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<h1 class="kratos-entry-title text-center">IO多路复用之select,poll,epoll总结</h1>
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<span>
<i class="fa fa-calendar"></i> 2019-04-13
<i class="fa fa-folder"></i> 分类 <a class="label-link" href="/categories/%E7%BC%96%E7%A8%8B%E5%9F%BA%E7%A1%80/">编程基础</a>, <a class="label-link" href="/categories/%E7%BC%96%E7%A8%8B%E5%9F%BA%E7%A1%80/%E7%BD%91%E7%BB%9C%E7%BC%96%E7%A8%8B/">网络编程</a>
<i class="fa fa-user"></i> 作者 云海鹰影
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本文最后编辑于 <time datetime="1615457057383"></time> 前，其中的内容可能需要更新。
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<p>前篇: <a href="/20190408-1.html">网络编程之IO模型和IO多路复用</a></p>
<p>select, poll, epoll 在历史上是先后按顺序出现的，后者的提出都是为了解决前者遗留的问题。select是POSIX早期提出的规范，windows和linux等各大标准库都有实现。poll和epoll都是linux上的实现，其中poll和select差别很小, 在高并发和性能上有很大局限，而epoll则解决了大部分问题。</p>
<h2 id="select"><a href="#select" class="headerlink" title="select"></a>select</h2><p>select原型函数:</p>
<figure class="highlight cpp" data-lang="cpp"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">/* </span></span><br><span class="line"><span class="comment"> * maxfdp1: 最大的fd加1 (fd是一个整形)</span></span><br><span class="line"><span class="comment"> * fd_set: 文件描述符fd的集合，通过FD_ZERO，FD_SET，FD_CLR，FD_ISSET等宏来控制</span></span><br><span class="line"><span class="comment"> * return: int 若有就绪描述符返回其数目，若超时则为0，若出错则为-1</span></span><br><span class="line"><span class="comment">*/</span></span><br><span class="line"><span class="function"><span class="type">int</span> <span class="title">select</span> <span class="params">(<span class="type">int</span> maxfdp1, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, <span class="keyword">struct</span> timeval *timeout)</span></span>;</span><br><span class="line"></span><br><span class="line"><span class="comment">/////////////////////////////////////////////////</span></span><br><span class="line"><span class="comment">//相关接口:</span></span><br><span class="line"><span class="function"><span class="type">void</span> <span class="title">FD_ZERO</span> <span class="params">(fd_set *fdset)</span></span>; <span class="comment">// clear all bits in fdset</span></span><br><span class="line"><span class="function"><span class="type">void</span> <span class="title">FD_SET</span> <span class="params">(<span class="type">int</span> fd,fd_set *fdset)</span></span>; <span class="comment">// turn on the bit for fd in fdset</span></span><br><span class="line"><span class="function"><span class="type">void</span> <span class="title">FD_CLR</span> <span class="params">(<span class="type">int</span> fd,fd_set *fdset)</span></span>; <span class="comment">// turn off the bit for fd in fdset</span></span><br><span class="line"><span class="function"><span class="type">int</span> <span class="title">FD_ISSET</span><span class="params">(<span class="type">int</span> fd,fd_set *fdset)</span></span>; <span class="comment">// is the bit for fd on in fdset</span></span><br></pre></td></tr></table></figure>
<p>select存在以下限制:</p>
<blockquote>
<p>1 调用select函数会阻塞进程，直到有描述副就绪，或者超时<br>2 单个进程能够监视的文件描述符的数量存在最大限制，通常是1024；想要修改限制只能通过修改内核宏定义;<br>3 由于select采用轮询的方式扫描文件描述符，文件描述符数量越多，性能越差;<br>4 每次调用select，都需要把fd_set集合从用户态拷贝到内核态，如果fd_set集合很大，这个开销也很大;<br>5 select返回的是含有整个fd集合，应用程序需要遍历整个数组才能发现哪些句柄发生了事件；<br>6 select的触发方式是水平触发，应用程序如果没有完成对一个已经就绪的文件描述符进行IO操作，那么之后每次select调用还是会将这些文件描述符通知进程。</p>
</blockquote>
<h2 id="poll"><a href="#poll" class="headerlink" title="poll"></a>poll</h2><p>poll 和 select 实现机制相似，只是内核用结构链表取代固定数组保存文件描述符集合，也就没有了最大文件描述数量的限制；但是select面临的其他问题，poll上依然存在。</p>
<p>poll原型函数:</p>
<figure class="highlight cpp" data-lang="cpp"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">/* </span></span><br><span class="line"><span class="comment"> * fds: pollfd数组，文件描述及其监听事件的结构体集合</span></span><br><span class="line"><span class="comment"> * nfds: pollfd数组的大小</span></span><br><span class="line"><span class="comment"> * timeout: 超时时间，-1表示永久阻塞，0表示立即返回</span></span><br><span class="line"><span class="comment"> * return: int 若有就绪描述符返回其数目，若超时则为0，若出错则为-1</span></span><br><span class="line"><span class="comment"> */</span></span><br><span class="line"><span class="function"><span class="type">int</span> <span class="title">poll</span> <span class="params">(<span class="keyword">struct</span> pollfd *fds, <span class="type">unsigned</span> <span class="type">int</span> nfds, <span class="type">int</span> timeout)</span></span>;</span><br><span class="line"></span><br><span class="line"><span class="comment">/////////////////////////////////////////////////</span></span><br><span class="line"><span class="comment">//相关结构:</span></span><br><span class="line"><span class="keyword">struct</span> <span class="title class_">pollfd</span> {</span><br><span class="line"> <span class="type">int</span> fd; <span class="comment">/* file descriptor */</span></span><br><span class="line"> <span class="type">short</span> events; <span class="comment">/* requested events to watch */</span></span><br><span class="line"> <span class="type">short</span> revents; <span class="comment">/* returned events witnessed */</span></span><br><span class="line">};</span><br></pre></td></tr></table></figure>
<p>可以看到，poll对select参数传值(readfds, writefds)稍有改进，pollfd结构包含了要监视的event和发生的event。和select函数一样，poll返回后，依然需要轮询pollfd数组来获取就绪的描述符。</p>
<h2 id="epoll"><a href="#epoll" class="headerlink" title="epoll"></a>epoll</h2><p>鉴于select和poll的缺陷，epoll采用了完全不同的事件驱动机制，上述select和poll存在的问题在epoll上不复存在。<br>epoll在Linux内核中建立一个简易的文件系统(文件系统一般用什么数据结构实现？B+树)，用来管理epoll对象。epoll对象封装了对于网络IO的所有操作，这样用户对IO的操作就转变为对epoll对象的操作。</p>
<p>epoll对用户提供三个调用接口:</p>
<figure class="highlight cpp" data-lang="cpp"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">/* 创建一个epoll对象</span></span><br><span class="line"><span class="comment"> * size: 监听的描述符数量</span></span><br><span class="line"><span class="comment"> * return: epoll句柄描述符 or -1 if failed</span></span><br><span class="line"><span class="comment"> */</span></span><br><span class="line"><span class="function"><span class="type">int</span> <span class="title">epoll_create</span><span class="params">(<span class="type">int</span> size)</span></span>;</span><br><span class="line"></span><br><span class="line"><span class="comment">/* 向epoll对象添加/删除/修改fd描述符，以及要监听的事件类型</span></span><br><span class="line"><span class="comment"> * epfd: epoll句柄描述符</span></span><br><span class="line"><span class="comment"> * op: fd操作类型: EPOLL_CTL_ADD, EPOLL_CTL_MOD, EPOLL_CTL_DEL</span></span><br><span class="line"><span class="comment"> * fd: 操作的fd描述符</span></span><br><span class="line"><span class="comment"> * event: 注册/取消监听的事件</span></span><br><span class="line"><span class="comment"> * return: 0 if success or -1 if failed</span></span><br><span class="line"><span class="comment"> */</span></span><br><span class="line"><span class="function"><span class="type">int</span> <span class="title">epoll_ctl</span><span class="params">(<span class="type">int</span> epfd, <span class="type">int</span> op, <span class="type">int</span> fd, <span class="keyword">struct</span> epoll_event *event)</span></span>;</span><br><span class="line"></span><br><span class="line"><span class="comment">/* 等待事件的就绪</span></span><br><span class="line"><span class="comment"> * epfd: epoll句柄描述符</span></span><br><span class="line"><span class="comment"> * events: 接收事件的数组</span></span><br><span class="line"><span class="comment"> * maxevents: 事件数组的大小</span></span><br><span class="line"><span class="comment"> * fd: 操作的fd描述符</span></span><br><span class="line"><span class="comment"> * timeout: 超时时间，-1表示永久阻塞，0表示立即返回</span></span><br><span class="line"><span class="comment"> * return: 就绪fd的数量 or -1 if failed</span></span><br><span class="line"><span class="comment"> */</span></span><br><span class="line"><span class="function"><span class="type">int</span> <span class="title">epoll_wait</span><span class="params">(<span class="type">int</span> epfd, <span class="keyword">struct</span> epoll_event * events, <span class="type">int</span> maxevents, <span class="type">int</span> timeout)</span></span>;</span><br><span class="line"></span><br><span class="line"><span class="comment">/////////////////////////////////////////////////</span></span><br><span class="line"><span class="comment">// 相关结构</span></span><br><span class="line"><span class="keyword">struct</span> <span class="title class_">epoll_event</span> {</span><br><span class="line"> <span class="type">__uint32_t</span> events; <span class="comment">/* Epoll events */</span></span><br><span class="line"> <span class="type">epoll_data_t</span> data; <span class="comment">/* User data variable */</span></span><br><span class="line">};</span><br><span class="line"></span><br><span class="line"><span class="keyword">typedef</span> <span class="keyword">union</span> <span class="title class_">epoll_data</span> {</span><br><span class="line"> <span class="type">void</span> *ptr;</span><br><span class="line"> <span class="type">int</span> fd;</span><br><span class="line"> <span class="type">__uint32_t</span> u32;</span><br><span class="line"> <span class="type">__uint64_t</span> u64;</span><br><span class="line">} <span class="type">epoll_data_t</span>;</span><br><span class="line"></span><br><span class="line"><span class="comment">//events可以是以下几个宏的集合:</span></span><br><span class="line">EPOLLIN :表示对应的文件描述符可以读(包括对端SOCKET正常关闭)；</span><br><span class="line">EPOLLOUT:表示对应的文件描述符可以写；</span><br><span class="line">EPOLLPRI:表示对应的文件描述符有紧急的数据可读(这里应该表示有带外数据到来)；</span><br><span class="line">EPOLLERR:表示对应的文件描述符发生错误；</span><br><span class="line">EPOLLHUP:表示对应的文件描述符被挂断；</span><br><span class="line">EPOLLET: 将EPOLL设为边缘触发(Edge Triggered)模式，这是相对于水平触发(Level Triggered)来说的。</span><br><span class="line">EPOLLONESHOT:只监听一次事件，当监听完这次事件之后，如果还需要继续监听这个socket的话，需要再次把这个socket加入到EPOLL队列里</span><br></pre></td></tr></table></figure>
<p>简单来说，epoll的工作机制是这样的:</p>
<ul>
<li>当调用epoll_create时，内核会创建一个epoll对象，epoll对象通过红黑树管理监听事件，通过双向链表存放发生的事件。</li>
<li>用户通过调用epoll_ctl向epoll对象注册/销毁监听事件；对于每一个添加到epoll对象中的事件，epoll都会向设备驱动程序注册回调(ep_poll_callback)。当驱动程序有事件发生时会调用该回调函数，它会将发生的事件添加到双向链表中。</li>
<li>当用户调用epoll_wait时，内核只需要检查epoll对象中的双向链表是否有事件。如果事件不为空，则把发生的事件复制到用户态，同时将事件数量返回给用户。(注意，epoll_wait依然可能造成阻塞。)</li>
</ul>
<p>由此可见，<strong>epoll通过红黑树，双向链表等数据结构，以及事件回调机制，保证了其高效和应对并发的能力。</strong></p>
<p>另外，epoll除了提供select/poll那种IO事件的水平触发(Level Triggered)外，还提供了边缘触发(Edge Triggered)，这就使得用户空间程序有可能缓存IO状态，减少epoll_wait/epoll_pwait的调用，提高应用程序效率。</p>
<ul>
<li><p>水平触发(LT):默认工作模式，即当epoll_wait检测到某描述符事件就绪并通知应用程序时，应用程序可以不立即处理该事件；下次调用epoll_wait时，会再次通知此事件。</p>
</li>
<li><p>边缘触发(ET): 当epoll_wait检测到某描述符事件就绪并通知应用程序时，应用程序必须立即处理该事件。如果不处理，下次调用epoll_wait时，不会再次通知此事件。(直到你做了某些操作导致该描述符变成未就绪状态了，也就是说边缘触发只在状态由未就绪变为就绪时只通知一次)。</p>
</li>
</ul>
<h2 id="总结"><a href="#总结" class="headerlink" title="总结"></a>总结</h2><p>可以用以下表格对比来看select, poll, epoll的区别:</p>
<table>
<thead>
<tr>
<th align="left"></th>
<th align="center">select</th>
<th align="center">poll</th>
<th align="center">epoll</th>
</tr>
</thead>
<tbody><tr>
<td align="left">操作方式</td>
<td align="center">遍历</td>
<td align="center">遍历</td>
<td align="center">回调</td>
</tr>
<tr>
<td align="left">底层实现</td>
<td align="center">数组</td>
<td align="center">链表</td>
<td align="center">红黑树</td>
</tr>
<tr>
<td align="left">IO效率</td>
<td align="center">每次调用都进行线性遍历，时间复杂度为O(n)</td>
<td align="center">每次调用都进行线性遍历，时间复杂度为O(n)</td>
<td align="center">事件通知方式，每当fd就绪，系统注册的回调函数就会被调用，将就绪fd放到readyList里面，时间复杂度O(1)</td>
</tr>
<tr>
<td align="left">最大连接数</td>
<td align="center">1024(x86)或2048(x64)</td>
<td align="center">无上限</td>
<td align="center">无上限</td>
</tr>
<tr>
<td align="left">fd拷贝</td>
<td align="center">每次调用select，都需要把fd集合从用户态拷贝到内核态</td>
<td align="center">每次调用poll，都需要把fd集合从用户态拷贝到内核态</td>
<td align="center">调用epoll_ctl时拷贝进内核并保存，之后每次epoll_wait不拷贝</td>
</tr>
</tbody></table>
<p>epoll是Linux目前大规模网络并发程序开发的首选模型。在绝大多数情况下性能远超select和poll。目前流行的高性能web服务器Nginx正式依赖于epoll提供的高效服务。但是，在并发连接不高的情况下，多线程+阻塞I/O方式可能性能更好。</p>
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