Ming

Posted on Oct 14, 2021 • Edited on Oct 19, 2021 • Originally published at kemingy.github.io

Why not multiprocessing

#python #multiprocessing #fork #spawn

During the development of a machine learning serving project Mosec, I used a lot of multiprocessing to make it more efficient. I want to share some experiences and researches related to Python multiprocessing.

start from a segment fault

Here is a code snippet that will run well on Darwin but trigger a segment fault on Unix.

import multiprocessing as mp
from time import sleep


def wait_for_event(event):
    while not event.is_set():
        sleep(0.1)


def trigger_segment_fault():
    event = mp.Event()
    p = mp.get_context("spawn").Process(target=wait_for_event, args=(event,))
    p.start()  # this will show the exitcode=-SIGSEGV
    sleep(1)
    print(p)
    event.set()
    p.terminate()


if __name__ == "__main__":
    trigger_segment_fault()

Yeah, the pure Python code can trigger a segment fault.

The reason is because of the new process start method. According to the Python document, spawn is the default one on macOS (start from Python 3.8) while fork is the default one on Unix. But the start method also affects the Event creation. Let's check the source code:

class Event(object):

    def __init__(self, *, ctx):
        self._cond = ctx.Condition(ctx.Lock())
        self._flag = ctx.Semaphore(0)

The initialization takes a ctx which is related to the start method. So when you try to access a forked event in a spawned process, this segment fault occurs. The way to solve this is simple -- using the same context. (Actually, you can use the spawn event in the forked process)

fork or spawn

Another question is that, which start method should I use?

spawn: The parent process starts a fresh python interpreter process. The child process will only inherit those resources necessary to run the process objects run() method. In particular, unnecessary file descriptors and handles from the parent process will not be inherited.

fork: The parent process uses os.fork() to fork the Python interpreter. The child process, when it begins, is effectively identical to the parent process. All resources of the parent are inherited by the child's process. Note that safely forking a multithreaded process is problematic.

We can see that spawn will create a new Python process and only inherit necessary resources. fork will call the underlying os.fork(), but the implementation in CPython is problematic.

When you are using spawn, accidentally access the main process variables may have some unexpected consequences.

import multiprocessing as mp
import os


class Dummy:
    def __init__(self) -> None:
        print(f"init in pid: {os.getpid()}")


Dummy()
x = None


def task():
    if x is None:
        print("x is None")


if __name__ == "__main__":
    p = mp.get_context("spawn").Process(target=task)
    p.start()
    p.join()

In the above code snippet, if the spawn process tries to access the variable x, it will trigger the initialization of both Dummy() and x = None. So you can see the terminal will print two "init in pid" with different PIDs.

So what kind of problem can the fork cause? Let's take a look at this article: Why your multiprocessing Pool is stuck (it’s full of sharks!).

import threading
import os
import time
import multiprocessing as mp


class AreYouOK:
    def __init__(self):
        print("init in:", os.getpid())
        self.lock = threading.Lock()

    def check(self):
        if self.lock.locked():
            return False
        return True

    def acquire(self):
        self.lock.acquire()

    def delay_release(self):
        time.sleep(1)
        self.lock.release()


greeter = AreYouOK()
greeter.acquire()

threading.Thread(target=greeter.delay_release, daemon=True).start()


def greeting():
    time.sleep(1)
    print(os.getpid(), greeter.check())


if __name__ == "__main__":
    mp.get_context("fork").Process(target=greeting).start()
    greeting()

In the above example, after the lock is released, the child process still cannot acquire the lock. Why?

The main point is that fork doesn't copy everything.

Let's check the man page of fork:

The child does not inherit its parent's memory locks

The child does not inherit semaphore adjustments from its parent

So what happens here is that the child process has a lock already been acquired, but no thread will release the lock because that running thread won't be copied to the fork process. These two locks are not the same (copied not shared). Here, the threading.Lock is obviously not process-safe and should be handled with cautions when it's used in some other libraries (queue.Queue).

If we use spawn instead of fork, everything related will be rebuilt in the new process (including the Thread). That's why we should use spawn instead of fork:

from multiprocessing import set_start_method
set_start_method("spawn")

The code snippet above may cause some problems when the code is executed more than once.

My suggestion is to use the start method context:

import multiprocessing as mp


context = mp.get_context("spawn")
context.Event()

garbage collection with deadlock

Let's take a look at another article: The tragic tale of the deadlocking Python queue.

This code snippet is copied from the above article.

from queue import Queue

q = Queue()


class Circular(object):
    def __init__(self):
        self.circular = self

    def __del__(self):
        print("Adding to queue in GC")
        q.put(1)


for i in range(1000000000):
    print("iteration", i)
    # Create an object that will be garbage collected
    # asynchronously, and therefore have its __del__
    # method called later:
    Circular()
    print("Adding to queue regularly")
    q.put(2)

Usually, we believe that Python runs one line at a time. But that's not true.

Garbage collection can interrupt Python functions at any point, and run arbitrary other Python code: __del__ methods and weakref callbacks. So can signal handlers, which happen e.g. when you hit Ctrl-C (your process gets the SIGINT signal) or a subprocess dies (your process gets the SIGCHLD signal).

So when we try to q.put(2), the queue needs to acquire the lock. Meanwhile, the GC will try to call the __del__ which also does the q.put(1). The q.put(2) is blocked by the GC, but the GC cannot acquire the lock because q.put(2) won't release it. Deadlock happens!

Thanks to the Python-dev team, this has been fixed in Python 3.7 by introducing SimpleQueue.

Copy on write

When running with multiprocessing, we hope the child process can share some data with the main process instead of copying from it. Especially when they are not used in the child process. This sounds reasonable. However, we missed another important part in Python: reference counting.

CPython contains two kinds of garbage collection methods: reference counting and generational garbage collection. The reference counting is the fundamental one and cannot be disabled. The generational garbage collection is mainly used to solve the reference cycles. Check this article for more details: Garbage collection in Python: things you need to know and Design of CPython’s Garbage Collector.

Let's take a look at the CPython implementation of PyObject:

typedef struct _object {
    _PyObject_HEAD_EXTRA
    Py_ssize_t ob_refcnt;
    PyTypeObject *ob_type;
} PyObject;

There is a class member called ob_refcnt which is used to track the reference counting. If we call fork() in the new process, the reference counting of all the Python objects will increase. This means the object itself has changed although the data accessed by the user is still the same.

To handle this problem, the Instagram Engineering team has come up with a solution: Copy-on-write friendly Python garbage collection.

static PyObject *
gc_freeze_impl(PyObject *module)
/*[clinic end generated code: output=502159d9cdc4c139 input=b602b16ac5febbe5]*/
{
    GCState *gcstate = get_gc_state();
    for (int i = 0; i < NUM_GENERATIONS; ++i) {
        gc_list_merge(GEN_HEAD(gcstate, i), &gcstate->permanent_generation.head);
        gcstate->generations[i].count = 0;
    }
    Py_RETURN_NONE;
}

Let's check the Python document for GC. In Python 3.7, it introduced a new method called gc.freeze:

Freeze all the objects tracked by gc - move them to a permanent generation and ignore all the future collections.

So will this solve the Copy-on-write problem? I'm not sure because I cannot come up with an example to reproduce it.

import time
import psutil
import multiprocessing as mp


def display_memory_usage(msg=""):
    process = psutil.Process()
    print(msg, ">", process.memory_info())


def processing():
    display_memory_usage("child ")


if __name__ == "__main__":
    data = list(range(10000000))

    p = mp.get_context("fork").Process(target=processing)
    p.start()

    time.sleep(0.1)
    display_memory_usage("parent")
    p.join()

The code snippet above will print the memory usage of the main process and child process. You may get something like this:

child  > pmem(rss=414748672, vms=427634688, shared=2969600, text=2035712, lib=0, data=411791360, dirty=0)
parent > pmem(rss=419000320, vms=427634688, shared=7221248, text=2035712, lib=0, data=411791360, dirty=0)

We can see that they don't share a lot. Although by default, the fork process should share the data with the parent process.

But if we change it to spawn, we will get something like this:

child  > pmem(rss=13848576, vms=23044096, shared=7069696, text=2035712, lib=0, data=7163904, dirty=0)
parent > pmem(rss=419139584, vms=428081152, shared=7196672, text=2035712, lib=0, data=412200960, dirty=0)

Since the data is not used by the spawn process, so this won't be copied to the new process.

I try to add the gc.freeze() before creating a new process, but it doesn't work at all. Not sure what I have missed.

I found that some discussion in the gc.freeze() PR. It looks that the untouched data should be able to share among processes. Also, it has been 4 years for Gunicorn to process this support for gc.freeze() for apps that use preloading. I cannot found a good example to demonstrate that this method works well.

To my understanding, the gc.freeze() will disable the generational garbage collection. But the reference counting cannot be disabled. So if we fork a new process, everything will be shared with the new process, which means it will change all the reference count.

If we change the start method from spawn to fork, it doesn't need the gc.freeze() to freeze the reference count, which has conflicts with the description in the Instagram blog.

Is there any method to avoid this? Yes. Check another blog written before the Instagram blog: Python vs Copy on Write. The solution is very straightforward: