Writing a parallel loop

Question

I am trying to run a parallel loop on a simple example.
What am I doing wrong?

from joblib import Parallel, delayed  
import multiprocessing

def processInpu         


        

Answer 1

On Windows, the multiprocessing module uses the 'spawn' method to start up multiple python interpreter processes. This is relatively slow. Parallel tries to be smart about running the code. In particular, it tries to adjust batch sizes so a batch takes about half a second to execute. (See the batch_size argument at https://pythonhosted.org/joblib/parallel.html)

Your processInput() function runs so fast that Parallel determines that it is faster to run the jobs serially on one processor than to spin up multiple python interpreters and run the code in parallel.

If you want to force your example to run on multiple cores, try setting batch_size to 1000 or making processInput() more complicated so it takes longer to execute.

Edit: Working example on windows that shows multiple processes in use (I'm using windows 7):

from joblib import Parallel, delayed
from os import getpid

def modfib(n):
    # print the process id to see that multiple processes are used, and
    # re-used during the job.
    if n%400 == 0:
        print(getpid(), n)  

    # fibonacci sequence mod 1000000
    a,b = 0,1
    for i in range(n):
        a,b = b,(a+b)%1000000
    return b

if __name__ == "__main__":
    Parallel(n_jobs=-1, verbose=5)(delayed(modfib)(j) for j in range(1000, 4000))