ProcessPoolExecutor deadlocks with large number of tasks

I'm trying to run multiple identical short tasks with ProcessPoolExecutor on 256 cores Ubuntu Linux machine. The tasks are independent and don't share any resources.

I initialize ProcessPoolExector with 252 cores, and submit it a large number of tasks (100K to 1 million)

What i get is that the tasks start running, but after ~1000 tasks the main process gets stuck, and no more tasks are executed

This reproduces on different machines (with the same number of cores)

A sample program:

from concurrent.futures import ProcessPoolExecutor
from time import sleep

import multiprocessing as mp


def run_function(id):
    for i in range(int(1e5)):
        i += 1
    print(f"{id}  Done!")


if __name__ == "__main__":
    proc_count = 252

    mp_ctx = mp.get_context('spawn')

    executor = ProcessPoolExecutor(max_workers=proc_count, mp_context=mp_ctx)
    futures = []

    for i in range(int(1e6)):
        task_future = executor.submit(run_function, i)
        futures.append(task_future)

    while not all([f.done() for f in futures]):
        sleep(2)

Is it a bug in ProcessPoolExecutor? The documentation lacks a description of such limitation.

I have a workaround that includes my implementation of a process pool with raw python multiprocessing objects, but i prefer to understand ProcessPoolExecutor and use it properly