Hi,

I think there is an issue with memory allocating with threading.Timer in 3.8.0/3.8.1.

When I run the attached code in Python 3.7.3 I get a memory consumption of approx. 10 MB. If I run the same code with python 3.8.0 or 3.8.1, it keeps consuming memory (several hundreds of MB). 

I've attached 3 images where I run the attached script under mprof.

By inspect of the output from tracemalloc, I can see that 
/usr/lib/python3.8/threading.py:908 keeps allocating memory.

I have tested this with python 3.8.0 from ubuntu 16.04 repository and python 3.8.1 from source. Both versions suffers from the memory consumption issue.

This issue also affects me, pyth. I tried it out with various combinations of sleep, and durations of tasks, but what I noticed is that threading_shutdown_locks continues to grow at each iteration.

for i in range(100000):
    for j in range(10):
        timer = threading.Thread(target = callback)
        timer.start()
    time.sleep(1)
    print(len(threading._shutdown_locks))

If I accumulate the Timer/Thread s and join them, then the _shutdown_locks will be cleared.

Please include the script as an attachment or text so that it will be easy to view them. The script as below from the attachment : 

import threading
import time
import tracemalloc

def callback():
    pass

tracemalloc.start()
for i in range(100000):
    snapshot1 = tracemalloc.take_snapshot()
    for i in range(100000):
        timer = threading.Timer(1, callback)
        timer.start()
    snapshot2 = tracemalloc.take_snapshot()
    top_stats = snapshot2.compare_to(snapshot1, 'lineno')
    print("[ Top 10 differences ]")
	
    for stat in top_stats[:10]:
        print(stat)

I filed a bug for this a few weeks ago, and then found another ticket about the same issue before:

https://bugs.python.org/issue37788

My ticket: 
https://bugs.python.org/issue39074

The memory leak was from a change introduced about 6 months ago:

https://github.com/python/cpython/commit/468e5fec8a2f534f1685d59da3ca4fad425c38dd

I close this issue as a duplicate of bpo-37788.