04 19:25:04.589814 [PROC_01] Finished
2012-12-04 19:25:04.589844 [PROC_07] Finished
2012-12-04 19:25:04.905065 [main ] Work_queue size is 0
2012-12-04 19:25:04.905098 [main ] Waiting until the result_queue is completely filled...
2012-12-04 19:25:04.905121 [main ] Result_queue size is 70000
2012-12-04 19:25:04.905140 [main ] Getting results...
2012-12-04 19:25:05.012083 [main ] All results loaded from result_queue
2012-12-04 19:25:05.012140 [main ] Storing test results in result_summary...
2012-12-04 19:25:05.020498 [main ] ERROR: Exception found: Traceback (most recent call last):
File "queue_bug.py", line 95, in main
test_result = result_dict[test_id]
KeyError: 10647
在第二次尝试时，我尝试从result_dict读取数据时出现 KeyError 。
这本字典中填充了从result_queue获得的数据，因此我怀疑它必须与之相关。
我还注意到，每当我使用失败的参数组合（例如70000/10）运行它时，KeyError会在不同的密钥上引发，这似乎表明存在一些并发/同步问题。
最后但同样重要的是，如果样本数据的大小或子过程的数量增加，则重现它的可能性会增加。
有什么想法吗？
答案 0 :(得分：3)
我为你的示例代码添加了一些额外的调试，我想我发现了问题。除了有关使用JoinableQueue的现有评论之外，主要问题是在处理结果时最后会执行以下操作：
try:
queue_data = result_queue.get_nowait()
except Queue.Empty:
break
但即使队列实际上不是空的，也可以引发Queue.Empty，但仅仅因为get_nowait()超时太快。相反，你应该尝试：
try:
queue_data = result_queue.get_nowait()
except Queue.Empty:
if result_queue.qsize() < 1:
break
换句话说，确保在断开循环之前队列实际上是空的。
答案 1 :(得分：3)
您的错误位于循环中，您可以从结果队列中获取数据
while result_queue.qsize() > 0:
try:
queue_data = result_queue.get()
test_id = queue_data[0]
test_data = queue_data[1]
test_result = queue_data[2]
result_dict[test_id] = test_result
except Queue.Empty:
log(' All results loaded from result_queue')
break
当剩下的结果还剩下时，您的get_nowait仍然是空的。我怀疑幕后有一些问题，因为在放弃之前未完成的线程数量会增加。