python 多进程在每个内核的处理器上运行？

Question

同时处理器只能运行一个进程，如果一个进程需要10秒完成工作，双进程完成相同的工作需要20秒（没有IO等待）。

但是当你运行它时，下面的代码让我很困惑。

#!python3
import time
import os, sys
from threading import Thread
from multiprocessing import Process
import logging
logger = logging.getLogger('TIMER')
formatter = logging.Formatter('%(asctime)s %(msecs)03d : %(message)s')
logging.basicConfig(level=logging.DEBUG,
                        format='%(asctime)s %(filename)s[line:%(lineno)d] %(levelname)s %(message)s',
                        #  datefmt='%a, %d %b %Y %H:%M:%S'
                    )
ProcessNum = 1 
def loop():
    start =time.process_time()
    mr = 300000000
    pr = 0
    for i in range(0,mr):
        pr = i
    end = time.process_time()
    logger.warning('pid {} coast time: {}'.format(os.getpid(),str(end-start)[:5] ))


def muti_process():
    for i in range(ProcessNum):
        t = Process(target=loop)
        t.start()
        logger.warning('start.... muti_process')

def muti_threads():
    for i in range(1):
        t = Thread(target=loop)
        t.start()
        logger.warning('start.... muti_threads')

if __name__ == '__main__':
    muti_process()

set ProcessNum = 1 运行你得到的程序

21:18:03,469 process.py[line:29] WARNING start.... muti_process
21:18:14,419 process.py[line:22] WARNING pid 3849 coast time: 10.89

set ProcessNum = 2 运行你得到的程序：

21:18:39,443 process.py[line:29] WARNING start.... muti_process
21:18:39,445 process.py[line:29] WARNING start.... muti_process
21:18:50,638 process.py[line:22] WARNING pid 3856 coast time: 11.14
21:18:50,644 process.py[line:22] WARNING pid 3857 coast time: 11.15

set ProcessNum = 3 运行你得到的程序：

21:19:01,319 process.py[line:29] WARNING start.... muti_process
21:19:01,321 process.py[line:29] WARNING start.... muti_process
21:19:01,324 process.py[line:29] WARNING start.... muti_process
21:19:17,286 process.py[line:22] WARNING pid 3864 coast time: 15.61
21:19:17,415 process.py[line:22] WARNING pid 3863 coast time: 15.78
21:19:17,466 process.py[line:22] WARNING pid 3862 coast time: 15.82

set ProcessNum = 4 运行你得到的程序：

21:19:28,140 process.py[line:29] WARNING start.... muti_process
21:19:28,143 process.py[line:29] WARNING start.... muti_process
21:19:28,147 process.py[line:29] WARNING start.... muti_process
21:19:28,157 process.py[line:29] WARNING start.... muti_process
21:19:48,927 process.py[line:22] WARNING pid 3867 coast time: 19.68
21:19:49,049 process.py[line:22] WARNING pid 3870 coast time: 19.68
21:19:49,085 process.py[line:22] WARNING pid 3869 coast time: 19.65
21:19:49,092 process.py[line:22] WARNING pid 3868 coast time: 19.64

ENV: osx Mojave ，CPU ：2.7G core i5(双核）python： Python 3.7.1

运行一个进程需要 10 秒，运行两个进程需要 11 秒。 结果看起来这两个进程在每个 cpu 核心上同时运行。为什么？

Answer 1

听起来你在问为什么更多进程需要更长的时间。

首先，每个流程的工作量都是固定的。多处理/多线程用于将大问题分解为小问题，然后在多个上下文（进程或线程）上运行这些小问题的解决方案。在您的代码中，您并没有这样做；你在所有进程上循环到 mr=300000000。如果您在一个进程上执行一次，所花费的时间与您在 4 个不同进程上执行 4 次所花费的时间相同。

当您增加进程数量时，导致时间增加的原因是 fork() 系统调用；在 Unix 机器上，这是创建新进程的调用。这个调用需要相对大量的时间，因为它必须复制父进程的整个地址空间（变量的内存等）。

希望能回答您的问题！