Shared variable in python's multiprocessing
https://www.programcreek.com/python/example/58176/multiprocessing.Value
https://docs.python.org/zh-cn/3.7/library/multiprocessing.html#multiprocessing-programming
在 Unix 上，如果一个进程执行完成但是没有被 join，就会变成僵尸进程。
https://www.geeksforgeeks.org/multiprocessing-python-set-1/
Once the processes start, the current program also keeps on executing. In order to stop execution of current program until a process is complete, we use join method.
p1.join() p2.join()
As a result, the current program will first wait for the completion of p1 and then p2. Once, they are completed, the next statements of current program are executed.