Question

我正在使用python进行频率字计数，即单进程版本：

#coding=utf-8
import string
import time
from collections import Counter
starttime = time.clock()
origin = open("document.txt", 'r').read().lower()
for_split = [',','\n','\t','\'','.','\"','!','?','-', '~']

#the words below will be ignoered when counting
ignored = ['the', 'and', 'i', 'to', 'of', 'a', 'in', 'was', 'that', 'had',
       'he', 'you', 'his','my', 'it', 'as', 'with', 'her', 'for', 'on']
i=0
for ch in for_split:
    origin = string.replace(origin, ch, ' ')
words = string.split(origin)
result = Counter(words).most_common(40)
for word, frequency in result:
    if not word in ignored and i < 10:
        print "%s : %d" % (word, frequency)
        i = i+1
print time.clock() - starttime

然后多处理版本如下：

#coding=utf-8
import time
import multiprocessing
from collections import Counter
for_split = [',','\n','\t','\'','.','\"','!','?','-', '~']
ignored = ['the', 'and', 'i', 'to', 'of', 'a', 'in', 'was', 'that', 'had',
       'he', 'you', 'his','my', 'it', 'as', 'with', 'her', 'for', 'on']
result_list = []

def worker(substr):
    result = Counter(substr)
    return result

def log_result(result):
    result_list.append(result)

def main():
    pool = multiprocessing.Pool(processes=5)
    origin = open("document.txt", 'r').read().lower()
 for ch in for_split:
         origin = origin.replace(ch, ' ')
    words = origin.split()
    step = len(words)/4
        substrs = [words[pos : pos+step] for pos in range(0, len(words), step)]
    result = Counter()
    for substr in substrs:
        pool.apply_async(worker, args=(substr,), callback = log_result)
    pool.close()
    pool.join()
    result = Counter()
    for item in result_list:
        result = result + item
    result = result.most_common(40)
    i=0
    for word, frequency in result:
        if not word in ignored and i < 10:
            print "%s : %d" % (word, frequency)
            i = i+1

if __name__ == "__main__":
        starttime = time.clock()
        main()
        print time.clock() - starttime

“document.txt”大约是22M，我的笔记本电脑有核心，2G内存，第一个版本的结果是3.27s，第二个是8.15s，我已经改变了进程数量（ pool = multiprocessing.Pool（processes = 5）），从2到10，结果几乎保持不变，为什么呢，我怎样才能使这个程序运行faser而不是单个进程版本？

Answer 1

我认为这是将单个字符串分发给worker并接收结果的开销。如果我使用示例文档（Dostojevski的“犯罪和惩罚”）运行上面给出的并行代码，则运行大约需要0.32秒，而单进程版本只需0.09秒。如果我修改worker函数只是处理字符串“test”而不是真实文档（仍然将真实字符串作为参数传递），运行时间将降至0.22秒。但是，如果我将“test”作为参数传递给map_async函数，则运行时间会减少到0.06秒。因此，我会说在您的情况下，程序的运行时受到进程间通信开销的限制。

使用以下代码，我将并行版本的运行时间缩短到0.08秒：首先，我将文件分区为多个（几乎）相等长度的块，确保各个块之间的边界确实与新队。然后，我只是将块的长度和偏移传递给每个工作进程，让它打开文件，读取块，处理它并返回结果。与通过map_async函数直接分发字符串相比，这似乎导致了更少的开销。对于较大的文件大小，您应该能够使用此代码查看运行时的改进。此外，如果您可以容忍小计数错误，则可以省略确定正确的块边界的步骤，并将文件拆分为相同的大块。在我的例子中，这将运行时间降低到0.04秒，使得mp代码比单进程代码更快。

#coding=utf-8
import time
import multiprocessing
import string
from collections import Counter
import os
for_split = [',','\n','\t','\'','.','\"','!','?','-', '~']
ignored = ['the', 'and', 'i', 'to', 'of', 'a', 'in', 'was', 'that', 'had',
       'he', 'you', 'his','my', 'it', 'as', 'with', 'her', 'for', 'on']
result_list = []

def worker(offset,length,filename):
    origin = open(filename, 'r')
    origin.seek(offset)
    content = origin.read(length).lower()

    for ch in for_split:
         content = content.replace(ch, ' ')

    words = string.split(content)
    result = Counter(words)
    origin.close()
    return result

def log_result(result):
    result_list.append(result)

def main():
    processes = 5
    pool = multiprocessing.Pool(processes=processes)
    filename = "document.txt"
    file_size = os.stat(filename)[6]
    chunks = []
    origin = open(filename, 'r')
    while True:
        lines = origin.readlines(file_size/processes)
        if not lines:
            break
        chunks.append("\n".join(lines))

    lengths = [len(chunk) for chunk in chunks]
    offset = 0

    for length in lengths:
        pool.apply_async(worker, args=(offset,length,filename,), callback = log_result)
        offset += length

    pool.close()
    pool.join()
    result = Counter()
    for item in result_list:
        result = result + item
    result = result.most_common(40)
    i=0
    for word, frequency in result:
        if not word in ignored and i < 10:
            print "%s : %d" % (word, frequency)
            i = i+1
if __name__ == "__main__":
    starttime = time.clock()
    main()
    print time.clock() - starttime

python top N字数，为什么multiprocess慢然后单个进程

1 个答案: