Nginx 的缓存模块 srcache
我们知道,Nginx的核心设计思想是事件驱动的非阻塞I/O。Nginx被设计为可以配置I/O多路复用策略,在Unix系统中传统的多路复用是采用select或poll,但是这两个方法的问题是随着监听socket的增加,性能会下降,因为在linux内核中是采用轮询的方式判断是否可以触
我们知道,Nginx的核心设计思想是事件驱动的非阻塞I/O。Nginx被设计为可以配置I/O多路复用策略,在Unix系统中传统的多路复用是采用select或poll,但是这两个方法的问题是随着监听socket的增加,性能会下降,因为在linux内核中是采用轮询的方式判断是否可以触发事件,换句话说算法的复杂度为O(N),而在较新的linux内核中引入了复杂度为O(1)的epoll,因此Nginx在Linux下默认采用epoll,而在FreeBSD下默认采用kqueue作为I/O策略。
即便是这样,传统的缓存策略仍可能造成效率低下,因为传统上是通过PHP操作memcache的,要执行PHP代码,Nginx就必然要和FastCGI通信,同时也要进入PHP的生命周期,因此SAPI、PHP Core和Zend Engine的一系列逻辑会被执行。更糟糕的是,fpm和PHP可能会阻塞,因此破坏了Nginx的非阻塞性。(原文中此处表述有误,fastcgi与nginx进行同步通信,但并不会破坏nginx i/o的非阻塞性,多谢agentzh给予指正)下图展示了在memcache命中时整个处理过程。
可以看到,即使memcache命中,还是要进入PHP的生命周期。我们知道,目前很多互联网应用都使用RESTful规范进行设计,在RESTful应用下,普遍使用uri和查询参数作为缓存的key,因此一种更高效的缓存策略是Nginx直接访问memcache,并用$uri和$args等Nginx内置变量设定缓存key规则,这样,当缓存命中时,Nginx可以跳过通过fastcgi和PHP通信的过程,直接从memcache中获取数据并返回。memc-nginx和srcache-nginx正是利用这种策略提高了缓存的效率。下图是这种高效缓存策略的示意图(当memcache命中时)。
模块介绍
memc-nginx和srcache-nginx模块均为前淘宝工程师agentzh(章亦春)开发。其中memc模块扩展了Nginx标准的memcache模块,增加了set、add、delete等memcache命令,而srcache则是为location增加了透明的基于subrequest的缓存层。两者配合使用,可以实现上一节提到的高效缓存机制。关于两个模块的详细信息可以参考它们Nginx官网的wiki(memc wiki,srcache wiki)页。
来源:http://www.oschina.net/p/srcache



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