What can multiprocessing and dill do together?

Question

I would like to use the multiprocessing library in Python. Sadly multiprocessing uses pickle which doesn't support functions with closures, lambdas, or functions in __main__. All three of these are important to me

In [1]: import pickle

In [2]: pickle.dumps(lambda x: x)
PicklingError: Can't pickle <function <lambda> at 0x23c0e60>: it's not found as __main__.<lambda>

Fortunately there is dill a more robust pickle. Apparently dill performs magic on import to make pickle work

In [3]: import dill

In [4]: pickle.dumps(lambda x: x)
Out[4]: "cdill.dill\n_load_type\np0\n(S'FunctionType'\np1 ...

This is very encouraging, particularly because I don't have access to the multiprocessing source code. Sadly, I still can't get this very basic example to work

import multiprocessing as mp
import dill

p = mp.Pool(4)
print p.map(lambda x: x**2, range(10))

Why is this? What am I missing? Exactly what are the limitations on the multiprocessing+dill combination?

Temporary Edit for J.F Sebastian

mrockli@mrockli-notebook:~/workspace/toolz$ python testmp.py 
    Temporary Edit for J.F Sebastian

mrockli@mrockli-notebook:~/workspace/toolz$ python testmp.py 
Exception in thread Thread-2:
Traceback (most recent call last):
  File "/home/mrockli/Software/anaconda/lib/python2.7/threading.py", line 808, in __bootstrap_inner
    self.run()
  File "/home/mrockli/Software/anaconda/lib/python2.7/threading.py", line 761, in run
    self.__target(*self.__args, **self.__kwargs)
  File "/home/mrockli/Software/anaconda/lib/python2.7/multiprocessing/pool.py", line 342, in _handle_tasks
    put(task)
PicklingError: Can't pickle <type 'function'>: attribute lookup __builtin__.function failed

^C
...lots of junk...

[DEBUG/MainProcess] cleaning up worker 3
[DEBUG/MainProcess] cleaning up worker 2
[DEBUG/MainProcess] cleaning up worker 1
[DEBUG/MainProcess] cleaning up worker 0
[DEBUG/MainProcess] added worker
[DEBUG/MainProcess] added worker
[INFO/PoolWorker-5] child process calling self.run()
[INFO/PoolWorker-6] child process calling self.run()
[DEBUG/MainProcess] added worker
[INFO/PoolWorker-7] child process calling self.run()
[DEBUG/MainProcess] added worker
[INFO/PoolWorker-8] child process calling self.run()Exception in thread Thread-2:
Traceback (most recent call last):
  File "/home/mrockli/Software/anaconda/lib/python2.7/threading.py", line 808, in __bootstrap_inner
    self.run()
  File "/home/mrockli/Software/anaconda/lib/python2.7/threading.py", line 761, in run
    self.__target(*self.__args, **self.__kwargs)
  File "/home/mrockli/Software/anaconda/lib/python2.7/multiprocessing/pool.py", line 342, in _handle_tasks
    put(task)
PicklingError: Can't pickle <type 'function'>: attribute lookup __builtin__.function failed

^C
...lots of junk...

[DEBUG/MainProcess] cleaning up worker 3
[DEBUG/MainProcess] cleaning up worker 2
[DEBUG/MainProcess] cleaning up worker 1
[DEBUG/MainProcess] cleaning up worker 0
[DEBUG/MainProcess] added worker
[DEBUG/MainProcess] added worker
[INFO/PoolWorker-5] child process calling self.run()
[INFO/PoolWorker-6] child process calling self.run()
[DEBUG/MainProcess] added worker
[INFO/PoolWorker-7] child process calling self.run()
[DEBUG/MainProcess] added worker
[INFO/PoolWorker-8] child process calling self.run()

Answer 1

multiprocessing makes some bad choices about pickling. Don't get me wrong, it makes some good choices that enable it to pickle certain types so they can be used in a pool's map function. However, since we have dill that can do the pickling, multiprocessing's own pickling becomes a bit limiting. Actually, if multiprocessing were to use pickle instead of cPickle... and also drop some of it's own pickling overrides, then dill could take over and give a much more full serialization for multiprocessing.

Until that happens, there's a fork of multiprocessing called pathos (the release version is a bit stale, unfortunately) that removes the above limitations. Pathos also adds some nice features that multiprocessing doesn't have, like multi-args in the map function. Pathos is due for a release, after some mild updating -- mostly conversion to python 3.x.

Python 2.7.5 (default, Sep 30 2013, 20:15:49) 
[GCC 4.2.1 (Apple Inc. build 5566)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import dill
>>> from pathos.multiprocessing import ProcessingPool    
>>> pool = ProcessingPool(nodes=4)
>>> result = pool.map(lambda x: x**2, range(10))
>>> result
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

and just to show off a little of what pathos.multiprocessing can do...

>>> def busy_add(x,y, delay=0.01):
...     for n in range(x):
...        x += n
...     for n in range(y):
...        y -= n
...     import time
...     time.sleep(delay)
...     return x + y
... 
>>> def busy_squared(x):
...     import time, random
...     time.sleep(2*random.random())
...     return x*x
... 
>>> def squared(x):
...     return x*x
... 
>>> def quad_factory(a=1, b=1, c=0):
...     def quad(x):
...         return a*x**2 + b*x + c
...     return quad
... 
>>> square_plus_one = quad_factory(2,0,1)
>>> 
>>> def test1(pool):
...     print pool
...     print "x: %s\n" % str(x)
...     print pool.map.__name__
...     start = time.time()
...     res = pool.map(squared, x)
...     print "time to results:", time.time() - start
...     print "y: %s\n" % str(res)
...     print pool.imap.__name__
...     start = time.time()
...     res = pool.imap(squared, x)
...     print "time to queue:", time.time() - start
...     start = time.time()
...     res = list(res)
...     print "time to results:", time.time() - start
...     print "y: %s\n" % str(res)
...     print pool.amap.__name__
...     start = time.time()
...     res = pool.amap(squared, x)
...     print "time to queue:", time.time() - start
...     start = time.time()
...     res = res.get()
...     print "time to results:", time.time() - start
...     print "y: %s\n" % str(res)
... 
>>> def test2(pool, items=4, delay=0):
...     _x = range(-items/2,items/2,2)
...     _y = range(len(_x))
...     _d = [delay]*len(_x)
...     print map
...     res1 = map(busy_squared, _x)
...     res2 = map(busy_add, _x, _y, _d)
...     print pool.map
...     _res1 = pool.map(busy_squared, _x)
...     _res2 = pool.map(busy_add, _x, _y, _d)
...     assert _res1 == res1
...     assert _res2 == res2
...     print pool.imap
...     _res1 = pool.imap(busy_squared, _x)
...     _res2 = pool.imap(busy_add, _x, _y, _d)
...     assert list(_res1) == res1
...     assert list(_res2) == res2
...     print pool.amap
...     _res1 = pool.amap(busy_squared, _x)
...     _res2 = pool.amap(busy_add, _x, _y, _d)
...     assert _res1.get() == res1
...     assert _res2.get() == res2
...     print ""
... 
>>> def test3(pool): # test against a function that should fail in pickle
...     print pool
...     print "x: %s\n" % str(x)
...     print pool.map.__name__
...     start = time.time()
...     res = pool.map(square_plus_one, x)
...     print "time to results:", time.time() - start
...     print "y: %s\n" % str(res)
... 
>>> def test4(pool, maxtries, delay):
...     print pool
...     m = pool.amap(busy_add, x, x)
...     tries = 0
...     while not m.ready():
...         time.sleep(delay)
...         tries += 1
...         print "TRY: %s" % tries
...         if tries >= maxtries:
...             print "TIMEOUT"
...             break
...     print m.get()
... 
>>> import time
>>> x = range(18)
>>> delay = 0.01
>>> items = 20
>>> maxtries = 20
>>> from pathos.multiprocessing import ProcessingPool as Pool
>>> pool = Pool(nodes=4)
>>> test1(pool)
<pool ProcessingPool(ncpus=4)>
x: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17]

map
time to results: 0.0553691387177
y: [0, 1, 4, 9, 16, 25, 36, 49, 64, 81, 100, 121, 144, 169, 196, 225, 256, 289]

imap
time to queue: 7.91549682617e-05
time to results: 0.102381229401
y: [0, 1, 4, 9, 16, 25, 36, 49, 64, 81, 100, 121, 144, 169, 196, 225, 256, 289]

amap
time to queue: 7.08103179932e-05
time to results: 0.0489699840546
y: [0, 1, 4, 9, 16, 25, 36, 49, 64, 81, 100, 121, 144, 169, 196, 225, 256, 289]

>>> test2(pool, items, delay)
<built-in function map>
<bound method ProcessingPool.map of <pool ProcessingPool(ncpus=4)>>
<bound method ProcessingPool.imap of <pool ProcessingPool(ncpus=4)>>
<bound method ProcessingPool.amap of <pool ProcessingPool(ncpus=4)>>

>>> test3(pool)
<pool ProcessingPool(ncpus=4)>
x: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17]

map
time to results: 0.0523059368134
y: [1, 3, 9, 19, 33, 51, 73, 99, 129, 163, 201, 243, 289, 339, 393, 451, 513, 579]

>>> test4(pool, maxtries, delay)
<pool ProcessingPool(ncpus=4)>
TRY: 1
TRY: 2
TRY: 3
TRY: 4
TRY: 5
TRY: 6
TRY: 7
[0, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34]