Python-並行性不起作用

Question

編輯：我在評論中得到了很好的反饋。 對代碼進行了一些更改，但仍然遇到類似的問題。 現在看來我的計時方式有問題，因為第二次操作要花費超過0秒的時間。

原文：我寫的代碼表面上看起來是並行的，但實際上運行起來並不快-並行和非並行版本都花費相同的時間。 對於我的一生，我不知道為什么。

我正在Windows 7上通過Anaconda使用Python 3.4，無論我在IDE（Spyder）還是命令提示符下提交作業，結果都是相同的。 這是我的代碼：

import multiprocessing
from multiprocessing import Pool
import time

def divide_by_two(n):
    time.sleep(.1)
    return n/2

if __name__ == '__main__':
    print("The number of cores is ", multiprocessing.cpu_count())
    pool = Pool(processes=multiprocessing.cpu_count())
    smallList = range(0,1000) #[0, 2, 4, 6, 8, 10]
    print('Checking the parallelized way', smallList[:5])
    start = time.time()
    result = pool.map(divide_by_two, smallList)
    end = time.time()
    cleaned = [x for x in result if not x is None]
    print('small List divided by two is ', str(cleaned[:5]))
    print('Parallel way takes ', str(end-start), ' seconds')

    #Now the dumb version
    print('Checking the slow way', smallList[:5])
    start2 = time.time()
    smallList = range(0,1000) #[0, 2, 4, 6, 8, 10]
    result2 = map(divide_by_two, smallList)
    end2 = time.time()
    cleaned2 = [x for x in result2 if not x is None]
    print('small List divided by two is ', str(cleaned2[:5]))
    print('The slow way takes', str(end2-start2), ' seconds')

這是輸出：

The number of cores is  4
Checking the parallelized way range(0, 5)
small List divided by two is  [0.0, 0.5, 1.0, 1.5, 2.0]
Parallel way takes  26.87681818008423  seconds
Checking the slow way range(0, 5)
small List divided by two is  [0.0, 0.5, 1.0, 1.5, 2.0]
The slow way takes 0.0  seconds

Answer 1

感謝評論者。 如果可以的話，我會投票贊成。 並行化問題的大小增加，這次使用了正確的變量（糟糕！），並且使用了不同的計時功能。 這里是：

import multiprocessing
from multiprocessing import Pool
import time

def divide_by_two(n):
    time.sleep(.1)
    return n/2

if __name__ == '__main__':
    print("The number of cores is ", multiprocessing.cpu_count())
    pool = Pool(processes=multiprocessing.cpu_count())
    smallList = range(0,1000) #[0, 2, 4, 6, 8, 10]
    print('Checking the parallelized way', smallList[:5])
    start = time.clock() #time.time()
    print('Start 1 is ', start)
    result = pool.map(divide_by_two, smallList)
    end = time.time()
    cleaned = [x for x in result if not x is None]
    print('small List divided by two is ', str(cleaned[:5]))
    print('Parallel way takes ', time.clock()-start, ' seconds')    

    #Now the dumb version
    print('Checking the slow way', smallList[:5])
    start = time.clock()
    print('Start 2 is ', start)
    result2 = map(divide_by_two, smallList)
    cleaned2 = [x for x in result2 if not x is None]
    print('small List divided by two is ', str(cleaned2[:5]))
    print('The slow way takes', time.clock()-start, ' seconds')