python concurrent.futures.ProcessPoolExecutor: Performance of .submit() vs .map()

Question

I am using concurrent.futures.ProcessPoolExecutor to find the occurrence of a number from a number range. The intent is to investigate the amount of speed-up performance gained from concurrency. To benchmark performance, I have a control - a serial code to perform said task (shown below). I have written 2 concurrent codes, one using concurrent.futures.ProcessPoolExecutor.submit() and the other using concurrent.futures.ProcessPoolExecutor.map() to perform the same task. They are shown below. Advice on drafting the former and latter can be seen here and here, respectively.

The task issued to all three codes was to find the number of occurrences of the number 5 in the number range of 0 to 1E8. Both .submit() and .map() were assigned 6 workers, and .map() had a chunksize of 10,000. The manner to discretise the workload were identical in the concurrent codes. However, the function used to find occurrences in both codes were different. This was because the way arguments were passed to a function called by .submit() and .map() were different.

All 3 codes reported the same number of occurrences, i.e. 56,953,279 times. However, the time taken to complete the task were very different. .submit() performed 2 times faster than the control while .map() took twice as long as the control to complete it's task.

Questions:

1. I would like to know if the slow performance of .map() is an artifact of my coding or it is inherently slow?" If the former, how can I improve it. I am just surprise that it performed slower than the control as there will be no much incentive to use it.
2. I like to know if there is anyway to make .submit() code perform even faster. A condition I have is that the function _concurrent_submit() must return an iterable with the numbers/occurrences containing the number 5.

Benchmark Results

concurrent.futures.ProcessPoolExecutor.submit()

#!/usr/bin/python3.5
# -*- coding: utf-8 -*-

import concurrent.futures as cf
from time import time
from traceback import print_exc

def _findmatch(nmin, nmax, number):
    '''Function to find the occurrence of number in range nmin to nmax and return
       the found occurrences in a list.'''
    print('\n def _findmatch', nmin, nmax, number)
    start = time()
    match=[]
    for n in range(nmin, nmax):
        if number in str(n):
            match.append(n)
    end = time() - start
    print("found {0} in {1:.4f}sec".format(len(match),end))
    return match

def _concurrent_submit(nmax, number, workers):
    '''Function that utilises concurrent.futures.ProcessPoolExecutor.submit to
       find the occurences of a given number in a number range in a parallelised
       manner.'''
    # 1. Local variables
    start = time()
    chunk = nmax // workers
    futures = []
    found =[]
    #2. Parallelization
    with cf.ProcessPoolExecutor(max_workers=workers) as executor:
        # 2.1. Discretise workload and submit to worker pool
        for i in range(workers):
            cstart = chunk * i
            cstop = chunk * (i + 1) if i != workers - 1 else nmax
            futures.append(executor.submit(_findmatch, cstart, cstop, number))
        # 2.2. Instruct workers to process results as they come, when all are
        #      completed or .....
        cf.as_completed(futures) # faster than cf.wait()
        # 2.3. Consolidate result as a list and return this list.
        for future in futures:
            for f in future.result():
                try:
                    found.append(f)
                except:
                    print_exc()
        foundsize = len(found)
        end = time() - start
        print('within statement of def _concurrent_submit():')
        print("found {0} in {1:.4f}sec".format(foundsize, end))
    return found

if __name__ == '__main__':
    nmax = int(1E8) # Number range maximum.
    number = str(5) # Number to be found in number range.
    workers = 6     # Pool of workers

    start = time()
    a = _concurrent_submit(nmax, number, workers)
    end = time() - start
    print('\n main')
    print('workers = ', workers)
    print("found {0} in {1:.4f}sec".format(len(a),end))

concurrent.futures.ProcessPoolExecutor.map()

#!/usr/bin/python3.5
# -*- coding: utf-8 -*-

import concurrent.futures as cf
import itertools
from time import time
from traceback import print_exc

def _findmatch(listnumber, number):    
    '''Function to find the occurrence of number in another number and return
       a string value.'''
    #print('def _findmatch(listnumber, number):')
    #print('listnumber = {0} and ref = {1}'.format(listnumber, number))
    if number in str(listnumber):
        x = listnumber
        #print('x = {0}'.format(x))
        return x 

def _concurrent_map(nmax, number, workers):
    '''Function that utilises concurrent.futures.ProcessPoolExecutor.map to
       find the occurrences of a given number in a number range in a parallelised
       manner.'''
    # 1. Local variables
    start = time()
    chunk = nmax // workers
    futures = []
    found =[]
    #2. Parallelization
    with cf.ProcessPoolExecutor(max_workers=workers) as executor:
        # 2.1. Discretise workload and submit to worker pool
        for i in range(workers):
            cstart = chunk * i
            cstop = chunk * (i + 1) if i != workers - 1 else nmax
            numberlist = range(cstart, cstop)
            futures.append(executor.map(_findmatch, numberlist,
                                        itertools.repeat(number),
                                        chunksize=10000))
        # 2.3. Consolidate result as a list and return this list.
        for future in futures:
            for f in future:
                if f:
                    try:
                        found.append(f)
                    except:
                        print_exc()
        foundsize = len(found)
        end = time() - start
        print('within statement of def _concurrent(nmax, number):')
        print("found {0} in {1:.4f}sec".format(foundsize, end))
    return found

if __name__ == '__main__':
    nmax = int(1E8) # Number range maximum.
    number = str(5) # Number to be found in number range.
    workers = 6     # Pool of workers

    start = time()
    a = _concurrent_map(nmax, number, workers)
    end = time() - start
    print('\n main')
    print('workers = ', workers)
    print("found {0} in {1:.4f}sec".format(len(a),end))

Serial Code:

#!/usr/bin/python3.5
# -*- coding: utf-8 -*-

from time import time

def _serial(nmax, number):    
    start = time()
    match=[]
    nlist = range(nmax)
    for n in nlist:
        if number in str(n):match.append(n)
    end=time()-start
    print("found {0} in {1:.4f}sec".format(len(match),end))
    return match

if __name__ == '__main__':
    nmax = int(1E8) # Number range maximum.
    number = str(5) # Number to be found in number range.

    start = time()
    a = _serial(nmax, number)
    end = time() - start
    print('\n main')
    print("found {0} in {1:.4f}sec".format(len(a),end))

Update 13th Feb 2017:

In addition to @niemmi answer, I have provide an answer following some personal research to show:

1. how to further speed-up @niemmi's .map() and .submit() solutions, and
2. when ProcessPoolExecutor.map() can led to more speed-up than ProcessPoolExecutor.submit().

Answer 1

Overview:

There are 2 parts to my answer:

• Part 1 shows how to gain more speed-up from @niemmi's ProcessPoolExecutor.map() solution.
  
• Part 2 shows when the ProcessPoolExecutor's subclasses .submit() and .map() yield non-equivalent compute times.

=======================================================================

Part 1: More Speed-up for ProcessPoolExecutor.map()

Background: This section builds on @niemmi's .map() solution, which by itself is excellent. While doing some research on his discretization scheme to better understand how that interact with .map() chunksizes arguement, I found this interesting solution.

I regard @niemmi's definition of chunk = nmax // workers to be a definition for chunksize, i.e. a smaller size of actual number range (given task) to be tackled by each worker in the worker pool. Now, this definition is premised on the assumption that if a computer has x number of workers, dividing the task equally among each worker will result in optimum use of each worker and hence the total task will be completed fastest. Therefore, the number of chunks to break up a given task into should always equal the number of pool workers. However, is this assumption correct?

Proposition: Here, I propose that the above assumption does not always lead to the fastest compute time when used with ProcessPoolExecutor.map(). Rather, discretising a task to an amount greater than the number of pool workers can lead to speed-up, i.e. faster completion of a given task.

Experiment: I have modified @niemmi's code to allow the number of discretized tasks to exceed the number of pool workers. This code is given below and used to fin the number of times the number 5 appears in the number range of 0 to 1E8. I have executed this code using 1, 2, 4, and 6 pool workers and for various ratio of number of discretized tasks vs the number of pool workers. For each scenario, 3 runs were made and the compute times were tabulated. "Speed-up" is defined here as the average compute time using equal number of chunks and pool workers over the average compute time of when the number of discretized tasks is greater than the number of pool workers.

Findings:

1. Figure on left shows the compute time taken by all the scenarios mentioned in the experiment section. It shows that the compute time taken by number of chunks / number of workers = 1 is always greater than the compute time taken by number of chunks > number of workers. That is, the former case is always less efficient than the latter.

2. Figure on right shows that a speed-up of 1.2 times or more was gained when the number of chunks / number of workers reach a threshold value of 14 or more. It is interesting to observe that the speed-up trend also occurred when ProcessPoolExecutor.map() was executed with 1 worker.

Conclusion: When customizing the number of discrete tasks that ProcessPoolExecutor.map()` should use to solve a given task, it is prudent to ensure that this number is greater than the number pool workers as this practice shortens compute time.

concurrent.futures.ProcessPoolExecutor.map() code. (revised parts only)

def _concurrent_map(nmax, number, workers, num_of_chunks):
    '''Function that utilises concurrent.futures.ProcessPoolExecutor.map to
       find the occurrences of a given number in a number range in a parallelised
       manner.'''
    # 1. Local variables
    start = time()
    chunksize = nmax // num_of_chunks
    futures = []
    found =[]
    #2. Parallelization
    with cf.ProcessPoolExecutor(max_workers=workers) as executor:
        # 2.1. Discretise workload and submit to worker pool
        cstart = (chunksize * i for i in range(num_of_chunks))
        cstop = (chunksize * i if i != num_of_chunks else nmax
                 for i in range(1, num_of_chunks + 1))
        futures = executor.map(_findmatch, cstart, cstop,
                               itertools.repeat(number))
        # 2.2. Consolidate result as a list and return this list.
        for future in futures:
            #print('type(future)=',type(future))
            for f in future:
                if f:
                    try:
                        found.append(f)
                    except:
                        print_exc()
        foundsize = len(found)
        end = time() - start
        print('\n within statement of def _concurrent(nmax, number):')
        print("found {0} in {1:.4f}sec".format(foundsize, end))
    return found

if __name__ == '__main__':
    nmax = int(1E8) # Number range maximum.
    number = str(5) # Number to be found in number range.
    workers = 4     # Pool of workers
    chunks_vs_workers = 14 # A factor of =>14 can provide optimum performance  
    num_of_chunks = chunks_vs_workers * workers

    start = time()
    a = _concurrent_map(nmax, number, workers, num_of_chunks)
    end = time() - start
    print('\n main')
    print('nmax={}, workers={}, num_of_chunks={}'.format(
          nmax, workers, num_of_chunks))
    print('workers = ', workers)
    print("found {0} in {1:.4f}sec".format(len(a),end))

=======================================================================

Part 2: Total compute time from using ProcessPoolExecutor subclasses .submit() and .map() can be dissimilar when returning a sorted/ordered result list.

Background: I have amended both the .submit() and .map() codes to allow an "apple-to-apple" comparison of their compute time and the ability to visualize the compute time of the main code, the compute time of the _concurrent method called by the main code to performs the concurrent operations, and the compute time for each discretized task/worker called by the _concurrent method. Furthermore, the concurrent method in these codes was structured to return an unordered and ordered list of the result directly from the future object of .submit() and the iterator of .map(). Source code is provided below (Hope it helps you.).

Experiments These two newly improved codes were used to perform the same experiment described in Part 1, save that only 6 pool workers were considered and the python built-in list and sorted methods were used to return an unordered and ordered list of the results to the main section of the code, respectively.

Findings:

1. From the _concurrent method's result, we can see the compute times of the _concurrent method used to create all the Future objects of ProcessPoolExecutor.submit(), and to create the iterator of ProcessPoolExecutor.map(), as a function of the number of discretized task over the number of pool workers, are equivalent. This result simply means that the ProcessPoolExecutor sub-classes .submit() and .map() are equally efficient/fast.
2. Comparing the compute times from main and it's _concurrent method, we can see that main ran longer than it's _concurrent method. This is to be expected as their time difference reflects the amount of compute times of the list and sorted methods (and that of the other methods encased within these methods). Clearly seen, the list method took less compute time to return a result list than the sorted method. The average compute times of the list method for both the .submit() and .map() codes were similar, at ~0.47sec. The average compute time of the sorted method for the .submit() and .map() codes was 1.23sec and 1.01sec, respectively. In other words, the list method performed 2.62 times and 2.15 times faster than sorted method for the .submit() and .map() codes, respectively.
3. It is not clear why the sorted method generated an ordered list from .map() faster than from .submit(), as the number of discretized tasks increased more than the number of pool workers, save when the number of discretized tasks equaled the number of pool workers. That said, these findings shows that the decision to use the equally fast .submit() or .map() sub-classes can be encumbered by the sorted method. For example, if the intent is to generate an ordered list in the shortest time possible, the use of ProcessPoolExecutor.map() should be preferred over ProcessPoolExecutor.submit() as .map() can allow the shortest total compute time.
4. The discretization scheme mentioned in Part 1 of my answer is shown here to speed-up the performance of both the .submit() and .map() sub-classes. The amount of speed-up can be as much as 20% over the case when the number of discretized tasks equaled the number of pool workers.

Improved .map() code

#!/usr/bin/python3.5
# -*- coding: utf-8 -*-

import concurrent.futures as cf
from time import time
from itertools import repeat, chain 


def _findmatch(nmin, nmax, number):
    '''Function to find the occurence of number in range nmin to nmax and return
       the found occurences in a list.'''
    start = time()
    match=[]
    for n in range(nmin, nmax):
        if number in str(n):
            match.append(n)
    end = time() - start
    #print("\n def _findmatch {0:<10} {1:<10} {2:<3} found {3:8} in {4:.4f}sec".
    #      format(nmin, nmax, number, len(match),end))
    return match

def _concurrent(nmax, number, workers, num_of_chunks):
    '''Function that utilises concurrent.futures.ProcessPoolExecutor.map to
       find the occurrences of a given number in a number range in a concurrent
       manner.'''
    # 1. Local variables
    start = time()
    chunksize = nmax // num_of_chunks
    #2. Parallelization
    with cf.ProcessPoolExecutor(max_workers=workers) as executor:
        # 2.1. Discretise workload and submit to worker pool
        cstart = (chunksize * i for i in range(num_of_chunks))
        cstop = (chunksize * i if i != num_of_chunks else nmax
                 for i in range(1, num_of_chunks + 1))
        futures = executor.map(_findmatch, cstart, cstop, repeat(number))
    end = time() - start
    print('\n within statement of def _concurrent_map(nmax, number, workers, num_of_chunks):')
    print("found in {0:.4f}sec".format(end))
    return list(chain.from_iterable(futures)) #Return an unordered result list
    #return sorted(chain.from_iterable(futures)) #Return an ordered result list

if __name__ == '__main__':
    nmax = int(1E8) # Number range maximum.
    number = str(5) # Number to be found in number range.
    workers = 6     # Pool of workers
    chunks_vs_workers = 30 # A factor of =>14 can provide optimum performance 
    num_of_chunks = chunks_vs_workers * workers

    start = time()
    found = _concurrent(nmax, number, workers, num_of_chunks)
    end = time() - start
    print('\n main')
    print('nmax={}, workers={}, num_of_chunks={}'.format(
          nmax, workers, num_of_chunks))
    #print('found = ', found)
    print("found {0} in {1:.4f}sec".format(len(found),end))    

Improved .submit() code.
This code is same as .map code except you replace the _concurrent method with the following:

def _concurrent(nmax, number, workers, num_of_chunks):
    '''Function that utilises concurrent.futures.ProcessPoolExecutor.submit to
       find the occurrences of a given number in a number range in a concurrent
       manner.'''
    # 1. Local variables
    start = time()
    chunksize = nmax // num_of_chunks
    futures = []
    #2. Parallelization
    with cf.ProcessPoolExecutor(max_workers=workers) as executor:
        # 2.1. Discretise workload and submit to worker pool
        for i in range(num_of_chunks):
            cstart = chunksize * i
            cstop = chunksize * (i + 1) if i != num_of_chunks - 1 else nmax
            futures.append(executor.submit(_findmatch, cstart, cstop, number))
    end = time() - start
    print('\n within statement of def _concurrent_submit(nmax, number, workers, num_of_chunks):')
    print("found in {0:.4f}sec".format(end))
    return list(chain.from_iterable(f.result() for f in cf.as_completed(
        futures))) #Return an unordered list
    #return list(chain.from_iterable(f.result() for f in cf.as_completed(
    #    futures))) #Return an ordered list

=======================================================================

Answer 2

You're comparing apples to oranges here. When using map you produce all the 1E8 numbers and transfer them to worker processes. This takes a lot of time compared to actual execution. When using submit you just create 6 sets of parameters that get transferred.

If you change map to operate with the same principle you'll get numbers that are close to each other:

def _findmatch(nmin, nmax, number):
    '''Function to find the occurrence of number in range nmin to nmax and return
       the found occurrences in a list.'''
    print('\n def _findmatch', nmin, nmax, number)
    start = time()
    match=[]
    for n in range(nmin, nmax):
        if number in str(n):
            match.append(n)
    end = time() - start
    print("found {0} in {1:.4f}sec".format(len(match),end))
    return match

def _concurrent_map(nmax, number, workers):
    '''Function that utilises concurrent.futures.ProcessPoolExecutor.map to
       find the occurrences of a given number in a number range in a parallelised
       manner.'''
    # 1. Local variables
    start = time()
    chunk = nmax // workers
    futures = []
    found =[]
    #2. Parallelization
    with cf.ProcessPoolExecutor(max_workers=workers) as executor:
        # 2.1. Discretise workload and submit to worker pool
        cstart = (chunk * i for i in range(workers))
        cstop = (chunk * i if i != workers else nmax for i in range(1, workers + 1))
        futures = executor.map(_findmatch, cstart, cstop, itertools.repeat(number))

        # 2.3. Consolidate result as a list and return this list.
        for future in futures:
            for f in future:
                try:
                    found.append(f)
                except:
                    print_exc()
        foundsize = len(found)
        end = time() - start
        print('within statement of def _concurrent(nmax, number):')
        print("found {0} in {1:.4f}sec".format(foundsize, end))
    return found

You could improve the performance of submit by using as_completed correctly. For given iterable of futures it will return an iterator that will yield futures in the order they complete.

You could also skip the copying of the data to another array and use itertools.chain.from_iterable to combine the results from futures to single iterable:

import concurrent.futures as cf
import itertools
from time import time
from traceback import print_exc
from itertools import chain

def _findmatch(nmin, nmax, number):
    '''Function to find the occurrence of number in range nmin to nmax and return
       the found occurrences in a list.'''
    print('\n def _findmatch', nmin, nmax, number)
    start = time()
    match=[]
    for n in range(nmin, nmax):
        if number in str(n):
            match.append(n)
    end = time() - start
    print("found {0} in {1:.4f}sec".format(len(match),end))
    return match

def _concurrent_map(nmax, number, workers):
    '''Function that utilises concurrent.futures.ProcessPoolExecutor.map to
       find the occurrences of a given number in a number range in a parallelised
       manner.'''
    # 1. Local variables
    chunk = nmax // workers
    futures = []
    found =[]
    #2. Parallelization
    with cf.ProcessPoolExecutor(max_workers=workers) as executor:
        # 2.1. Discretise workload and submit to worker pool
        for i in range(workers):
            cstart = chunk * i
            cstop = chunk * (i + 1) if i != workers - 1 else nmax
            futures.append(executor.submit(_findmatch, cstart, cstop, number))

    return chain.from_iterable(f.result() for f in cf.as_completed(futures))

if __name__ == '__main__':
    nmax = int(1E8) # Number range maximum.
    number = str(5) # Number to be found in number range.
    workers = 6     # Pool of workers

    start = time()
    a = _concurrent_map(nmax, number, workers)
    end = time() - start
    print('\n main')
    print('workers = ', workers)
    print("found {0} in {1:.4f}sec".format(sum(1 for x in a),end))