一次打开网站，发现502，第一反应肯定是php-fpm没启动,尝试启动还是502.
1.首先查询Nginx日志发现如下连接PHP失败：
2016/07/29 15:56:04 [error] 23760#0: *186 connect() failed (111: Connection refused) while connecting to upstream, client:
11.10.8.252.18, server: xuliangwei.com request: "GET /index.php?route=rest/crontab HTTP/1.1", upstream: "fastcgi://127.0.0.1:9000", host: "xuliangwei.com"
2.通过dmesg 查询至如下:
[6819070.815161]init: php5-fpm main process ended, respawning
[6819070.861943]init: php5-fpm main process (25401) terminated with status 78
[6819070.861955]init: php5-fpm main process ended, respawning
[6819070.910152]init: php5-fpm main process (25409) terminated with status 78
3.通过php-fpm检测配置文件
root@trttest:/etc/php5/fpm/pool.d# /usr/sbin/php5-fpm --fpm-config /etc/php5/fpm/php-fpm.conf
[29-Jul-2016 15:57:14] ERROR: [/etc/php5/fpm/php-fpm.conf:27] unknown entry 'catch_workers_output'
[29-Jul-2016 15:57:14] ERROR: failed to load configuration file '/etc/php5/fpm/php-fpm.conf'
[29-Jul-2016 15:57:14] ERROR: FPM initialization failed
4.注释catch_workers_output
root@trttest:/etc/php5/fpm/pool.d# vim /etc/php5/fpm/php-fpm.conf +27
root@trttest:/etc/php5/fpm/pool.d# service php5-fpm start
php5-fpm start/running, process 25545
5.启动成功后，网站已正常访问
root@trttest:/etc/php5/fpm/pool.d# ps aux|grep php
root 25545 0.6 0.1 342748 18812 ? Ss 15:58 0:00 php-fpm: master process (/etc/php5/fpm/php-fpm.conf)
ubuntu 25548 0.0 0.0 342748 5644 ? S 15:58 0:00 php-fpm: pool www
ubuntu 25549 0.0 0.0 342748 5644 ? S 15:58 0:00 php-fpm: pool www
ubuntu 25550 0.0 0.0 342748 5644 ? S 15:58 0:00 php-fpm: pool www
ubuntu 25551 0.0 0.0 342748 5644 ? S 15:58 0:00 php-fpm: pool www
ubuntu 25552 0.0 0.0 342748 5644 ? S 15:58 0:00 php-fpm: pool www
6.谷歌给出的解决方案有以下2种：
1.php-fpm没有运行
执行如下命令查看是否启动了php-fpm，如果没有则启动你的php-fpm即可
netstat -ant | grep 9000
2.php-fpm队列满了
php-fpm.conf配置文件pm.max_children修改大一点,重启php-fpm并观察日志情况