1、request_time
官網描述：request processing time in seconds with a milliseconds resolution; time elapsed between the first bytes were read from the client and the log write after the last bytes were sent to the client 。
指的就是從接受用戶請求的第一個字節到發送完響應數據的時間，即包括接收請求數據時間、程序響應時間、輸出
響應數據時間。
2、upstream_response_time
官網描述：keeps times of responses obtained from upstream servers; times are kept in seconds with a milliseconds resolution. Several response times are separated by commas and colons like addresses in the $upstream_addr variable
是指從Nginx向後端（php-cgi)建立連接開始到接受完數據然後關閉連接爲止的時間。
從上面的描述可以看出，$request_time肯定比$upstream_response_time值大，特別是使用POST方式傳遞參數時，因爲Nginx會把request body緩存住，接受完畢後纔會把數據一起發給後端。所以如果用戶網絡較差，或者傳遞數據較大時，$request_time會比$upstream_response_time大很多。
所以如果使用nginx的accesslog查看php程序中哪些接口比較慢的話，記得在log_format中加入$upstream_response_time。
轉自： http://wuzhangshu927.blog.163.com/blog/static/114224687201310674652147/