python muti process runs on processor each core ？ - python-3.x

Question

In the same time processor could only run one process,if one process take 10 seconds to finish work, double process finish the same work it will take 20 seconds( without IO wait).
But the following code when you run it confused me.
#!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 run program you get
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 run program you get:
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 run program you get:
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 run program you get:
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(double core）python： Python 3.7.1
When you run one process it take 10 seconds, when you run two processes it take 11 seconds .
The result looks like that the two processes runs on the same time , at each cpu core. why?

Answer 1

It sounds like you’re asking why it takes longer for more processes.
First of all, your workload is fixed for each process. Multiprocessing/multithreading is used to break big problems into smaller problems, and then running the solutions to those smaller problems on multiple contexts (processes or threads.) In your code, you’re not doing that; you’re looping up to mr=300000000 on all the processes. If you do it once on one process, it will take the same amount of time as it would if you were to do it 4 times on 4 different processes.
What’s contributing to the increase in time when you increase the number of processes is the fork() system call; this, on a Unix machine, is the call to create a new process. This call takes relatively a lot of time because it has to copy the parent process’ whole address space (memory for variables and such.)
Hope that answers your question!