https://blog.wongcw.com/2020/01/01/%e5%91%8a%e5%88%a532%e4%bd%8d%e7%89%88%e6%9c%ac%ef%bc%9a%e5%9f%ba%e6%96%bcgentoo%e7%9a%84calculate-linux-20%e7%8f%be%e5%b7%b2%e9%96%8b%e6%94%be%e4%b8%8b%e8%bc%89/