In a recent application, a student of mine tried to create child
processes inside of a multiprocessing Pool worker (for security and
convenience reasons, we wanted to run some code inside of a child
process). Here is some test code for python 2.7:
=============================
import multiprocessing as mp
def run_computation(x):
print 2*x
def f(x):
curr_proc=mp.current_process()
# uncomment following line to get this to work
#curr_proc.daemon=False
p = mp.Process(target=run_computation, args=(x,))
p.start()
p.join()
pool = mp.Pool(processes=4)
pool.map(f, range(10))
===============================
The result is:
Traceback (most recent call last):
File "daemon.py", line 17, in <module>
pool.map(f, range(10))
File
"/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/multiprocessing/pool.py",
line 199, in map
return self.map_async(func, iterable, chunksize).get()
File
"/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/multiprocessing/pool.py",
line 491, in get
raise self._value
AssertionError: daemonic processes are not allowed to have children
The problem appears to be that multiprocessing sets its workers to have
the daemon flag set to True, which prevents workers from creating child
processes. If I uncomment the line indicated in the code, I can create
child processes and the program works (it prints out the even numbers
from 0 to 18).
It makes me nervous to just change the daemon status of the process like
that, especially when I don't know the reason the workers have
daemon=True to begin with. What is the reasoning behind that decision?
What issues do we need to worry about if we just set the daemon mode
flag like in the above code?
Thanks,
Jason
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