I have a very large (read only) array of data that I want to be processed by multiple processes in parallel.
I like the Pool.map function and would like to use it to calculate functions on that data in parallel.
I saw that one can use the Value or Array class to use shared memory data between processes. But when I try to use this I get a RuntimeError: 'SynchronizedString objects should only be shared between processes through inheritance when using the Pool.map function:
Here is a simplified example of what I am trying to do:
from sys import stdin
from multiprocessing import Pool, Array
def count_it( arr, key ):
count = 0
for c in arr:
if c == key:
count += 1
return count
if __name__ == '__main__':
testData = "abcabcs bsdfsdf gdfg dffdgdfg sdfsdfsd sdfdsfsdf"
# want to share it using shared memory
toShare = Array('c', testData)
# this works
print count_it( toShare, "a" )
pool = Pool()
# RuntimeError here
print pool.map( count_it, [(toShare,key) for key in ["a", "b", "s", "d"]] )
Can anyone tell me what I am doing wrong here?
So what I would like to do is pass info about a newly created shared memory allocated array to the processes after they have been created in the process pool.
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Answer
Trying again as I just saw the bounty ;)
Basically I think the error message means what it said – multiprocessing shared memory Arrays can’t be passed as arguments (by pickling). It doesn’t make sense to serialise the data – the point is the data is shared memory. So you have to make the shared array global. I think it’s neater to put it as the attribute of a module, as in my first answer, but just leaving it as a global variable in your example also works well. Taking on board your point of not wanting to set the data before the fork, here is a modified example. If you wanted to have more than one possible shared array (and that’s why you wanted to pass toShare as an argument) you could similarly make a global list of shared arrays, and just pass the index to count_it (which would become for c in toShare[i]:).
from sys import stdin
from multiprocessing import Pool, Array, Process
def count_it( key ):
count = 0
for c in toShare:
if c == key:
count += 1
return count
if __name__ == '__main__':
# allocate shared array - want lock=False in this case since we
# aren't writing to it and want to allow multiple processes to access
# at the same time - I think with lock=True there would be little or
# no speedup
maxLength = 50
toShare = Array('c', maxLength, lock=False)
# fork
pool = Pool()
# can set data after fork
testData = "abcabcs bsdfsdf gdfg dffdgdfg sdfsdfsd sdfdsfsdf"
if len(testData) > maxLength:
raise ValueError, "Shared array too small to hold data"
toShare[:len(testData)] = testData
print pool.map( count_it, ["a", "b", "s", "d"] )
[EDIT: The above doesn’t work on windows because of not using fork. However, the below does work on Windows, still using Pool, so I think this is the closest to what you want:
from sys import stdin
from multiprocessing import Pool, Array, Process
import mymodule
def count_it( key ):
count = 0
for c in mymodule.toShare:
if c == key:
count += 1
return count
def initProcess(share):
mymodule.toShare = share
if __name__ == '__main__':
# allocate shared array - want lock=False in this case since we
# aren't writing to it and want to allow multiple processes to access
# at the same time - I think with lock=True there would be little or
# no speedup
maxLength = 50
toShare = Array('c', maxLength, lock=False)
# fork
pool = Pool(initializer=initProcess,initargs=(toShare,))
# can set data after fork
testData = "abcabcs bsdfsdf gdfg dffdgdfg sdfsdfsd sdfdsfsdf"
if len(testData) > maxLength:
raise ValueError, "Shared array too small to hold data"
toShare[:len(testData)] = testData
print pool.map( count_it, ["a", "b", "s", "d"] )
Not sure why map won’t Pickle the array but Process and Pool will – I think perhaps it has be transferred at the point of the subprocess initialization on windows. Note that the data is still set after the fork though.