我读过的对epoll最好的讲解

深碍√TFBOYSˉ_ 2022-07-16 13:10 161阅读 0赞

问：epoll 或者 kqueue 的原理是什么？为什么 [epoll][] 和 kqueue 可以用基于事件的方式，单线程的实现并发？我没看过 linux 内核，对这方面一直有疑问……

必须从很多基础的概念开始构建这个答案，并且可能引申到很多别的问题。

首先我们来定义流的概念，一个流可以是文件，socket，pipe 等等可以进行I/O操作的内核对象。不管是文件，还是套接字，还是管道，我们都可以把他们看作流。

之后我们来讨论I/O的操作，通过read，我们可以从流中读入数据；通过write，我们可以往流写入数据。现在假定一个情形，我们需要从流中读数据，但是流中还没有数据，（典型的例子为，客户端要从 socket 读入数据，但是服务器还没有把数据传回来），这时候该怎么办？

*  阻塞。阻塞是个什么概念呢？比如某个时候你在等快递，但是你不知道快递什么时候过来，而且你没有别的事可以干（或者说接下来的事要等快递来了才能做）；那么你可以去睡觉了，因为你知道快递把货送来时一定会给你打个电话（假定一定能叫醒你）。
 *  非阻塞忙轮询。接着上面等快递的例子，如果用忙轮询的方法，那么你需要知道快递员的手机号，然后每分钟给他挂个电话：“你到了没？”

很明显一般人不会用第二种做法，不仅显很无脑，浪费话费不说，还占用了快递员大量的时间。大部分程序也不会用第二种做法，因为第一种方法经济而简单，经济是指消耗很少的CPU时间，如果线程睡眠了，就掉出了系统的调度队列，暂时不会去瓜分CPU宝贵的时间片了。

为了解释阻塞是如何进行的，我们来讨论缓冲区，以及内核缓冲区，最终把I/O事件解释清楚。缓冲区的引入是为了减少频繁I/O操作而引起频繁的系统调用（你知道它很慢的），当你操作一个流时，更多的是以缓冲区为单位进行操作，这是相对于用户空间而言。对于内核来说，也需要缓冲区。

假设有一个管道，进程A为管道的写入方，B为管道的读出方。

1.  假设一开始内核缓冲区是空的，B作为读出方，被[阻塞][Link 1]着。然后首先A往管道写入，这时候内核缓冲区由空的状态变到非空状态，内核就会产生一个事件告诉B该醒来了，这个事件姑且称之为“缓冲区非空”。
2.  但是“缓冲区非空”事件通知B后，B却还没有读出数据；且内核许诺了不能把写入管道中的数据丢掉这个时候，A写入的数据会滞留在内核缓冲区中，如果内核也缓冲区满了，B仍未开始读数据，最终内核缓冲区会被填满，这个时候会产生一个I/O事件，告诉进程A，你该等等（阻塞）了，我们把这个事件定义为“缓冲区满”。
3.  假设后来B终于开始读数据了，于是内核的缓冲区空了出来，这时候内核会告诉A，内核缓冲区有空位了，你可以从长眠中醒来了，继续写数据了，我们把这个事件叫做“缓冲区非满”
4.  也许事件Y1已经通知了A，但是A也没有数据写入了，而B继续读出数据，知道内核缓冲区空了。这个时候内核就告诉B，你需要阻塞了！，我们把这个时间定为“缓冲区空”。

这四个情形涵盖了四个I/O事件，缓冲区满，缓冲区空，缓冲区非空，缓冲区非满（注都是说的内核缓冲区，且这四个术语都是我生造的，仅为解释其原理而造）。这四个I/O事件是进行阻塞同步的根本。（如果不能理解“同步”是什么概念，请学习操作系统的锁，信号量，条件变量等任务同步方面的相关知识）。

然后我们来说说阻塞I/O的缺点。但是阻塞I/O模式下，一个线程只能处理一个流的I/O事件。如果想要同时处理多个流，要么多进程(fork)，要么多线程(pthread\_create)，很不幸这两种方法效率都不高。

于是再来考虑非阻塞忙轮询的I/O方式，我们发现我们可以同时处理多个流了（把一个流从阻塞模式切换到非阻塞模式再此不予讨论）：

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我们只要不停的把所有流从头到尾问一遍，又从头开始。这样就可以处理多个流了，但这样的做法显然不好，因为如果所有的流都没有数据，那么只会白白浪费CPU。这里要补充一点，阻塞模式下，内核对于I/O事件的处理是阻塞或者唤醒，而非阻塞模式下则把I/O事件交给其他对象（后文介绍的 select 以及 epoll）处理甚至直接忽略。

为了避免CPU空转，可以引进了一个代理（一开始有一位叫做select的代理，后来又有一位叫做poll的代理，不过两者的本质是一样的）。这个代理比较厉害，可以同时观察许多流的I/O事件，在空闲的时候，会把当前线程阻塞掉，当有一个或多个流有I/O事件时，就从阻塞态中醒来，于是我们的程序就会轮询一遍所有的流（于是我们可以把“忙”字去掉了）。代码长这样：

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  <tr style="margin:0px!important; padding:0px!important; border:0px!important; outline:0px!important; vertical-align:baseline!important; float:none!important; position:static!important; left:auto!important; top:auto!important; right:auto!important; bottom:auto!important; height:auto!important; width:auto!important; line-height:1.1em!important; min-height:auto!important; background:none!important"> 
   <td style="margin:0px!important; padding:0px!important; border:0px!important; outline:0px!important; vertical-align:top!important; float:none!important; position:static!important; left:auto!important; top:auto!important; right:auto!important; bottom:auto!important; height:auto!important; width:3em!important; line-height:1.1em!important; font-family:'Courier New',Courier,monospace!important; min-height:auto!important; color:rgb(175,175,175)!important; background:none!important"> <code style="letter-spacing:0px; margin:0px!important; padding:0px 0.3em 0px 0px!important; border:0px!important; outline:0px!important; vertical-align:baseline!important; font-family:'Courier New',Courier,monospace!important; text-align:right!important; float:none!important; position:static!important; left:auto!important; top:auto!important; right:auto!important; bottom:auto!important; height:auto!important; width:2.7em!important; line-height:1.1em!important; min-height:auto!important; display:block!important; background:none!important">3</code></td> 
   <td style="margin:0px!important; padding:0px 0px 0px 0.5em!important; border-width:0px 0px 0px 3px!important; border-left-style:solid!important; border-left-color:rgb(108,226,108)!important; outline:0px!important; vertical-align:top!important; float:none!important; position:static!important; left:auto!important; top:auto!important; right:auto!important; bottom:auto!important; height:auto!important; width:auto!important; line-height:1.1em!important; font-family:'Courier New',Courier,monospace!important; min-height:auto!important; background:none!important"> <code style="letter-spacing:0px; margin:0px!important; padding:0px!important; border:0px!important; outline:0px!important; vertical-align:baseline!important; font-family:'Courier New',Courier,monospace!important; float:none!important; position:static!important; left:auto!important; top:auto!important; right:auto!important; bottom:auto!important; height:auto!important; width:auto!important; line-height:1.1em!important; min-height:auto!important; background:none!important">&nbsp;&nbsp;&nbsp;&nbsp;</code><code style="letter-spacing:0px; margin:0px!important; padding:0px!important; border:0px!important; outline:0px!important; vertical-align:baseline!important; font-family:'Courier New',Courier,monospace!important; float:none!important; position:static!important; left:auto!important; top:auto!important; right:auto!important; bottom:auto!important; height:auto!important; width:auto!important; line-height:1.1em!important; font-weight:bold!important; min-height:auto!important; color:rgb(0,102,153)!important; background:none!important">for</code>&nbsp;<code style="letter-spacing:0px; margin:0px!important; padding:0px!important; border:0px!important; outline:0px!important; vertical-align:baseline!important; font-family:'Courier New',Courier,monospace!important; float:none!important; position:static!important; left:auto!important; top:auto!important; right:auto!important; bottom:auto!important; height:auto!important; width:auto!important; line-height:1.1em!important; min-height:auto!important; background:none!important">i in streams[] { </code></td> 
  </tr> 
 </tbody> 
</table>

<table style="border-spacing:0px; margin:0px!important; padding:0px!important; border:0px!important; outline:0px!important; font-size:12px!important; vertical-align:baseline!important; border-collapse:collapse!important; width:auto!important; float:none!important; position:static!important; left:auto!important; top:auto!important; right:auto!important; bottom:auto!important; height:auto!important; line-height:1.1em!important; min-height:auto!important; background:none!important"> 
 <tbody style="margin:0px!important; padding:0px!important; border:0px!important; outline:0px!important; vertical-align:baseline!important; float:none!important; position:static!important; left:auto!important; top:auto!important; right:auto!important; bottom:auto!important; height:auto!important; width:auto!important; line-height:1.1em!important; min-height:auto!important; background:none!important"> 
  <tr style="margin:0px!important; padding:0px!important; border:0px!important; outline:0px!important; vertical-align:baseline!important; float:none!important; position:static!important; left:auto!important; top:auto!important; right:auto!important; bottom:auto!important; height:auto!important; width:auto!important; line-height:1.1em!important; min-height:auto!important; background:none!important"> 
   <td style="margin:0px!important; padding:0px!important; border:0px!important; outline:0px!important; vertical-align:top!important; float:none!important; position:static!important; left:auto!important; top:auto!important; right:auto!important; bottom:auto!important; height:auto!important; width:3em!important; line-height:1.1em!important; font-family:'Courier New',Courier,monospace!important; min-height:auto!important; color:rgb(175,175,175)!important; background:none!important"> <code style="letter-spacing:0px; margin:0px!important; padding:0px 0.3em 0px 0px!important; border:0px!important; outline:0px!important; vertical-align:baseline!important; font-family:'Courier New',Courier,monospace!important; text-align:right!important; float:none!important; position:static!important; left:auto!important; top:auto!important; right:auto!important; bottom:auto!important; height:auto!important; width:2.7em!important; line-height:1.1em!important; min-height:auto!important; display:block!important; background:none!important">5</code></td> 
   <td style="margin:0px!important; padding:0px 0px 0px 0.5em!important; border-width:0px 0px 0px 3px!important; border-left-style:solid!important; border-left-color:rgb(108,226,108)!important; outline:0px!important; vertical-align:top!important; float:none!important; position:static!important; left:auto!important; top:auto!important; right:auto!important; bottom:auto!important; height:auto!important; width:auto!important; line-height:1.1em!important; font-family:'Courier New',Courier,monospace!important; min-height:auto!important; background:none!important"> <code style="letter-spacing:0px; margin:0px!important; padding:0px!important; border:0px!important; outline:0px!important; vertical-align:baseline!important; font-family:'Courier New',Courier,monospace!important; float:none!important; position:static!important; left:auto!important; top:auto!important; right:auto!important; bottom:auto!important; height:auto!important; width:auto!important; line-height:1.1em!important; min-height:auto!important; background:none!important">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code><code style="letter-spacing:0px; margin:0px!important; padding:0px!important; border:0px!important; outline:0px!important; vertical-align:baseline!important; font-family:'Courier New',Courier,monospace!important; float:none!important; position:static!important; left:auto!important; top:auto!important; right:auto!important; bottom:auto!important; height:auto!important; width:auto!important; line-height:1.1em!important; min-height:auto!important; background:none!important">read until unavailable</code></td> 
  </tr> 
 </tbody> 
</table>

于是，如果没有I/O事件产生，我们的程序就会阻塞在select处。但是依然有个问题，我们从select那里仅仅知道了，有I/O事件发生了，但却并不知道是那几个流（可能有一个，多个，甚至全部），我们只能无差别轮询所有流，找出能读出数据，或者写入数据的流，对他们进行操作。

但是使用select，我们有O(n)的无差别轮询复杂度，同时处理的流越多，每一次无差别轮询时间就越长。

说了这么多，终于能好好解释epoll了。

epoll 可以理解为event poll，不同于忙轮询和无差别轮询，epoll之会把哪个流发生了怎样的I/O事件通知我们。此时我们对这些流的操作都是有意义的。（复杂度降低到了O(k)，k为产生I/O事件的流的个数，也有认为O(1)的\[原文为O(1)，但实际上O(k)更为准确\]）

在讨论epoll的实现细节之前，先把epoll的相关操作列出\[原文所列第二点说法让人产生EPOLLIN/EPOLLOUT等同于“缓冲区非空”和“缓冲区非满”的事件，但并非如此，详细可以Google关于epoll的边缘触发和水平触发。\]：

*  epoll\_create 创建一个epoll对象，一般epollfd = epoll\_create()
 *  epoll\_ctl （epoll\_add/epoll\_del的合体），往epoll对象中增加/删除某一个流的某一个事件。比如 epoll\_ctl(epollfd, EPOLL\_CTL\_ADD, socket, EPOLLIN);//有缓冲区内有数据时epoll\_wait返回，epoll\_ctl(epollfd, EPOLL\_CTL\_DEL, socket, EPOLLOUT);//缓冲区可写入时epoll\_wait返回
 *  epoll\_wait(epollfd,...)等待直到注册的事件发生

注：当对一个非阻塞流的读写发生缓冲区满或缓冲区空，write/read会返回-1，并设置errno=EAGAIN。而epoll只关心缓冲区非满和缓冲区非空事件）

一个epoll模式的代码大概的样子是：

<table style="border-spacing:0px; margin:0px!important; padding:0px!important; border:0px!important; outline:0px!important; font-size:12px!important; vertical-align:baseline!important; border-collapse:collapse!important; width:auto!important; float:none!important; position:static!important; left:auto!important; top:auto!important; right:auto!important; bottom:auto!important; height:auto!important; line-height:1.1em!important; min-height:auto!important; background:none!important"> 
 <tbody style="margin:0px!important; padding:0px!important; border:0px!important; outline:0px!important; vertical-align:baseline!important; float:none!important; position:static!important; left:auto!important; top:auto!important; right:auto!important; bottom:auto!important; height:auto!important; width:auto!important; line-height:1.1em!important; min-height:auto!important; background:none!important"> 
  <tr style="margin:0px!important; padding:0px!important; border:0px!important; outline:0px!important; vertical-align:baseline!important; float:none!important; position:static!important; left:auto!important; top:auto!important; right:auto!important; bottom:auto!important; height:auto!important; width:auto!important; line-height:1.1em!important; min-height:auto!important; background:none!important"> 
   <td style="margin:0px!important; padding:0px!important; border:0px!important; outline:0px!important; vertical-align:top!important; float:none!important; position:static!important; left:auto!important; top:auto!important; right:auto!important; bottom:auto!important; height:auto!important; width:3em!important; line-height:1.1em!important; font-family:'Courier New',Courier,monospace!important; min-height:auto!important; color:rgb(175,175,175)!important; background:none!important"> <code style="letter-spacing:0px; margin:0px!important; padding:0px 0.3em 0px 0px!important; border:0px!important; outline:0px!important; vertical-align:baseline!important; font-family:'Courier New',Courier,monospace!important; text-align:right!important; float:none!important; position:static!important; left:auto!important; top:auto!important; right:auto!important; bottom:auto!important; height:auto!important; width:2.7em!important; line-height:1.1em!important; min-height:auto!important; display:block!important; background:none!important">2</code></td> 
   <td style="margin:0px!important; padding:0px 0px 0px 0.5em!important; border-width:0px 0px 0px 3px!important; border-left-style:solid!important; border-left-color:rgb(108,226,108)!important; outline:0px!important; vertical-align:top!important; float:none!important; position:static!important; left:auto!important; top:auto!important; right:auto!important; bottom:auto!important; height:auto!important; width:auto!important; line-height:1.1em!important; font-family:'Courier New',Courier,monospace!important; min-height:auto!important; background:none!important"> <code style="letter-spacing:0px; margin:0px!important; padding:0px!important; border:0px!important; outline:0px!important; vertical-align:baseline!important; font-family:'Courier New',Courier,monospace!important; float:none!important; position:static!important; left:auto!important; top:auto!important; right:auto!important; bottom:auto!important; height:auto!important; width:auto!important; line-height:1.1em!important; min-height:auto!important; background:none!important">&nbsp;&nbsp;&nbsp;&nbsp;</code><code style="letter-spacing:0px; margin:0px!important; padding:0px!important; border:0px!important; outline:0px!important; vertical-align:baseline!important; font-family:'Courier New',Courier,monospace!important; float:none!important; position:static!important; left:auto!important; top:auto!important; right:auto!important; bottom:auto!important; height:auto!important; width:auto!important; line-height:1.1em!important; min-height:auto!important; background:none!important">active_stream[] = epoll_wait(epollfd)</code></td> 
  </tr> 
 </tbody> 
</table>

<table style="border-spacing:0px; margin:0px!important; padding:0px!important; border:0px!important; outline:0px!important; font-size:12px!important; vertical-align:baseline!important; border-collapse:collapse!important; width:auto!important; float:none!important; position:static!important; left:auto!important; top:auto!important; right:auto!important; bottom:auto!important; height:auto!important; line-height:1.1em!important; min-height:auto!important; background:none!important"> 
 <tbody style="margin:0px!important; padding:0px!important; border:0px!important; outline:0px!important; vertical-align:baseline!important; float:none!important; position:static!important; left:auto!important; top:auto!important; right:auto!important; bottom:auto!important; height:auto!important; width:auto!important; line-height:1.1em!important; min-height:auto!important; background:none!important"> 
  <tr style="margin:0px!important; padding:0px!important; border:0px!important; outline:0px!important; vertical-align:baseline!important; float:none!important; position:static!important; left:auto!important; top:auto!important; right:auto!important; bottom:auto!important; height:auto!important; width:auto!important; line-height:1.1em!important; min-height:auto!important; background:none!important"> 
   <td style="margin:0px!important; padding:0px!important; border:0px!important; outline:0px!important; vertical-align:top!important; float:none!important; position:static!important; left:auto!important; top:auto!important; right:auto!important; bottom:auto!important; height:auto!important; width:3em!important; line-height:1.1em!important; font-family:'Courier New',Courier,monospace!important; min-height:auto!important; color:rgb(175,175,175)!important; background:none!important"> <code style="letter-spacing:0px; margin:0px!important; padding:0px 0.3em 0px 0px!important; border:0px!important; outline:0px!important; vertical-align:baseline!important; font-family:'Courier New',Courier,monospace!important; text-align:right!important; float:none!important; position:static!important; left:auto!important; top:auto!important; right:auto!important; bottom:auto!important; height:auto!important; width:2.7em!important; line-height:1.1em!important; min-height:auto!important; display:block!important; background:none!important">3</code></td> 
   <td style="margin:0px!important; padding:0px 0px 0px 0.5em!important; border-width:0px 0px 0px 3px!important; border-left-style:solid!important; border-left-color:rgb(108,226,108)!important; outline:0px!important; vertical-align:top!important; float:none!important; position:static!important; left:auto!important; top:auto!important; right:auto!important; bottom:auto!important; height:auto!important; width:auto!important; line-height:1.1em!important; font-family:'Courier New',Courier,monospace!important; min-height:auto!important; background:none!important"> <code style="letter-spacing:0px; margin:0px!important; padding:0px!important; border:0px!important; outline:0px!important; vertical-align:baseline!important; font-family:'Courier New',Courier,monospace!important; float:none!important; position:static!important; left:auto!important; top:auto!important; right:auto!important; bottom:auto!important; height:auto!important; width:auto!important; line-height:1.1em!important; min-height:auto!important; background:none!important">&nbsp;&nbsp;&nbsp;&nbsp;</code><code style="letter-spacing:0px; margin:0px!important; padding:0px!important; border:0px!important; outline:0px!important; vertical-align:baseline!important; font-family:'Courier New',Courier,monospace!important; float:none!important; position:static!important; left:auto!important; top:auto!important; right:auto!important; bottom:auto!important; height:auto!important; width:auto!important; line-height:1.1em!important; font-weight:bold!important; min-height:auto!important; color:rgb(0,102,153)!important; background:none!important">for</code>&nbsp;<code style="letter-spacing:0px; margin:0px!important; padding:0px!important; border:0px!important; outline:0px!important; vertical-align:baseline!important; font-family:'Courier New',Courier,monospace!important; float:none!important; position:static!important; left:auto!important; top:auto!important; right:auto!important; bottom:auto!important; height:auto!important; width:auto!important; line-height:1.1em!important; min-height:auto!important; background:none!important">i in active_stream[] { </code></td> 
  </tr> 
 </tbody> 
</table>

<table style="border-spacing:0px; margin:0px!important; padding:0px!important; border:0px!important; outline:0px!important; font-size:12px!important; vertical-align:baseline!important; border-collapse:collapse!important; width:auto!important; float:none!important; position:static!important; left:auto!important; top:auto!important; right:auto!important; bottom:auto!important; height:auto!important; line-height:1.1em!important; min-height:auto!important; background:none!important"> 
 <tbody style="margin:0px!important; padding:0px!important; border:0px!important; outline:0px!important; vertical-align:baseline!important; float:none!important; position:static!important; left:auto!important; top:auto!important; right:auto!important; bottom:auto!important; height:auto!important; width:auto!important; line-height:1.1em!important; min-height:auto!important; background:none!important"> 
  <tr style="margin:0px!important; padding:0px!important; border:0px!important; outline:0px!important; vertical-align:baseline!important; float:none!important; position:static!important; left:auto!important; top:auto!important; right:auto!important; bottom:auto!important; height:auto!important; width:auto!important; line-height:1.1em!important; min-height:auto!important; background:none!important"> 
   <td style="margin:0px!important; padding:0px!important; border:0px!important; outline:0px!important; vertical-align:top!important; float:none!important; position:static!important; left:auto!important; top:auto!important; right:auto!important; bottom:auto!important; height:auto!important; width:3em!important; line-height:1.1em!important; font-family:'Courier New',Courier,monospace!important; min-height:auto!important; color:rgb(175,175,175)!important; background:none!important"> <code style="letter-spacing:0px; margin:0px!important; padding:0px 0.3em 0px 0px!important; border:0px!important; outline:0px!important; vertical-align:baseline!important; font-family:'Courier New',Courier,monospace!important; text-align:right!important; float:none!important; position:static!important; left:auto!important; top:auto!important; right:auto!important; bottom:auto!important; height:auto!important; width:2.7em!important; line-height:1.1em!important; min-height:auto!important; display:block!important; background:none!important">4</code></td> 
   <td style="margin:0px!important; padding:0px 0px 0px 0.5em!important; border-width:0px 0px 0px 3px!important; border-left-style:solid!important; border-left-color:rgb(108,226,108)!important; outline:0px!important; vertical-align:top!important; float:none!important; position:static!important; left:auto!important; top:auto!important; right:auto!important; bottom:auto!important; height:auto!important; width:auto!important; line-height:1.1em!important; font-family:'Courier New',Courier,monospace!important; min-height:auto!important; background:none!important"> <code style="letter-spacing:0px; margin:0px!important; padding:0px!important; border:0px!important; outline:0px!important; vertical-align:baseline!important; font-family:'Courier New',Courier,monospace!important; float:none!important; position:static!important; left:auto!important; top:auto!important; right:auto!important; bottom:auto!important; height:auto!important; width:auto!important; line-height:1.1em!important; min-height:auto!important; background:none!important">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code><code style="letter-spacing:0px; margin:0px!important; padding:0px!important; border:0px!important; outline:0px!important; vertical-align:baseline!important; font-family:'Courier New',Courier,monospace!important; float:none!important; position:static!important; left:auto!important; top:auto!important; right:auto!important; bottom:auto!important; height:auto!important; width:auto!important; line-height:1.1em!important; min-height:auto!important; background:none!important">read or write till unavailable</code></td> 
  </tr> 
 </tbody> 
</table>

限于篇幅，我只说这么多，以揭示原理性的东西，至于epoll的使用细节，请参考man和google，实现细节，请参阅linux kernel source。

[epoll]: http://www.nowamagic.net/academy/tag/epoll
[Link 1]: http://www.nowamagic.net/academy/tag/%E9%98%BB%E5%A1%9E