This is very similar to issue35715. But this is happen for thread.

After the func in thread finished, the memory is still hold and accumulate.

import asyncio
import time
import concurrent
import threading

loop = asyncio.get_event_loop()

def prepare_a_giant_list():
    m = []
    for i in range(1000*1000):
        m.append("There's a fat fox jump over a sheep" + str(i))

    th_num = threading.active_count()
    print("Thread number is {}".format(th_num))
    return m

@asyncio.coroutine
def main():
    global loop
    global counter
    async_executor = concurrent.futures.ThreadPoolExecutor(max_workers=20)
    loop.run_in_executor(async_executor, prepare_a_giant_list)
    time.sleep(15)
    loop.run_in_executor(async_executor, prepare_a_giant_list)
    time.sleep(15)
    loop.run_in_executor(async_executor, prepare_a_giant_list)
    time.sleep(15)
    loop.run_in_executor(async_executor, prepare_a_giant_list)
    time.sleep(15)

if __name__ == "__main__":
    loop.run_until_complete(main())
    loop.close()