何时对进程调用.join（）？

Question

我正在阅读关于Python中多处理模块的各种教程，并且很难理解为什么/何时调用process.join()。例如，我偶然发现了这个例子：

nums = range(100000)
nprocs = 4

def worker(nums, out_q):
    """ The worker function, invoked in a process. 'nums' is a
        list of numbers to factor. The results are placed in
        a dictionary that's pushed to a queue.
    """
    outdict = {}
    for n in nums:
        outdict[n] = factorize_naive(n)
    out_q.put(outdict)

# Each process will get 'chunksize' nums and a queue to put his out
# dict into
out_q = Queue()
chunksize = int(math.ceil(len(nums) / float(nprocs)))
procs = []

for i in range(nprocs):
    p = multiprocessing.Process(
            target=worker,
            args=(nums[chunksize * i:chunksize * (i + 1)],
                  out_q))
    procs.append(p)
    p.start()

# Collect all results into a single result dict. We know how many dicts
# with results to expect.
resultdict = {}
for i in range(nprocs):
    resultdict.update(out_q.get())

# Wait for all worker processes to finish
for p in procs:
    p.join()

print resultdict

据我所知，process.join()将阻塞调用进程，直到调用其join方法的进程完成执行。我还相信，在上述代码示例中启动的子进程在完成目标函数时即在将结果推送到out_q之后完成执行。最后，我相信out_q.get()会阻塞调用过程，直到有结果被拉出来。因此，如果您考虑代码：

resultdict = {}
for i in range(nprocs):
    resultdict.update(out_q.get())

# Wait for all worker processes to finish
for p in procs:
    p.join()

主进程被out_q.get()调用阻塞，直到每个工作进程完成将其结果推送到队列。因此，当主进程退出for循环时，每个子进程都应该已经完成了执行，对吗？

如果是这样，那么此时是否有理由调用p.join()方法？不是所有的工作进程都已经完成了，那么这是如何导致主进程“等待所有的工作进程完成”的呢我问这个问题主要是因为我在多个不同的例子中看到了这一点，我很好奇自己是否理解不了什么。

Answer 1

尝试运行此：

import math
import time
from multiprocessing import Queue
import multiprocessing

def factorize_naive(n):
    factors = []
    for div in range(2, int(n**.5)+1):
        while not n % div:
            factors.append(div)
            n //= div
    if n != 1:
        factors.append(n)
    return factors

nums = range(100000)
nprocs = 4

def worker(nums, out_q):
    """ The worker function, invoked in a process. 'nums' is a
        list of numbers to factor. The results are placed in
        a dictionary that's pushed to a queue.
    """
    outdict = {}
    for n in nums:
        outdict[n] = factorize_naive(n)
    out_q.put(outdict)

# Each process will get 'chunksize' nums and a queue to put his out
# dict into
out_q = Queue()
chunksize = int(math.ceil(len(nums) / float(nprocs)))
procs = []

for i in range(nprocs):
    p = multiprocessing.Process(
            target=worker,
            args=(nums[chunksize * i:chunksize * (i + 1)],
                  out_q))
    procs.append(p)
    p.start()

# Collect all results into a single result dict. We know how many dicts
# with results to expect.
resultdict = {}
for i in range(nprocs):
    resultdict.update(out_q.get())

time.sleep(5)

# Wait for all worker processes to finish
for p in procs:
    p.join()

print resultdict

time.sleep(15)

打开任务管理器。您应该能够看到4个子进程在被操作系统终止（由于连接调用）之前处于僵尸状态几秒钟：

在更复杂的情况下，子进程可能永远处于僵尸状态（就像您在另一个question中询问的情况），如果创建了足够多的子进程，则可以填充进程表，从而给操作系统带来麻烦（这可能会杀死主进程以避免失败）。

Answer 2

在您调用join之前，所有工作人员都已将其结果放入其队列中，但他们不一定返回，而且他们的进程可能尚未终止。他们可能这样做，也可能没有这样做，这取决于时机。

调用join确保所有进程都有时间正确终止。