Issue 35715: ProcessPool workers hold onto return value of last task in memory (original) (raw)

ProcessPoolExecutor workers will hold onto the return value of their last task in memory until the next task is received. Since the return value has already been propagated to the parent process's Future or else effectively discarded, this is holding onto objects unnecessarily.

Simple case to reproduce:

import concurrent.futures
import time

executor = concurrent.futures.ProcessPoolExecutor(max_workers=1)

def big_val():
    return [{1:1} for i in range(1, 1000000)]

executor.submit(big_val)

# Observe the memory usage of the process worker during the sleep interval
time.sleep(10)

This should be easily fixed by having the worker explicitly del r after calling _sendback_result as it already does this for call_item