Setup

This section describes the different ways of creating a mpire.WorkerPool object.

Starting a WorkerPool

The mpire.WorkerPool class controls a pool of worker processes similarly to a multiprocessing.Pool. It contains all the map like functions (with the addition of mpire.WorkerPool.map_unordered()), but currently lacks the apply and apply_async functions (if you wish to add it, feel free to do so).

An mpire.WorkerPool can be started in two different ways. The first and recommended way to do so is using a context manager:

from mpire import WorkerPool

# Start a pool of 4 workers
with WorkerPool(n_jobs=4) as pool:
    # Do some processing here
    pass

The with statement takes care of properly joining/terminating the spawned worker processes after the block has ended.

The other way is to do it manually:

# Start a pool of 4 workers
pool = WorkerPool(n_jobs=4)

# Do some processing here
pass

# Clean up pool (this will block until all processing has completed)
pool.stop_and_join()

# In the case you want to kill the processes even though they are still busy
pool.terminate()

When using n_jobs=None MPIRE will spawn as many processes as there are CPUs on your system. Specifying more jobs than you have CPUs is, of course, possible as well.

Warning

The results queue should be drained first before joining the workers, otherwise you can get a deadlock. If you want to join either way, use mpire.WorkerPool.terminate(). For more information, see the warnings in the Python docs here.

Nested WorkerPools

Normally, the mpire.WorkerPool class spawns daemon child processes who are not able to create child processes themselves, so nested pools are not allowed. However, there’s an option to create normal child processes, instead of daemon, to allow for nested structures:

def job(...)
    with WorkerPool(n_jobs=4) as p:
        # Do some work
        results = p.map(...)

with WorkerPool(n_jobs=4, daemon=True) as pool:
    # This will raise an AssertionError telling you daemon processes can't start child processes
    pool.map(job, ...)

with WorkerPool(n_jobs=4, daemon=False) as pool:
    # This will work just fine
    pool.map(job, ...)

Do make sure all your non-daemon processes are terminated correctly. If a nested child process is interrupted, for example when the user triggers a KeyboardInterrupt, the process will remain active and will have to be terminated manually.

CPU pinning

If desired you can pin the child processes of mpire.WorkerPool to specific CPUs by using the cpu_ids parameter in the constructor:

# Pin the two child processes to CPUs 2 and 3
with WorkerPool(n_jobs=2, cpu_ids=[2, 3]) as pool:
    ...

# Pin the child processes to CPUs 40-59
with WorkerPool(n_jobs=20, cpu_ids=list(range(40, 60))) as pool:
    ...

# All child processes have to share a single core:
with WorkerPool(n_jobs=4, cpu_ids=[0]) as pool:
    ...

# All child processes have to share multiple cores, namely 4-7:
with WorkerPool(n_jobs=4, cpu_ids=[[4, 5, 6, 7]]) as pool:
    ...

# Each child process can use two distinctive cores:
with WorkerPool(n_jobs=4, cpu_ids=[[0, 1], [2, 3], [4, 5], [6, 7]]) as pool:
    ...

CPU IDs have to be positive integers, not exceeding the number of CPUs available (which can be retrieved by using mpire.cpu_count()). Use None to disable CPU pinning (which is the default).

Shared objects

MPIRE allows you to provide shared objects to the workers in a similar way as is possible with the multiprocessing.Process class. These shared objects are treated as copy-on-write, they are only copied once changes are made to them, otherwise they share the same memory address. This is convenient if you want to let workers access a large dataset that wouldn’t fit in memory when copied multiple times. When shared objects are copied they are only copied once for each worker, in contrast to copying it for each task which is done when using a regular multiprocessing.Pool.

By using a multiprocessing.Array, multiprocessing.Value, or another object with multiprocessing.Manager you could even store results in the same object from multiple processes. However, be aware of the possible locking behavior that comes with it. However, in some cases you can safely disable locking, as is shown here:

from multiprocessing import Array

def square_with_index(shared_objects, idx, x):
    # Even though the shared objects is a single container, we 'unpack' it
    # (only to be consistent with the function below)
    results_container = shared_objects

    # Square
    results_container[idx] = x * x

def square_add_and_modulo_with_index(shared_objects, idx, x):
    # Unpack results containers
    square_results_container, add_results_container = shared_objects

    # Square, add and modulo
    square_results_container[idx] = x * x
    add_results_container[idx] = x + x
    return x % 2


# 1. Use a shared array of size 100 and type float to store the results
results_container = Array('f', 100, lock=False)
with WorkerPool(n_jobs=4, shared_objects=results_container) as pool:

    # Square the results and store them in the results container
    pool.map_unordered(square_with_index, enumerate(range(100)),
                       iterable_len=100)

# 2, Use a shared array of size 100 and type float to store the results
square_results_container = Array('f', 100, lock=False)
add_results_container = Array('f', 100, lock=False)
with WorkerPool(n_jobs=4,
                shared_objects=(square_results_container, add_results_container)) as pool:

    # Square, add and modulo the results and store them in the results containers
    modulo_results = pool.map(square_add_and_modulo_with_index,
                              enumerate(range(100)), iterable_len=100)

Multiple objects can be provided by placing them, for example, in a tuple container as is done in example two above. When providing shared objects the provided function pointer in the map functions should receive the shared objects as its first argument (or the second argument when the worker ID is passed on as well, see Accessing the worker ID).

In the first example (marked #1) we create a results container and disable locking. We can safely disable locking here as each task writes to a different index in the array, so no race conditions can occur. Disabling locking is, of course, a lot faster than enabling it.

In the second example we create two different results containers, one for squaring and for adding the given value. Additionally, we also return a value, even though we use shared objects for storing results.

Instead of passing the shared objects to the mpire.WorkerPool constructor you can also use the mpire.WorkerPool.set_shared_objects() function:

results_container = Array('f', 100, lock=False)

with WorkerPool(n_jobs=4) as pool:
    pool.set_shared_objects(results_container)
    pool.map_unordered(square_with_index, enumerate(range(100)),
                       iterable_len=100)

Worker state

If you want to let each worker have its own state you can use the use_worker_state flag:

import numpy as np
import pickle

def load_big_model():
    # Load a model which takes up a lot of memory
    with open('./a_really_big_model.p3', 'rb') as f:
        return pickle.load(f)

def model_predict(worker_state, x):
    # Load model
    if 'model' not in worker_state:
        worker_state['model'] = load_big_model()

    # Predict
    return worker_state['model'].predict(x)

with WorkerPool(n_jobs=4, use_worker_state=True) as pool:
    # Let the model predict
    data = np.array([[...]])
    results = pool.map(model_predict, data)

Instead of passing the flag to the mpire.WorkerPool constructor you can also make use of mpire.WorkerPool.set_use_worker_state():

with WorkerPool(n_jobs=4) as pool:
    pool.set_use_worker_state()
    results = pool.map(model_predict, data)

Accessing the worker ID

Each worker in MPIRE is given an integer ID to distinguish them. Worker #1 will have ID 0, #2 will have ID 1, etc. Sometimes it can be useful to have access to this ID. For example, when you have a shared array of which the size equals the number of workers and you want worker #1 only to access the first element, and worker #2 only to access the second element, and so on.

By default, the worker ID is not passed on. You can enable/disable this by setting the pass_worker_id flag:

def square_sum(worker_id, shared_objects, x):
    # Even though the shared objects is a single container, we 'unpack' it anyway
    results_container = shared_objects

    # Square and sum
    results_container[worker_id] += x * x

# Use a shared array of size equal to the number of jobs to store the results
results_container = Array('f', 4, lock=False)

with WorkerPool(n_jobs=4, shared_objects=results_container, pass_worker_id=True) as pool:
    # Square the results and store them in the results container
    pool.map_unordered(square_sum, range(100))

The worker ID will always be the first passed on argument to the provided function pointer.

Instead of passing the flag to the mpire.WorkerPool constructor you can also make use of mpire.WorkerPool.pass_on_worker_id():

with WorkerPool(n_jobs=4, shared_objects=results_container) as pool:
    pool.pass_on_worker_id()
    pool.map_unordered(square_sum, range(100))

Process start method

The multiprocessing package allows you to start processes using a few different methods: 'fork', 'spawn' or 'forkserver'. Threading is also available by using 'threading'. For detailed information on the multiprocessing contexts, please refer to the multiprocessing documentation and caveats section. In short:

• 'fork' (the default) copies the parent process such that the child process is effectively identical. This includes copying everything currently in memory. This is sometimes useful, but other times useless or even a serious bottleneck.

• 'spawn' starts a fresh python interpreter where only those resources necessary are inherited.

• 'forkserver' first starts a server process. Whenever a new process is needed the parent process requests the server to fork a new process.

• 'threading' starts child threads.

The 'spawn' and 'forkserver' methods have some caveats. All resources needed for running the child process should be picklable. This can sometimes be a hassle when you heavily rely on lambdas or are trying to run MPIRE in an interactive shell. To remedy most of these problems MPIRE can use dill as a replacement for pickle. Simply install the required dependencies and you’re good to go.

Additionally, global variables (constants are fine) might have a different value than you might expect. You also have to import packages within the called function:

import os

def failing_job(folder, filename):
    return os.path.join(folder, filename)

# This will fail because 'os' is not copied to the child processes
with WorkerPool(n_jobs=2, start_method='spawn') as pool:
    pool.map(failing_job, [('folder', '0.p3'), ('folder', '1.p3')])

def working_job(folder, filename):
    import os
    return os.path.join(folder, filename)

# This will work
with WorkerPool(n_jobs=2, start_method='spawn') as pool:
    pool.map(working_job, [('folder', '0.p3'), ('folder', '1.p3')])