Python3 compare the speed between multiple thread and single thread issue

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I met an issue with multiple threads in Python3. I want to compare the speed between single and multiple threads. But the time cost for a single thread is not correct I think, in the code below. It should be 2 seconds at least (because of GIL, the speed between single and multiple threads should be nearly the same in the code below), but it was nearly 0 seconds in fact. I found that the program does not enter to thread_job() function when execute main() function, why does this happen?

Python

import threading
import time

def thread_job():
    for _ in range(5):
        time.sleep(2)

def main():
    for _ in range(5): #Activate 5 threads to execute the time wait job.        
        added_thread = threading.Thread(target = thread_job)
        added_thread.start()


if __name__ == '__main__':
    start = time.time()
    main()
    end = time.time()
    print("Mutiple thread takes " + str(end - start) + '\n') #calculate the time cost for multiple thread. it should be nearly 2 seconds in theory.

    start_1 = time.time()
    thread_job()
    end_1 = time.time()
    print("single thread takes " + str(end_1 - start_1) + '\n') #calculate the time cost for single thread. it should be nearly 10 seconds in theory.

What I have tried:

<pre>
Output:

Mutiple thread takes 0.0005013942718505859

single thread takes 10.002868175506592

The code was execute on windows Visual studio code .

Posted 7-Jul-19 19:46pm

chenghuang

Updated 7-Jul-19 21:17pm

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2 solutions

Solution 2

Python

import threading
import time

def thread_job():
    for _ in range(5):
        time.sleep(2)

def main():
    for _ in range(5): #Activate 5 threads to execute the time wait job.        
        added_thread = threading.Thread(target = thread_job)
        added_thread.start()
        threads.append(added_thread)    

if __name__ == '__main__':
    threads = []
    start = time.time()
    main()
    end = time.time()
    for thread in threads:
        thread.join()      #should add join there wait all threads finished.
    print("Mutiple thread takes " + str(end - start) + '\n') #calculate the time cost for multiple thread. it should be nearly 2 seconds in theory.
    start_1 = time.time()
    thread_job()
    end_1 = time.time()
    print("single thread takes " + str(end_1 - start_1) + '\n') #calculate the time cost for single thread. it should be nearly 10 seconds in theory.

Posted 7-Jul-19 21:18pm

chenghuang

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Richard MacCutchan · Accepted Answer · 2019-07-07T20:41:00

Solution 1

You are measuring the end time of the multi threaded test when all threads have started, not when they have finished. You need to add some sort of wait action in main to check when all the threads are complete.

Posted 7-Jul-19 20:41pm

Richard MacCutchan