早在12年左右開始用linux服務器時安裝的就是LNMP環境，當時服務器配置很低，經常碰到502錯誤，此文就是收藏在那個時候。現在使用的是寶塔面板，環境優化很方便，服務器配置也好得多，所以很少再碰到這種問題了。以下為原文，轉自楊俊偉博客：http://yangjunwei.com/a/723.html

最近VPS總是出現 Nginx 502 Bad Gateway 錯誤，導致網頁無法正常訪問，但FTP和SSH正常連接，很是傷腦筋！這次好好整治一下！ 根據問題，應該是 php-fpm 出了問題，先查看日志文件 /usr/local/php/logs/php-fpm.log 日志內容大致如下（借用了一下28、29兩天的記錄）：

Jan 28 22:50:00.309235 [NOTICE] fpm_unix_init_main(), line 284: getrlimit(nofile): max:1024, cur:1024
Jan 28 22:50:00.309552 [NOTICE] fpm_event_init_main(), line 88: libevent: using epoll
Jan 28 22:50:00.309617 [NOTICE] fpm_init(), line 52: fpm is running, pid 7967
Jan 28 22:50:00.310444 [NOTICE] fpm_children_make(), line 352: child 7968 (pool default) started
Jan 28 22:50:00.311328 [NOTICE] fpm_children_make(), line 352: child 7969 (pool default) started
Jan 28 22:50:00.312208 [NOTICE] fpm_children_make(), line 352: child 7970 (pool default) started
Jan 28 22:50:00.313161 [NOTICE] fpm_children_make(), line 352: child 7971 (pool default) started
Jan 28 22:50:00.314210 [NOTICE] fpm_children_make(), line 352: child 7972 (pool default) started
Jan 28 22:50:00.314242 [NOTICE] fpm_event_loop(), line 107: libevent: entering main loop
Jan 29 16:58:30.845059 [NOTICE] fpm_got_signal(), line 70: received SIGUSR2
Jan 29 16:58:30.856418 [NOTICE] fpm_pctl(), line 256: switching to 'reloading' state
Jan 29 16:58:30.856449 [NOTICE] fpm_pctl_kill_all(), line 172: sending signal 3 SIGQUIT to child 7972 (pool default)
Jan 29 16:58:30.856463 [NOTICE] fpm_pctl_kill_all(), line 172: sending signal 3 SIGQUIT to child 7971 (pool default)
Jan 29 16:58:30.856475 [NOTICE] fpm_pctl_kill_all(), line 172: sending signal 3 SIGQUIT to child 7970 (pool default)
Jan 29 16:58:30.856487 [NOTICE] fpm_pctl_kill_all(), line 172: sending signal 3 SIGQUIT to child 7969 (pool default)
Jan 29 16:58:30.856537 [NOTICE] fpm_pctl_kill_all(), line 172: sending signal 3 SIGQUIT to child 7968 (pool default)
Jan 29 16:58:30.856546 [NOTICE] fpm_pctl_kill_all(), line 181: 5 children are still alive
Jan 29 16:58:35.845629 [NOTICE] fpm_pctl_kill_all(), line 172: sending signal 15 SIGTERM to child 7972 (pool default)
Jan 29 16:58:35.845662 [NOTICE] fpm_pctl_kill_all(), line 172: sending signal 15 SIGTERM to child 7971 (pool default)
Jan 29 16:58:35.845671 [NOTICE] fpm_pctl_kill_all(), line 172: sending signal 15 SIGTERM to child 7970 (pool default)
Jan 29 16:58:35.845678 [NOTICE] fpm_pctl_kill_all(), line 172: sending signal 15 SIGTERM to child 7969 (pool default)
Jan 29 16:58:35.845686 [NOTICE] fpm_pctl_kill_all(), line 172: sending signal 15 SIGTERM to child 7968 (pool default)
Jan 29 16:58:35.845691 [NOTICE] fpm_pctl_kill_all(), line 181: 5 children are still alive
Jan 29 16:58:36.450929 [NOTICE] fpm_got_signal(), line 48: received SIGCHLD
Jan 29 16:58:36.451016 [WARNING] fpm_children_bury(), line 215: child 7968 (pool default) exited on signal 15 SIGTERM after 65319.297391 seconds from start
Jan 29 16:58:36.451062 [WARNING] fpm_children_bury(), line 215: child 7971 (pool default) exited on signal 15 SIGTERM after 65319.294705 seconds from start
Jan 29 16:58:36.451088 [WARNING] fpm_children_bury(), line 215: child 7972 (pool default) exited on signal 15 SIGTERM after 65319.293684 seconds from start
Jan 29 16:58:36.451103 [NOTICE] fpm_got_signal(), line 48: received SIGCHLD
Jan 29 16:58:36.451834 [NOTICE] fpm_got_signal(), line 48: received SIGCHLD
Jan 29 16:58:36.451868 [WARNING] fpm_children_bury(), line 215: child 7969 (pool default) exited on signal 15 SIGTERM after 65319.297343 seconds from start
Jan 29 16:58:36.451891 [WARNING] fpm_children_bury(), line 215: child 7970 (pool default) exited on signal 15 SIGTERM after 65319.296487 seconds from start
Jan 29 16:58:36.451903 [NOTICE] fpm_pctl_exec(), line 95: reloading: execvp("/usr/local/php/bin/php-cgi", {"/usr/local/php/bin/php-cgi", "--fpm", "--fpm-config", "/usr/local/php/etc/php-fpm.conf"})

滿眼的NOTICE錯誤，據觀察至后幾天，錯誤日志都是如此！據網絡資料分析說，這類錯誤大都是由于php線程打開文件句柄受限導致的錯誤，這里綜合各位童鞋的分析，整理記錄如下，希望能解決此類 502 問題！

首先檢查一下ulimit -n的值，SSH輸入命令：

# ulimit -n
返回：65535

1、提升服務器的文件句柄打開

SSH命令：# vi /etc/security/limits.conf，在結尾處添加以下內容：

* soft nofile 65535
* hard nofile 65535

2、提升nginx的進程文件打開數

# vi /usr/local/nginx/conf/nginx.conf
查看 worker_rlimit_nofile 51200;

3、修改 php-fpm.conf 配置文件

前面確認了 ulimit -n 值為 65535，/usr/local/php/etc/php-fpm.conf 中的選項 rlimit_files 確保和此數值一致。

<value name="rlimit_files">65535</value>
<value name="max_requests">10240</value>

4、修改 sysctl.conf

# vi /etc/sysctl.conf

底部添加

fs.file-max=65535

至此，重啟 /root/lnmp restart 生效，看看還有沒有類似錯誤信息出現！

ps.為減小php-fpm.log文件大小，可將 /usr/local/php/etc/php-fpm.conf?中的 Log level 由 notice?修改為 ERROR，這樣能降低日志的生成速度！

Log level
??? <value name="log_level">Error</value>

2012.02.20更新：貌似 502 Bad Gateway 已經消失了！！