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Python Parallel computation

From WikiOD

Remarks[edit | edit source]

Due to the GIL (Global interpreter lock) only one instance of the python interpreter executes in a single process. So in general, using multi-threading only improves IO bound computations, not CPU-bound ones. The multiprocessing module is recommended if you wish to parallelise CPU-bound tasks.

GIL applies to CPython, the most popular implementation of Python, as well as PyPy. Other implementations such as Jython and IronPython have no GIL.

Using the multiprocessing module to parallelise tasks[edit | edit source]

import multiprocessing

def fib(n):
    """computing the Fibonacci in an inefficient way
    was chosen to slow down the CPU."""
    if n <= 2:
        return 1
    else:
        return fib(n-1)+fib(n-2) 
p = multiprocessing.Pool() 
print(p.map(fib,[38,37,36,35,34,33]))

# Out: [39088169, 24157817, 14930352, 9227465, 5702887, 3524578]

As the execution of each call to fib happens in parallel, the time of execution of the full example is 1.8× faster than if done in a sequential way on a dual processor.

Python 2.2+

Using a C-extension to parallelize tasks[edit | edit source]

The idea here is to move the computationally intensive jobs to C (using special macros), independent of Python, and have the C code release the GIL while it's working.

#include "Python.h"
...
PyObject *pyfunc(PyObject *self, PyObject *args) {
    ...
    Py_BEGIN_ALLOW_THREADS
    // Threaded C code
    ...
    Py_END_ALLOW_THREADS
    ...
}

Using Parent and Children scripts to execute code in parallel[edit | edit source]

child.py

import time

def main():
    print "starting work"
    time.sleep(1)
    print "work work work work work"
    time.sleep(1)
    print "done working"

if __name__ == '__main__':
    main()

parent.py

import os

def main():
    for i in range(5):
        os.system("python child.py &")

if __name__ == '__main__':
    main()

This is useful for parallel, independent HTTP request/response tasks or Database select/inserts. Command line arguments can be given to the child.py script as well. Synchronization between scripts can be achieved by all scripts regularly checking a separate server (like a Redis instance).

Using PyPar module to parallelize[edit | edit source]

PyPar is a library that uses the message passing interface (MPI) to provide parallelism in Python. A simple example in PyPar (as seen at https://github.com/daleroberts/pypar) looks like this:

import pypar as pp

ncpus = pp.size()
rank = pp.rank()
node = pp.get_processor_name()

print 'I am rank %d of %d on node %s' % (rank, ncpus, node)

if rank == 0:
  msh = 'P0'
  pp.send(msg, destination=1)
  msg = pp.receive(source=rank-1)
  print 'Processor 0 received message "%s" from rank %d' % (msg, rank-1)
else:
  source = rank-1
  destination = (rank+1) % ncpus
  msg = pp.receive(source)
  msg = msg + 'P' + str(rank)
  pypar.send(msg, destination)
pp.finalize()

Credit:Stack_Overflow_Documentation