Nginx+；php5.2 fpm 502错误

我在服务器中发现了两种502错误：

connection reset by peer, it happens as request_time reach to 10s.
connection timeout, it happens as request_time reach to 3s.

服务器是：

centos , kernel(2.6.32)
php5.2 php-fpm
nginx 1.6.3.

访问日志输出：

[28/Nov/2015:14:41:08 +0800] "GET /ben2.php HTTP/1.1" 502 172 "-" "Apache-HttpClient/4.2.6 (java 1.5)" "-" 3.000

[28/Nov/2015:14:41:11 +0800] "GET /ben2.php HTTP/1.1" 502 172 "-" "Apache-HttpClient/4.2.6 (java 1.5)" "-" 10.000

错误日志：

2015/11/28 14:41:11 [error] 12981#0: *798323 recv() failed (104: Connection reset by peer) while reading response header from upstream, client: xx.xx.xx.xx, server: xx.xx.xx, request: "GET /ben2.php HTTP/1.1", upstream: "fastcgi://127.0.0.1:9000", host: "xx.xx.xx"

2015/11/28 14:41:08 [error] 12981#0: *798215 connect() failed (110: Connection timed out) while connecting to upstream, client: xx.xx.xx.xx, server: xx.xx.xx, request: "GET /ben2.php HTTP/1.1", upstream: "fastcgi://127.0.0.1:9000", host: "xx.xx.xx"

我尝试过“请求\终止\超时、快速CGI\连接\超时、快速CGI\读/写\超时”，但没有任何帮助

下一步我能做什么

编辑： 我编写了一个运行11秒的php文件，没有502个错误（因此我认为nginx和php没有限制）

502错误可能仅在压力测试时发生

在出现502个错误时，php脚本仍能成功运行