How python manager.dict() locking works:

Question

A managers.dict() allow to share a dictionary across process and perform thread-safe operation. In my case each a coordinator process create the shared dict with m elements and n worker processes read and write to/from a single dict key.

Do managers.dict() have one single lock for the dict or m locks, one for every key in it?

Is there an alternative way to share m elements to n workers, other than a shared dict, when the workers do not have to communicate with each other?

Related python-manager-dict-is-very-slow-compared-to-regular-dict

Answer 1

After some tries I can say there is only one lock per managers dict. Here is the code that proves it:

import time
import multiprocessing as mp


def process_f(key, shared_dict):
        values = [i for i in range(64 * 1024 * 1024)]
        print "Writing {}...".format(key)
        a = time.time()
        shared_dict[key] = values
        b = time.time()
        print "released {} in {}ms".format(key, (b-a)*1000)


def main():
    process_manager = mp.Manager()
    n = 5
    keys = [i for i in range(n)]
    shared_dict = process_manager.dict({i: i * i for i in keys})

    pool = mp.Pool(processes=n)

    for i in range(n):
        pool.apply_async(process_f, (keys[i], shared_dict))
    time.sleep(20)


if __name__ == '__main__':
    main()

output:

Writing 4...
Writing 3...
Writing 1...
Writing 2...
Writing 0...
released 4 in 3542.7968502ms
released 0 in 4416.22900963ms
released 1 in 6247.48706818ms
released 2 in 7926.97191238ms
released 3 in 9973.71196747ms

Process finished with exit code 0

The increasing time for writing show the waiting which is happening.