問題描述

我有一個(gè) python 程序，它運(yùn)行蒙特卡羅模擬來尋找概率問題的答案.我正在使用多處理，這里是偽代碼

I have a python program that runs a Monte Carlo simulation to find answers to probability questions. I am using multiprocessing and here it is in pseudo code

import multiprocessing

def runmycode(result_queue):
    print "Requested..."
    while 1==1:
       iterations +=1
    if "result found (for example)":
        result_queue.put("result!")

    print "Done"

processs = []
result_queue = multiprocessing.Queue()

for n in range(4): # start 4 processes
    process = multiprocessing.Process(target=runmycode, args=[result_queue])
    process.start()
    processs.append(process)

print "Waiting for result..."

result = result_queue.get() # wait

for process in processs: # then kill them all off
    process.terminate()

print "Got result:", result

我想對(duì)此進(jìn)行擴(kuò)展，以便統(tǒng)一計(jì)算已運(yùn)行的迭代次數(shù).就像如果線程 1 已經(jīng)運(yùn)行了 100 次，線程 2 已經(jīng)運(yùn)行了 100 次，那么我想總共顯示 200 次迭代，作為控制臺(tái)的打印.我指的是線程進(jìn)程中的 iterations 變量.如何確保所有線程都添加到同一個(gè)變量?我認(rèn)為使用 iterations 的 Global 版本會(huì)起作用，但事實(shí)并非如此.

I'd like to extend this so that I can keep a unified count of the number of iterations that have been run. Like if thread 1 has run 100 times and thread 2 has run 100 times then I want to show 200 iterations total, as a print to the console. I am referring to the iterations variable in the thread process. How can I make sure that ALL threads are adding to the same variable? I thought that using a Global version of iterations would work but it does not.

推薦答案

正常的全局變量在進(jìn)程之間的共享方式與線程之間的共享方式不同.您需要使用流程感知數(shù)據(jù)結(jié)構(gòu).對(duì)于您的用例，multiprocessing.Value 應(yīng)該可以正常工作:

Normal global variables are not shared between processes the way they are shared between threads. You need to use a process-aware data structure. For your use-case, a multiprocessing.Value should work fine:

import multiprocessing

def runmycode(result_queue, iterations):
   print("Requested...")
   while 1==1: # This is an infinite loop, so I assume you want something else here
       with iterations.get_lock(): # Need a lock because incrementing isn't atomic
           iterations.value += 1
   if "result found (for example)":
       result_queue.put("result!")

   print("Done")


if __name__ == "__main__":
    processs = []
    result_queue = multiprocessing.Queue()

    iterations = multiprocessing.Value('i', 0)
    for n in range(4): # start 4 processes
        process = multiprocessing.Process(target=runmycode, args=(result_queue, iterations))
        process.start()
        processs.append(process)

    print("Waiting for result...")

    result = result_queue.get() # wait

    for process in processs: # then kill them all off
        process.terminate()

    print("Got result: {}".format(result))
    print("Total iterations {}".format(iterations.value))

幾點(diǎn)說明:

1. 我明確地將 Value 傳遞給孩子，以保持代碼與 Windows 兼容，Windows 無法在父子之間共享讀/寫全局變量.
2. 我用鎖保護(hù)了增量，因?yàn)樗皇窃硬僮鳎⑶胰菀资艿礁?jìng)爭(zhēng)條件的影響.
3. 我添加了一個(gè) if __name__ == "__main__": 保護(hù)，再次幫助提高 Windows 兼容性，并作為一般最佳實(shí)踐.

1. I explicitly passed the Value to the children, to keep the code compatible with Windows, which can't share read/write global variables between parent and children.
2. I protected the increment with a lock, because its not an atomic operation, and is susceptible to race conditions.
3. I added an if __name__ == "__main__": guard, again to help with Windows compatibility, and just as a general best practice.

這篇關(guān)于在多處理期間保持統(tǒng)一計(jì)數(shù)?的文章就介紹到這了，希望我們推薦的答案對(duì)大家有所幫助，也希望大家多多支持html5模板網(wǎng)！