如果偶尔出现502，回源时间很小，需要检查源服务器nginx的keepalive_timeout是否大于50s 

#user nobody; worker_processes 4; #error_log logs/error.log; #error_log logs/error.log notice; #error_log logs/error.log info; #pid logs/nginx.pid; events { worker_connections 1024...