Based on this question I assumed that creating new process should be almost as fast as creating new thread in Linux. However, little test showed very different result. Here's my code:

from multiprocessing import Process, Pool
from threading import Threadtimes = 1000def inc(a):b = 1return a + bdef processes():for i in xrange(times):p = Process(target=inc, args=(i, ))p.start()p.join()def threads():for i in xrange(times):t = Thread(target=inc, args=(i, ))t.start()t.join()

Tests:

>>> timeit processes() 
1 loops, best of 3: 3.8 s per loop>>> timeit threads() 
10 loops, best of 3: 98.6 ms per loop

So, processes are almost 40 times slower to create! Why does it happen? Is it specific to Python or these libraries? Or did I just misinterpreted the answer above?

UPD 1. To make it more clear. I understand that this piece of code doesn't actually introduce any concurrency. The goal here is to test the time needed to create a process and a thread. To use real concurrency with Python one can use something like this:

def pools():pool = Pool(10)pool.map(inc, xrange(times))

which really runs much faster than threaded version.

UPD 2. I have added version with os.fork():

for i in xrange(times):child_pid = os.fork()if child_pid:os.waitpid(child_pid, 0)else:exit(-1)

Results are:

$ time python test_fork.py real    0m3.919s
user    0m0.040s
sys     0m0.208s$ time python test_multiprocessing.py real    0m1.088s
user    0m0.128s
sys     0m0.292s$ time python test_threadings.pyreal    0m0.134s
user    0m0.112s
sys     0m0.048s

The question you linked to is comparing the cost of just calling fork(2) vs. pthread_create(3), whereas your code does quite a bit more, e.g. using join() to wait for the processes/threads to terminate.

If, as you say...

The goal here is to test the time needed to create a process and a thread.

...then you shouldn't be waiting for them to complete. You should be using test programs more like these...

fork.py

import os
import timedef main():for i in range(100):pid = os.fork()if pid:#print 'created new process %d' % pidcontinueelse:time.sleep(1)returnif __name__ == '__main__':main()

thread.py

import thread
import timedef dummy():time.sleep(1)def main():for i in range(100):tid = thread.start_new_thread(dummy, ())#print 'created new thread %d' % tidif __name__ == '__main__':main()

...which give the following results...

$ time python fork.py
real    0m0.035s
user    0m0.008s
sys     0m0.024s$ time python thread.py
real    0m0.032s
user    0m0.012s
sys     0m0.024s

...so there's not much difference in the creation time of threads and processes.