参考：https://pdf-lib.org/Home/Details/5262
执行命令：gunicorn -w 4 -b 0.0.0.0:8000 wsgi:app，遇到如下错误：
[2019-01-10 17:17:13 +0800] [31665] [INFO] Starting gunicorn 19.9.0
[2019-01-10 17:17:13 +0800] [31665] [ERROR] Connection in use: ('0.0.0.0', 8000)
[2019-01-10 17:17:13 +0800] [31665] [ERROR] Retrying in 1 second.
[2019-01-10 17:17:14 +0800] [31665] [ERROR] Connection in use: ('0.0.0.0', 8000)
[2019-01-10 17:17:14 +0800] [31665] [ERROR] Retrying in 1 second.
[2019-01-10 17:17:15 +0800] [31665] [ERROR] Connection in use: ('0.0.0.0', 8000)
[2019-01-10 17:17:15 +0800] [31665] [ERROR] Retrying in 1 second.
[2019-01-10 17:17:16 +0800] [31665] [ERROR] Connection in use: ('0.0.0.0', 8000)
[2019-01-10 17:17:16 +0800] [31665] [ERROR] Retrying in 1 second.
[2019-01-10 17:17:17 +0800] [31665] [ERROR] Connection in use: ('0.0.0.0', 8000)
[2019-01-10 17:17:17 +0800] [31665] [ERROR] Retrying in 1 second.
[2019-01-10 17:17:18 +0800] [31665] [ERROR] Can't connect to ('0.0.0.0', 8000)
这是因为之前执行：supervisorctl start wsgi，占用了8000端口。
使用命令：supervisorctl stop wsgi，关闭该进程即可。