Well, that's a common issue when using asyncio: you forgot await.

async def main(_loop):
    while True:
        with futures.ThreadPoolExecutor() as pool:
            _loop.run_in_executor(pool, nop)
        sys.stdout.write(f'\r{get_mem():0.3f}MB')

It should be: "await _loop.run_in_executor(pool, nop)" ;-)

Sadly, PYTHONASYNCIODEBUG=1 env var doesn't complain on this bug.

See: https://docs.python.org/dev/library/asyncio-dev.html#debug-mode