python - Multiprocessing pool with an iterator

Question

I would like to use multiprocessing pool with an iterator in order to execute a function in a thread splitting the iterator in N elements until the iterator is finish.

import arcpy
from multiprocessing import Pool

def insert(rows):
    with arcpy.da.InsertCursor("c:emp2.gdbest" fields=["*"]) as i_cursor:
        #i_cursor is an iterator
        for row in rows:
            i_cursor.insertRow(row)

input_rows = []
count = 0
pool = Pool(4)
with arcpy.da.SearchCursor("c:emp.gdbest", fields=["*"]) as s_cursor:
    #s_cursor is an iterator
    for row in s_cursor:
        if (count < 100):
            input_rows.append(row)
            count += 1
        else:
            #send 100 rows to the insert function in a new thread
            pool.apply_async(insert, input_rows)
            #reset count and input_rows
            count = 1
            input_rows = [row]


pool.join()
pool.close()

My question, is this script the right way to do it? Is there a better way?

Probably something is wrong with that script, because I got the following AssertionError at the pool.join()

Traceback (most recent call last):
  File "G:Maximeruncate_append_pool.py", line 50, in <module>
    pool.join()
  File "C:AppPython27ArcGIS10.3libmultiprocessingpool.py", line 460, in join
    assert self._state in (CLOSE, TERMINATE)
AssertionError

Answer 1

If I have to guess what's primarily wrong with your code, I'd say it's in passing your input_rows to your process function insert() - the way multiprocessing.Pool.apply_async() works is to unpack the arguments passed to it, so your insert() function actually retreives 100 arguments instead of one argument with a list of 100 elements. This causes an immediate error before your process function even gets the chance to start. If you change your call to pool.apply_async(insert, [input_rows]) it might start working... You also would be defeating the purpose of iterators and you just might convert your whole input iterator into a list and feed slices of 100 to multiprocessing.Pool.map() and be done with it.

But you asked if there is a 'better' way to do it. While 'better' is a relative category, in an ideal world, multiprocessing.Pool comes with a handy imap() (and imap_unordered()) method intended to consume iterables and spread them over the selected pool in a lazy fashion (so no running over the whole iterator before processing), so all you need to build are your iterator slices and pass it to it for processing, i.e.:

import arcpy
import itertools
import multiprocessing

# a utility function to get us a slice of an iterator, as an iterator
# when working with iterators maximum lazyness is preferred 
def iterator_slice(iterator, length):
    iterator = iter(iterator)
    while True:
        res = tuple(itertools.islice(iterator, length))
        if not res:
            break
        yield res

def insert(rows):
    with arcpy.da.InsertCursor("c:emp2.gdbest" fields=["*"]) as i_cursor:
        for row in rows:
            i_cursor.insertRow(row)

if __name__ == "__main__":  # guard for multi-platform use
    with arcpy.da.SearchCursor("c:emp.gdbest", fields=["*"]) as s_cursor:
        pool = multiprocessing.Pool(processes=4)  # lets use 4 workers
        for result in pool.imap_unordered(insert, iterator_slice(s_cursor, 100)):
            pass  # do whatever you want with your result (return from your process function)
        pool.close()  # all done, close cleanly

(btw. your code wouldn't give you the last slice for all s_cursor sizes that are not multiples of 100)

But... it would be wonderful if it actually worked as advertised. While a lot of it has been fixed over the years, imap_unordered() will still take a large sample of your iterator (far larger than the actual pool processes' number) when producing its own iterator, so if that's a concern you'll have to get down and dirty yourself, and you're on the right track - apply_async() is the way to go when you want to control how to feed your pool, you just need to make sure you feed your pool properly:

if __name__ == "__main__":
    with arcpy.da.SearchCursor("c:emp.gdbest", fields=["*"]) as s_cursor:
        pool = multiprocessing.Pool(processes=4)  # lets use 4 workers
        cursor_iterator = iterator_slice(s_cursor, 100)  # slicer from above, for convinience
        queue = []  # a queue for our current worker async results, a deque would be faster
        while cursor_iterator or queue:  # while we have anything to do...
            try:
                # add our next slice to the pool:
                queue.append(pool.apply_async(insert, [next(cursor_iterator)])) 
            except (StopIteration, TypeError):  # no more data, clear out the slice iterator
                cursor_iterator = None
            # wait for a free worker or until all remaining finish
            while queue and (len(queue) >= pool._processes or not cursor_iterator):
                process = queue.pop(0)  # grab a process response from the top
                process.wait(0.1)  # let it breathe a little, 100ms should be enough
                if not process.ready():  # a sub-process has not finished execution
                    queue.append(process)  # add it back to the queue
                else:
                    # you can use process.get() to get the result if needed
                    pass
        pool.close()

And now your s_cursor iterator will be called only when the next 100 results are needed (when your insert() process function exits cleanly or not).

UPDATE - The previously posted code had a bug in it on closing queues in the end if a captured result is desired, this one should do the job nicely. We can easily test it with some mock functions:

import random
import time

# just an example generator to prove lazy access by printing when it generates
def get_counter(limit=100):
    for i in range(limit):
        if not i % 3:  # print every third generation to reduce verbosity
            print("Generated: {}".format(i))
        yield i

# our process function, just prints what's passed to it and waits for 1-6 seconds
def test_process(values):
    time_to_wait = 1 + random.random() * 5
    print("Processing: {}, waiting: {:0.2f} seconds".format(values, time_to_wait))
    time.sleep(time_to_wait)
    print("Processed: {}".format(values))

Now we can intertwine them like:

if __name__ == "__main__":
    pool = multiprocessing.Pool(processes=2)  # lets use just 2 workers
    count = get_counter(30)  # get our counter iterator set to iterate from 0-29
    count_iterator = iterator_slice(count, 7)  # we'll process them in chunks of 7
    queue = []  # a queue for our current worker async results, a deque would be faster
    while count_iterator or queue:
        try:
            # add our next slice to the pool:
            queue.append(pool.apply_async(test_process, [next(count_iterator)]))
        except (StopIteration, TypeError):  # no more data, clear out the slice iterator
            count_iterator = None
        # wait for a free worker or until all remaining workers finish
        while queue and (len(queue) >= pool._processes or not count_iterator):
            process = queue.pop(0)  # grab a process response from the top
            process.wait(0.1)  # let it breathe a little, 100ms should be enough
            if not process.ready():  # a sub-process has not finished execution
                queue.append(process)  # add it back to the queue
            else:
                # you can use process.get() to get the result if needed
                pass
    pool.close()

And the result is (of course, it will differ from system to system):

Generated: 0
Generated: 3
Generated: 6
Generated: 9
Generated: 12
Processing: (0, 1, 2, 3, 4, 5, 6), waiting: 3.32 seconds
Processing: (7, 8, 9, 10, 11, 12, 13), waiting: 2.37 seconds
Processed: (7, 8, 9, 10, 11, 12, 13)
Generated: 15
Generated: 18
Processing: (14, 15, 16, 17, 18, 19, 20), waiting: 1.85 seconds
Processed: (0, 1, 2, 3, 4, 5, 6)
Generated: 21
Generated: 24
Generated: 27
Processing: (21, 22, 23, 24, 25, 26, 27), waiting: 2.55 seconds
Processed: (14, 15, 16, 17, 18, 19, 20)
Processing: (28, 29), waiting: 3.14 seconds
Processed: (21, 22, 23, 24, 25, 26, 27)
Processed: (28, 29)

Proving that our generator/iterator is used to collect data only when there's a free slot in the pool to do the work ensuring a minimal memory usage (and/or I/O pounding if your iterators ultimately do that). You won't get it much more streamlined than this. The only additional, albeit marginal, speed up you can obtain is to reduce the wait time (but your main process will then eat more resources) and to increase the allowed queue size (at the expense of memory) which is locked to the number of processes in the above code - if you use while queue and (len(queue) >= pool._processes + 3 or not count_iterator): it will load 3 more iterator slices ensuring lesser latency in situations when a process ends and a slot in the pool frees up.