• 1   Deadpool
• 2   Licence
• 3   Installation
• 4   Why would I want to use this?
• 5   What differs from ProcessPoolExecutor?
  • 5.1   Similarities
  • 5.2   Differences in existing behaviour
  • 5.3   New features in Deadpool
• 6   Show me some code
  • 6.1   Simple case
  • 6.2   Timeouts
  • 6.3   Handling OOM killed situations
• 7   Design Details
  • 7.1   Typical Example - with timeouts
  • 7.2   Deadpool tasks have priority
  • 7.3   Controlling the backlog of submitted tasks
  • 7.4   More about shutdown
• 8   Developer Workflow
  • 8.1   nox
  • 8.2   tests
  • 8.3   style
  • 8.4   docs
  • 8.5   release

1   Deadpool

Deadpool is a process pool that is really hard to kill.

Deadpool is an implementation of the Executor interface in the concurrent.futures standard library. Deadpool is a process pool executor, quite similar to the stdlib's ProcessPoolExecutor.

This document assumes that you are familiar with the stdlib ProcessPoolExecutor. If you are not, it is important to understand that Deadpool makes very specific tradeoffs that can result in quite different behaviour to the stdlib implementation.

2   Licence

This project can be licenced either under the terms of the Apache 2.0 licence, or the Affero GPL 3.0 licence. The choice is yours.

3   Installation

The python package name is deadpool-executor, so to install you must type $ pip install deadpool-executor. The import name is deadpool, so in your Python code you must type import deadpool to use it.

I try quite hard to keep dependencies to a minimum. Currently Deadpool has no dependencies other than psutil which is simply too useful to avoid for this library.

4   Why would I want to use this?

I created Deadpool because I became frustrated with the stdlib ProcessPoolExecutor, and various other community implementations of process pools. In particular, I had a use-case that required a high server uptime, but also had variable and unpredictable memory requirements such that certain tasks could trigger the OOM killer, often resulting in a "broken" process pool. I also needed task-specific timeouts that could kill a "hung" task, which the stdlib executor doesn't provide.

You might wonder, isn't it bad to just kill a task like that? In my use-case, we had extensive logging and monitoring to alert us if any tasks failed; but it was paramount that our services continue to operate even when tasks got killed in OOM scenarios, or specific tasks took too long. This is the primary trade-off that Deadpool offers: the pool will not break, but tasks can receive SIGKILL under certain conditions. This trade-off is likely fine if you've seen many OOMs break your pools.

I also tried using the Pebble community process pool. This is a cool project, featuring several of the properties I've been looking for such as timeouts, and more resilient operation. However, during testing I found several occurrences of a mysterious RuntimeError that caused the Pebble pool to become broken and no longer accept new tasks.

My goal with Deadpool is that the pool must never enter a broken state. Any means by which that can happen will be considered a bug.

5   What differs from ProcessPoolExecutor?

Deadpool is generally similar to ProcessPoolExecutor since it executes tasks in subprocesses, and implements the standard Executor abstract interface. We can draw a few comparisons to the stdlib pool to guide your decision process about whether this makes sense for your use-case:

5.1   Similarities

• Deadpool also supports the max_tasks_per_child parameter (a new feature in Python 3.11, although it was available in multiprocessing.Pool since Python 3.2).
• The "initializer" callback in Deadpool works the same.
• Deadpool defaults to the forkserver multiprocessing context, unlike the stdlib pool which defaults to fork on Linux. It's just a setting though, you can change it in the same way as with the stdlib pool. Like the stdlib, I strongly advise you to avoid using fork because propagation threads and locks via fork is going to ruin your day eventually. While this is a difference to the default behaviour of the stdlib pool, it's not a difference in behaviour to the stdlib pool when you use the forkserver context which is the recommended context for multiprocessing.

5.2   Differences in existing behaviour

Deadpool differs from the stdlib pool in the following ways:

• If a Deadpool subprocess in the pool is killed by some external actor, for example, the OS runs out of memory and the OOM killer kills a pool subprocess that is using too much memory, Deadpool does not care and further operation is unaffected. Deadpool will not, and indeed cannot raise BrokenProcessPool or BrokenExecutor.
• Deadpool precreates all subprocesses up to the pool size on creation.
• Deadpool tasks can have priorities. When the executor chooses the next pending task to schedule to a subprocess, it chooses the pending task with the highest priority. This gives you a way of prioritizing certain kinds of tasks. For example, you might give UI-sensitive tasks a higher priority to deliver a more snappy user experience to your users. The priority can be specified in the submit call.
• The shutdown parameters wait and cancel_futures can behave differently to how they work in the ProcessPoolExecutor. This is discussed in more detail later in this document.
• Deadpool currently only works on Linux. There isn't any specific reason it can't work on other platforms. The malloc trim feature also requires a glibc system, so probably won't work on Alpine.

5.3   New features in Deadpool

Deadpool has the following features that are not present in the stdlib pool:

• With Deadpool you can provider a "finalizer" callback that will fire before a subprocess is shut down or killed. The finalizer callback might be executed in a different thread than the main thread of the subprocess, so don't rely on the callback running in the main subprocess thread. There are certain circumstances where the finalizer will not run at all, such as when the subprocess is killed by the OS due to an out-of-memory (OOM) condition. So don't design your application such that the finalizer is required to run for correct operation.
• Even though Deadpool typically uses a hard kill to remove subprocesses, it does still run any handlers registered with atexit.
• Deadpool tasks can have timeouts. When a task hits the timeout, the underlying subprocess in the pool is killed with SIGKILL. The entire process tree of that subprocess is killed. Your application logic needs to handle this. The finalizer will not run.
• Deadpool also allows a finalizer, with corresponding finalargs, that will be called after a task is executed on a subprocess, but before the subprocess terminates. It is analogous to the initializer and initargs parameters. Just like the initializer callable, the finalizer callable is executed inside the subprocess. It is not guaranteed that the finalizer will always run. If a process is killed, e.g. due to a timeout or any other reason, the finalizer will not run. The finalizer could be used for things like flushing pending monitoring messages, such as traces and so on.
• Deadpool can ask the system allocator (Linux only) to return unused memory back to the OS based on exceeding a max threshold RSS. For long-running pools and modern kernels, the system memory allocator can hold onto unused memory for a surprisingly long time, and coupled with bloat due to memory fragmentation, this can result in carrying very large RSS values in your pool. The max_tasks_per_child helps with this because a subprocess is entirely erased when the max is reached, but it does mean that periodically there will be a small latency penalty from constructing the replacement subprocess. In my opinion, max_tasks_per_child is appropriate for when you know or suspect there's a real memory leak somewhere in your code (or a 3rd-party package!), and the easiest way to deal with that right now is just to periodically remove a process.

6   Show me some code

6.1   Simple case

The simple case works exactly the same as with ProcessPoolExecutor:

import deadpool

def f():
    return 123

with deadpool.Deadpool() as exe:
    fut = exe.submit(f)
    result = fut.result()

assert result == 123

It is intended that all the basic behaviour should "just work" in the same way, and Deadpool should be a drop-in replacement for ProcessPoolExecutor; but there are some subtle differences so you should read all of this document to see if any of those will affect you.

6.2   Timeouts

If a timeout is reached on a task, the subprocess running that task will be killed, as in SIGKILL. Deadpool doesn't mind, but your own application should: if you use timeouts it is likely important that your tasks be idempotent, especially if your application will restart tasks, or restart them after application deployment, and other similar scenarios.

import time
import deadpool

def f():
    time.sleep(10.0)

with deadpool.Deadpool() as exe:
    fut = exe.submit(f, deadpool_timeout=1.0)

    with pytest.raises(deadpool.TimeoutError)
        fut.result()

The parameter deadpool_timeout is special and consumed by Deadpool in the call. You can't use a parameter with this name in your function kwargs.

6.3   Handling OOM killed situations

import time
import deadpool

def f():
    x = list(range(10**100))

with deadpool.Deadpool() as exe:
    fut = exe.submit(f, deadpool_timeout=1.0)

    try:
        result = fut.result()
    except deadpool.ProcessError:
        print("Oh no someone killed my task!")

As long as the OOM killer terminates merely a subprocess (and not the main process), which is likely because it'll be your subprocess that is using too much memory, this will not hurt the pool, and it will be able to receive and process more tasks. Note that this event will show up as a ProcessError exception when accessing the future, so you have a way of at least tracking these events.

7   Design Details

7.1   Typical Example - with timeouts

Here's a typical example of how code using Deadpool might look. The output of the code further below should be similar to the following:

$ python examples/entrypoint.py
...................xxxxxxxxxxx.xxxxxxx.x.xxxxxxx.x
$

Each . is a successfully completed task, and each x is a task that timed out. Below is the code for this example.

import random, time
import deadpool


def work():
    time.sleep(random.random() * 4.0)
    print(".", end="", flush=True)
    return 1


def main():
    with deadpool.Deadpool() as exe:
        futs = (exe.submit(work, deadpool_timeout=2.0) for _ in range(50))
        for fut in deadpool.as_completed(futs):
            try:
                assert fut.result() == 1
            except deadpool.TimeoutError:
                print("x", end="", flush=True)


if __name__ == "__main__":
    main()
    print()

• The work function will be busy for a random time period between 0 and 4 seconds.
• There is a deadpool_timeout kwarg given to the submit method. This kwarg is special and will be consumed by Deadpool. You cannot use this kwarg name for your own task functions.
• When a task completes, it prints out . internally. But when a task raises a deadpool.TimeoutError, a x will be printed out instead.
• When a task times out, keep in mind that the underlying process that is executing that task is killed, literally with the SIGKILL signal.

7.2   Deadpool tasks have priority

The example below is similar to the previous one for timeouts. In fact this example retains the timeouts to show how the different features compose together. In this example we create tasks with different priorities, and we change the printed character of each task to show that higher priority items are executed first.

The code example will print something similar to the following:

$ python examples/priorities.py
!!!!!xxxxxxxxxxx!x..!...x.xxxxxxxx.xxxx.x...xxxxxx

You can see how the ! characters, used for indicating higher priority tasks, appear towards the front indicating that they were executed sooner. Below is the code.

import random, time
import deadpool


def work(symbol):
    time.sleep(random.random() * 4.0)
    print(symbol, end="", flush=True)
    return 1


def main():
    with deadpool.Deadpool(max_backlog=100) as exe:
        futs = []
        for _ in range(25):
            fut = exe.submit(work, ".",deadpool_timeout=2.0, deadpool_priority=10)
            futs.append(fut)
            fut = exe.submit(work, "!",deadpool_timeout=2.0, deadpool_priority=0)
            futs.append(fut)

        for fut in deadpool.as_completed(futs):
            try:
                assert fut.result() == 1
            except deadpool.TimeoutError:
                print("x", end="", flush=True)


if __name__ == "__main__":
    main()
    print()

• When the tasks are submitted, they are given a priority. The default value for the deadpool_priority parameter is 0, but here we'll write them out explicity. Half of the tasks will have priority 10 and half will have priority 0.
• A lower value for the deadpool_priority parameters means a higher priority. The highest priority allowed is indicated by 0. Negative priority values are not allowed.
• I also specified the max_backlog parameter when creating the Deadpool instance. This is discussed in more detail next, but quickly: task priority can only be enforced on what is in the submitted backlog of tasks, and the max_backlog parameter controls the depth of that queue. If max_backlog is too low, then the window of prioritization will not include tasks submitted later which might have higher priorities than earlier-submitted tasks. The submit call will in fact block once the max_backlog depth has been reached.

7.3   Controlling the backlog of submitted tasks

By default, the max_backlog parameter is set to 5. This parameter is used to create the "submit queue" size. The submit queue is the place where submitted tasks are held before they are executed in background processes.

If the submit queue is large (max_backlog), it will mean that a large number of tasks can be added to the system with the submit method, even before any tasks have finished exiting. Conversely, a low max_backlog parameter means that the submit queue will fill up faster. If the submit queue is full, it means that the next call to submit will block.

This kind of blocking is fine, and typically desired. It means that backpressure from blocking is controlling the amount of work in flight. By using a smaller max_backlog, it means that you'll also be limiting the amount of memory in use during the execution of all the tasks.

However, if you nevertheless still accumulate received futures as my example code above is doing, that accumulation, i.e., the list of futures, will contribute to memory growth. If you have a large amount of work, it will be better to set a callback function on each of the futures rather than processing them by iterating over as_completed.

The example below illustrates this technique for keeping memory consumption down:

import random, time
import deadpool


def work():
    time.sleep(random.random() * 4.0)
    print(".", end="", flush=True)
    return 1


def cb(fut):
    try:
        assert fut.result() == 1
    except deadpool.TimeoutError:
        print("x", end="", flush=True)


def main():
    with deadpool.Deadpool() as exe:
        for _ in range(50):
            exe.submit(work, deadpool_timeout=2.0).add_done_callback(cb)


if __name__ == "__main__":
    main()
    print()

With this callback-based design, we no longer have an accumulation of futures in a list. We get the same kind of output as in the "typical example" from earlier:

$ python examples/callbacks.py
.....xxx.xxxxxxxxx.........x..xxxxx.x....x.xxxxxxx

Speaking of callbacks, the customized Future class used by Deadpool lets you set a callback for when the task begins executing on a real system process. That can be configured like so:

with deadpool.Deadpool() as exe:
    f = exe.submit(work)

    def cb(fut: deadpool.Future):
        print(f"My task is running on process {fut.pid}")

    f.add_pid_callback(cb)

Obviously, both kinds of callbacks can be added:

with deadpool.Deadpool() as exe:
    f = exe.submit(work)
    f.add_pid_callback(lambda fut: f"Started on {fut.pid=}")
    f.add_done_callback(lambda fut: f"Completed {fut.pid=}")

7.4   More about shutdown

In the documentation for ProcessPoolExecutor, the following function signature is given for the shutdown method of the executor interface:

shutdown(wait=True, *, cancel_futures=False)

I want to honor this, but it presents some difficulties because the semantics of the wait and cancel_futures parameters need to be somewhat different for Deadpool.

In Deadpool, this is what the combinations of those flags mean:

Shutdown flags

wait	cancel_futures	effect
True	True	Wait for already-running tasks to complete; the shutdown() call will unblock (return) when they're done. Cancel all pending tasks that are in the submit queue, but have not yet started running. The fut.cancelled() method will return True for such cancelled tasks.
True	False	Wait for already-running tasks to complete. Pending tasks in the submit queue that have not yet started running will not be cancelled, and will all continue to execute. The shutdown() call will return only after all submitted tasks have completed.
False	True	Already-running tasks will be cancelled and this means the underlying subprocesses executing these tasks will receive SIGKILL. Pending tasks on the submit queue that have not yet started running will also be cancelled.
False	False	This is a strange one. What to do if the caller doesn't want to wait, but also doesn't want to cancel things? In this case, already-running tasks will be allowed to complete, but pending tasks on the submit queue will be cancelled. This is the same outcome as as wait==True and cancel_futures==True. An alternative design might have been to allow all tasks, both running and pending, to just keep going in the background even after the shutdown() call returns. Does anyone have a use-case for this?

If you're using Deadpool as a context manager, you might be wondering how exactly to set these parameters in the shutdown call, since that call is made for you automatically when the context manager exits.

For this, Deadpool provides additional parameters that can be provided when creating the instance:

# This is pseudocode
import deadpool

with deadpool.DeadPool(
        shutdown_wait=True,
        shutdown_cancel_futures=True
):
    fut = exe.submit(...)

8   Developer Workflow

8.1   nox

This project uses nox. Follow the instructions for installing nox at their page, and then come back here.

While nox can be configured so that all the tools for each of the tasks can be installed automatically when run, this takes too much time and so I've decided that you should just have the following tools in your environment, ready to go. They do not need to be installed in the same venv or anything like that. I've found a convenient way to do this is with pipx. For example, to install black using pipx you can do the following:

$ pipx install black

You must do the same for isort and ruff. See the following sections for actually using nox to perform dev actions.

8.2   tests

To run the tests:

$ nox -s test

To run tests for a particular version, and say with coverage:

$ nox -s testcov-3.11

To pass additional arguments to pytest, use the -- separator:

$ nox -s testcov-3.11 -- -k test_deadpool -s <etc>

This is nonstandard above, but I customized the noxfile.py to allow this.

8.3   style

To apply style fixes, and check for any remaining lints,

$ nox -t style

8.4   docs

The only docs currently are this README, which uses RST. Github uses docutils to render RST.

8.5   release

This project uses flit to release the package to pypi. The whole process isn't as automated as I would like, but this is what I currently do:

1. Ensure that main branch is fully up to date with all to be released, and all the tests succeed.

2. Change the __version__ field in deadpool.py. Flit uses this to stamp the version.

3. Verify that flit build succeeds. This will produce a wheel in the dist/ directory. You can inspect this wheel to ensure it contains only what is necessary. This wheel will be what is uploaded to PyPI.

4. Commit the changed ``__version__``. Easy to forget this step, resulting in multiple awkward releases to try to get the state all correct again.

5. Now create the git tag and push to github:

   $ git tag YYYY.MM.patch
   $ git push --tags origin main

6. Now deploy to PyPI:

   $ flit publish