Do a pile of work

Wednesday 19 August 2020

I had a large pile of data to feed through an expensive function. The concurrent.futures module in the Python standard library has always worked well for me as a simple way to farm out work across threads or processes.

Update: this code swallows exceptions. An improved version is at Do a pile of work better.

For example, if my work function is “workfn”, and it takes tuples of arguments as produced by “argsfn()”, this is how you could run them all:

for args in argsfn():

This is how you would run them on a number of threads:

import concurrent.futures as cf

with cf.ThreadPoolExecutor(max_workers=nthreads) as executor:
    for args in argsfn():
        executor.submit(workfn, *args)

But this will generate all of the arguments up-front. If I have millions of work invocations, this could be a problem. I wanted a way to feed the tasks in as they are processed, to keep the queue small. And I wanted a progress bar.

I started from this Stack Overflow answer, added in tqdm for a progress bar, and made this:

import concurrent.futures as cf
from tqdm import tqdm

def wait_first(futures):
    Wait for the first future to complete.

        (done, not_done): two sets of futures.

    return cf.wait(futures, return_when=cf.FIRST_COMPLETED)

def do_work(nthreads, argsfn, workfn):
    Do a pile of work, maybe in threads, with a progress bar.

    Two callables are provided: `workfn` is the unit of work to be done,
    many times.  Its arguments are provided by calling `argsfn`, which
    must produce a sequence of tuples.  `argsfn` will be called a few
    times, and must produce the same sequence each time.

        nthreads: the number of threads to use.
        argsfn: a callable that produces tuples, the arguments to `workfn`.
        workfn: a callable that does work.

    total = sum(1 for _ in argsfn())
    with tqdm(total=total, smoothing=0.1) as progressbar:
        if nthreads:
            limit = 2 * nthreads
            not_done = set()
            with cf.ThreadPoolExecutor(max_workers=nthreads) as executor:
                for args in argsfn():
                    if len(not_done) >= limit:
                        done, not_done = wait_first(not_done)
                    not_done.add(executor.submit(workfn, *args))
                while not_done:
                    done, not_done = wait_first(not_done)
            for args in argsfn():

There might be a better way. I don’t like the duplication of the wait_first call, but this works, and produces the right results.

BTW: my actual work function spawned subprocesses, which is why a thread pool worked to give me parallelism. A pure-Python work function wouldn’t get a speed-up this way, but a ProcessPoolExecutor could help.


Here's a different take using multiprocessing.dummy.

I really, really wanted it to be shorter, but without with_backpressure, the imap_unordered() call will consume the whole iterator immediately (it's lazy only in some ways, see for details).

Similar to your example, pure-Python code can be sped up by using multiprocessing.Pool instead.
from multiprocessing.dummy import Pool
from queue import Queue
from tqdm import tqdm

def with_backpressure(it, maxsize):
    """Consume at most maxsize elements at a time from an iterator,
    then block until additional elements can be consumed.

        (wrapped iterator, advance function):
            An iterator with the same elements as the original one,
            and an advance() function; call advance() when another 
            element can be consumed.

    queue = Queue(maxsize)
    sentinel = object()
    it = iter(it)

    def advance():
        queue.put(next(it, sentinel))

    # "prime" the queue
    for _ in range(maxsize):

    return iter(queue.get, sentinel), advance

def do_work(nthreads, argsfn, workfn):
    total = sum(1 for _ in argsfn())

    def worker(args):
        return workfn(*args)

    pool = Pool(nthreads)
    args, advance = with_backpressure(argsfn(), nthreads * 2)
    progressbar = tqdm(total=total, smoothing=0.1)
    with pool, progressbar:
        for _ in pool.imap_unordered(worker, args):

if __name__ == '__main__':
    import time
    def argsfn():
        for i in range(10):
            yield 1,

    do_work(3, argsfn, time.sleep)

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