为什么单例模式在多线程环境下有效，而在多进程环境下无效？

import multiprocessing
import threading
import time

def singleton(cls):
    _instance = {}
    def inner():
        if cls not in _instance:
            _instance[cls] = cls()
        return _instance[cls]
    return inner

@singleton
class Cls:
    count = 0
    def __init__(self):
        self.count += 1

def run(thread_or_process_id):
    for i in range(100):
        a = Cls()
        a.count += 1
        b = Cls()
        b.count += 1
        b = Cls()
        b.count += 1
        time.sleep(1)
        print(f"{thread_or_process_id}: a.count = {a.count}, id(b) = {id(b)}")

if __name__ == '__main__':
    # 多线程测试
    threading.Thread(target=run, args=("Thread 1",)).start()
    threading.Thread(target=run, args=("Thread 2",)).start()

    # 多进程测试 (注释掉多线程部分后运行)
    # multiprocessing.Process(target=run, args=("Process 1",)).start()
    # multiprocessing.Process(target=run, args=("Process 2",)).start()