HTTP keep-alive

今天在阅读Django performance tips时发现关闭keepalive可以提高效率，原文如下：

Turn off KeepAlive I don’t totally understand how KeepAlive works, but turning it off on our Django servers increased performance by something like 50%. Of course, don’t do this if the same server is also serving media… but you’re not doing that, right?

HTTP keep-alive即是HTTP persistent connection，维基百科有详细说明：http://en.wikipedia.org/wiki/HTTP_persistent_connection

如果使用keep-alive功能，多个http请求会使用同一个tcp连接，这样可节省多次建立连接的时间及资源消耗，

下图显示了是否使用keep-alive功能的tcp连接对比图：

http 1.0默认不开启，可进行指定，http 1.1默认开启。

上面我们看到启用keep-alive可节省多次建立连接的时间，但是服务器端要保持连接状态(nginx默认保持74s)，这样会影响服务器端的性能。而且目前网络带宽比较大，建立连接的延时比较少，所以keep-alive在很多情况下会导致性能下降。这就验证了一开始那篇文章指出的在django servers中关闭keep-alive 功能可大约提升50%的性能。

所以建议关系keep-alive功能，除非有特殊需求。

下面介绍如何nginx中关系keep-alive功能

nginx文档中有详细说明:

keepalive_timeout

Syntax: keepalive_timeout timeout [ header_timeout ]

Default: 75s

Context: http server location

Reference: keepalive_timeout

The first parameter assigns the timeout for keep-alive connections with the client. The server will close connections after this time.

The optional second parameter assigns the time value in the header Keep-Alive: timeout=time of the response. This header can convince some browsers to close the connection, so that the server does not have to. Without this parameter, nginx does not send a Keep-Alive header (though this is not what makes a connection "keep-alive").

如果我们要关闭keep-alive功能，可在http, server或location中设置keepalive_timeout 0;

如：

server {
  listen          port;
  server_name     address;
  access_log      /var/log/nginx/mysite-access.log ;
  error_log       /var/log/nginx/mysite-error.log ;
  keepalive_timeout  0;
  location / {
    ........
  }
}