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class="tag-list-link" href="/tags/Synchronized/" rel="tag">Synchronized</a><span class="tag-list-count">2</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/Thread-dump/" rel="tag">Thread dump</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/ThreadLocal/" rel="tag">ThreadLocal</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/ThreadPool/" rel="tag">ThreadPool</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/Tomcat/" rel="tag">Tomcat</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/Trapping-Rain-Water/" rel="tag">Trapping Rain Water</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/WeakReference/" rel="tag">WeakReference</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/Web/" rel="tag">Web</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/Webhook/" rel="tag">Webhook</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/WordPress/" rel="tag">WordPress</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/aqs/" rel="tag">aqs</a><span class="tag-list-count">3</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/await/" rel="tag">await</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/bloom-filter/" rel="tag">bloom filter</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/c/" rel="tag">c++</a><span class="tag-list-count">14</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/cglib/" rel="tag">cglib</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/cgroup/" rel="tag">cgroup</a><span class="tag-list-count">2</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/cluster/" rel="tag">cluster</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/condition/" rel="tag">condition</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/docker/" rel="tag">docker</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/dp/" rel="tag">dp</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/dubbo/" rel="tag">dubbo</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/echo/" rel="tag">echo</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/environment/" rel="tag">environment</a><span class="tag-list-count">2</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/gap-lock/" rel="tag">gap lock</a><span class="tag-list-count">2</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/gc/" rel="tag">gc</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/git/" rel="tag">git</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/grep/" rel="tag">grep</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/hadoop/" rel="tag">hadoop</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/icu4c/" rel="tag">icu4c</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/im/" rel="tag">im</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/is-not-null/" rel="tag">is not null</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/is-null/" rel="tag">is null</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/j-u-c/" rel="tag">j.u.c</a><span class="tag-list-count">2</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/java/" rel="tag">java</a><span class="tag-list-count">33</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/jvm/" rel="tag">jvm</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/leetcode/" rel="tag">leetcode</a><span class="tag-list-count">40</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/leetcode-155/" rel="tag">leetcode 155</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/linked-list/" rel="tag">linked list</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/linux/" rel="tag">linux</a><span class="tag-list-count">3</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/lock/" rel="tag">lock</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/mfc/" rel="tag">mfc</a><span class="tag-list-count">2</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/min-stack/" rel="tag">min stack</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/mq/" rel="tag">mq</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/mvcc/" rel="tag">mvcc</a><span class="tag-list-count">3</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/mysql/" rel="tag">mysql</a><span class="tag-list-count">6</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/namespace/" rel="tag">namespace</a><span class="tag-list-count">3</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/next-key-lock/" rel="tag">next-key lock</a><span class="tag-list-count">2</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/nginx/" rel="tag">nginx</a><span class="tag-list-count">3</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/nullsfirst/" rel="tag">nullsfirst</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/openresty/" rel="tag">openresty</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/php/" rel="tag">php</a><span class="tag-list-count">4</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/procedure/" rel="tag">procedure</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/python/" rel="tag">python</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/read-view/" rel="tag">read view</a><span class="tag-list-count">3</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/redis/" rel="tag">redis</a><span class="tag-list-count">12</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/scp/" rel="tag">scp</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/signal/" rel="tag">signal</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/sort/" rel="tag">sort</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/spark/" rel="tag">spark</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/ssh/" rel="tag">ssh</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/stack/" rel="tag">stack</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/stream/" rel="tag">stream</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/string/" rel="tag">string</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/swoole/" rel="tag">swoole</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/top/" rel="tag">top</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/uname/" rel="tag">uname</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/unlock/" rel="tag">unlock</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/utf8/" rel="tag">utf8</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/utf8mb4/" rel="tag">utf8mb4</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/utf8mb4-0900-ai-ci/" rel="tag">utf8mb4_0900_ai_ci</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/utf8mb4-general-ci/" rel="tag">utf8mb4_general_ci</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/utf8mb4-unicode-ci/" rel="tag">utf8mb4_unicode_ci</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/value/" rel="tag">value</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/websocket/" rel="tag">websocket</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/zookeeper/" rel="tag">zookeeper</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/zsh/" rel="tag">zsh</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E4%B8%89%E9%98%B6%E6%AE%B5%E6%8F%90%E4%BA%A4/" rel="tag">三阶段提交</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E4%B8%8D%E5%8F%AF%E5%8F%98%E5%BC%95%E7%94%A8/" rel="tag">不可变引用</a><span class="tag-list-count">2</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E4%B8%9C%E4%BA%AC%E5%A5%A5%E8%BF%90%E4%BC%9A/" rel="tag">东京奥运会</a><span class="tag-list-count">2</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E4%B8%A4%E9%98%B6%E6%AE%B5%E6%8F%90%E4%BA%A4/" rel="tag">两阶段提交</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E4%B8%AD%E5%B1%B1%E8%B7%AF/" rel="tag">中山路</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E4%B8%AD%E5%BA%8F/" rel="tag">中序</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E4%B8%AD%E9%97%B4%E4%BB%B6/" rel="tag">中间件</a><span class="tag-list-count">4</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E4%B8%BE%E9%87%8D/" rel="tag">举重</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E4%B9%92%E4%B9%93%E7%90%83/" rel="tag">乒乓球</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E4%BA%8B%E5%8A%A1/" rel="tag">事务</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E4%BA%8C%E5%8F%89%E6%A0%91/" rel="tag">二叉树</a><span class="tag-list-count">3</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E4%BA%92%E6%96%A5%E9%94%81/" rel="tag">互斥锁</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E4%BB%A3%E7%A0%81%E9%A2%98%E8%A7%A3/" rel="tag">代码题解</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E4%BF%AE%E7%94%B5%E8%84%91%E7%9A%84/" rel="tag">修电脑的</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%81%8F%E5%90%91%E9%94%81/" rel="tag">偏向锁</a><span class="tag-list-count">2</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%81%A5%E5%BA%B7%E7%A0%81/" rel="tag">健康码</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%85%AC%E4%BA%A4/" rel="tag">公交</a><span class="tag-list-count">3</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%85%AC%E4%BA%A4%E8%BD%A6/" rel="tag">公交车</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%86%85%E5%AD%98%E5%88%86%E5%B8%83/" rel="tag">内存分布</a><span class="tag-list-count">3</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%86%85%E5%AD%98%E6%B3%84%E6%BC%8F/" rel="tag">内存泄漏</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%87%86%E5%A4%87/" rel="tag">准备</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%87%8F%E8%82%A5/" rel="tag">减肥</a><span class="tag-list-count">6</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%88%86%E5%B8%83%E5%BC%8F%E4%BA%8B%E5%8A%A1/" rel="tag">分布式事务</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%88%86%E5%B8%83%E5%BC%8F%E9%94%81/" rel="tag">分布式锁</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%88%87%E7%89%87/" rel="tag">切片</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%88%9D%E5%A7%8B%E5%8C%96/" rel="tag">初始化</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%89%8A%E5%B3%B0%E5%A1%AB%E8%B0%B7/" rel="tag">削峰填谷</a><span class="tag-list-count">4</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%89%8D%E5%BA%8F/" rel="tag">前序</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%8A%A0%E5%A1%9E/" rel="tag">加塞</a><span class="tag-list-count">3</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%8A%A0%E8%BD%BD/" rel="tag">加载</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%8D%95%E4%BE%8B/" rel="tag">单例</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%8D%9A%E5%AE%A2%EF%BC%8C%E6%96%87%E7%AB%A0/" rel="tag">博客，文章</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%8E%A6%E9%97%A8/" rel="tag">厦门</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%8F%8C%E4%BA%B2%E5%A7%94%E6%B4%BE/" rel="tag">双亲委派</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%8F%91%E8%A1%8C%E7%89%88/" rel="tag">发行版</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%8F%A3%E7%BD%A9/" rel="tag">口罩</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%8F%AF%E5%8F%98%E5%BC%95%E7%94%A8/" rel="tag">可变引用</a><span class="tag-list-count">2</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%90%90%E6%A7%BD/" rel="tag">吐槽</a><span class="tag-list-count">2</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%9B%A4%E7%89%A9%E8%B5%84/" rel="tag">囤物资</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%9E%83%E5%9C%BE%E5%9B%9E%E6%94%B6/" rel="tag">垃圾回收</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%9F%BA%E7%A1%80%E8%AE%BE%E6%96%BD/" rel="tag">基础设施</a><span class="tag-list-count">2</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%A4%A7%E6%89%AB%E9%99%A4/" rel="tag">大扫除</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%AD%97%E7%AC%A6%E9%9B%86/" rel="tag">字符集</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%AE%89%E5%85%A8/" rel="tag">安全</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%AE%B9%E9%94%99%E6%9C%BA%E5%88%B6/" rel="tag">容错机制</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%AF%84%E7%94%9F%E8%99%AB/" rel="tag">寄生虫</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%B0%84%E5%87%BB/" rel="tag">射击</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%B0%8F%E6%8A%80%E5%B7%A7/" rel="tag">小技巧</a><span class="tag-list-count">5</span></li><li class="tag-list-item"><a class="tag-list-link" 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href="https://nicksxs.me/2020/04/18/redis%E7%B3%BB%E5%88%97%E4%BB%8B%E7%BB%8D%E5%85%AB/"><span hidden itemprop="author" itemscope itemtype="http://schema.org/Person"><meta itemprop="image" content="/uploads/avatar.jpg"><meta itemprop="name" content="Nicksxs"></span><span hidden itemprop="publisher" itemscope itemtype="http://schema.org/Organization"><meta itemprop="name" content="Nicksxs's Blog"><meta itemprop="description" content="learn from zero,技术博客，Nicksxs，史学森"></span><span hidden itemprop="post" itemscope itemtype="http://schema.org/CreativeWork"><meta itemprop="name" content="redis系列介绍八-淘汰策略 | Nicksxs's Blog"><meta itemprop="description" content=""></span><header class="post-header"><h1 class="post-title" itemprop="name headline">redis系列介绍八-淘汰策略</h1><div class="post-meta-container"><div class="post-meta"><span class="post-meta-item"><span class="post-meta-item-icon"><i class="far fa-calendar"></i> </span><span class="post-meta-item-text">发表于</span> <time title="创建时间：2020-04-18 23:23:41" itemprop="dateCreated datePublished" datetime="2020-04-18T23:23:41+08:00">2020-04-18</time> </span><span class="post-meta-item"><span class="post-meta-item-icon"><i class="far fa-calendar-check"></i> </span><span class="post-meta-item-text">更新于</span> <time title="修改时间：2022-06-22 23:20:15" itemprop="dateModified" datetime="2022-06-22T23:20:15+08:00">2022-06-22</time> </span><span class="post-meta-item"><span class="post-meta-item-icon"><i class="far fa-folder"></i> </span><span class="post-meta-item-text">分类于</span> <span itemprop="about" itemscope itemtype="http://schema.org/Thing"><a href="/categories/Redis/" itemprop="url" rel="index"><span itemprop="name">Redis</span></a> </span>， <span itemprop="about" itemscope itemtype="http://schema.org/Thing"><a href="/categories/Redis/%E6%95%B0%E6%8D%AE%E7%BB%93%E6%9E%84/" itemprop="url" rel="index"><span itemprop="name">数据结构</span></a> </span>， <span itemprop="about" itemscope itemtype="http://schema.org/Thing"><a href="/categories/Redis/%E6%BA%90%E7%A0%81/" itemprop="url" rel="index"><span itemprop="name">源码</span></a> </span>， <span itemprop="about" itemscope itemtype="http://schema.org/Thing"><a href="/categories/C/" itemprop="url" rel="index"><span itemprop="name">C</span></a> </span>， <span itemprop="about" itemscope itemtype="http://schema.org/Thing"><a href="/categories/C/Redis/" itemprop="url" rel="index"><span itemprop="name">Redis</span></a> </span></span><span id="/2020/04/18/redis%E7%B3%BB%E5%88%97%E4%BB%8B%E7%BB%8D%E5%85%AB/" class="post-meta-item leancloud_visitors" data-flag-title="redis系列介绍八-淘汰策略" title="阅读次数"><span class="post-meta-item-icon"><i class="far fa-eye"></i> </span><span class="post-meta-item-text">阅读次数：</span> <span class="leancloud-visitors-count"></span> </span><span class="post-meta-item" title="阅读次数" id="busuanzi_container_page_pv"><span class="post-meta-item-icon"><i class="far fa-eye"></i> </span><span class="post-meta-item-text">阅读次数：</span> <span id="busuanzi_value_page_pv"></span> </span><span class="post-meta-item"><span class="post-meta-item-icon"><i class="far fa-comment"></i> </span><span class="post-meta-item-text">Disqus：</span> <a title="disqus" href="/2020/04/18/redis%E7%B3%BB%E5%88%97%E4%BB%8B%E7%BB%8D%E5%85%AB/#disqus_thread" itemprop="discussionUrl"><span class="post-comments-count disqus-comment-count" data-disqus-identifier="2020/04/18/redis系列介绍八/" itemprop="commentCount"></span></a></span></div></div></header><div class="post-body" itemprop="articleBody"><h3 id="LRU"><a href="#LRU" class="headerlink" title="LRU"></a>LRU</h3><p>说完了过期策略再说下淘汰策略，redis 使用的策略是近似的 lru 策略，为什么是近似的呢，先来看下什么是 lru，看下 wiki 的介绍<br><img data-src="https://img.nicksxs.com/uPic/z5cDpo.jpg">，图中一共有四个槽的存储空间，依次访问顺序是 A B C D E D F，<br>当第一次访问 D 时刚好占满了坑，并且值是 4，这个值越小代表越先被淘汰，当 E 进来时，看了下已经存在的四个里 A 是最小的，代表是最早存在并且最早被访问的，那就先淘汰它了，E 占领了 A 的位置，并设置值为 4，然后又访问 D 了，D 已经存在了，不过又被访问到了，得更新值为 5，然后是 F 进来了，这时 B 是最老的且最近未被访问，所以就淘汰它了。以上是一个 lru 的简要说明，但是 redis 没有严格按照这个去执行，理由跟前面过期策略一致，最严格的过期策略应该是每个 key 都有对应的定时器，当超时时马上就能清除，但是问题是这样的cpu 消耗太大，所换来的内存效率不太值得，淘汰策略也是这样，类似于上图，要维护所有 key 的一个有序 lru 值，并且遍历将最小的淘汰，redis 采用的是抽样的形式，最初的实现方式是随机从 dict 抽取 5 个 key，淘汰一个 lru 最小的，这样子勉强能达到淘汰的目的，但是效果不是特别好，后面在 redis 3.0开始，将随机抽取改成了维护一个 pool，pool 的大小默认是 16，每次放入的都是按lru 值有序排列好，每一次放入的必须是 lru小于 pool 中最小的 lru 才允许放入，直到放满，后面再有新的就会将大的踢出。<br>redis 针对这个策略的改进做了一个实验，这里借用下图<br><img data-src="https://img.nicksxs.com/uPic/lEx4Ug.jpg"><br>首先背景是这图中的所有点都对应一个 redis 的 key，灰色部分加入后被顺序访问过一遍，然后又加入了绿色部分，那么按照理论的 lru 算法，应该是图左上中，浅灰色部分全都被淘汰，那么对比来看看图右上，左下和右下，左下表示 2.8 版本就是随机抽样 5 个 key，淘汰其中 lru 最小的一个，发现是灰色和浅灰色的都有被淘汰的，右下的 3.0 版本抽样数量不变的情况下，稍好一些，当 3.0 版本的抽样数量调整成 10 后，已经较为接近理论上的 lru 策略了，通过代码来简要分析下</p><pre class="line-numbers language-C" data-language="C"><code class="language-C">typedef struct redisObject {
|
|
unsigned type:4;
|
|
unsigned encoding:4;
|
|
unsigned lru:LRU_BITS; /* LRU time (relative to global lru_clock) or
|
|
* LFU data (least significant 8 bits frequency
|
|
* and most significant 16 bits access time). */
|
|
int refcount;
|
|
void *ptr;
|
|
} robj;<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre><p>对于 lru 策略来说，lru 字段记录的就是<a target="_blank" rel="noopener" href="https://github.com/antirez/redis/blob/unstable/src/server.h#L603"><code>redisObj</code></a> 的LRU time，<br>redis 在访问数据时，都会调用<a target="_blank" rel="noopener" href="https://github.com/antirez/redis/blob/unstable/src/db.c#L55"><code>lookupKey</code></a>方法</p><pre class="line-numbers language-C" data-language="C"><code class="language-C">/* Low level key lookup API, not actually called directly from commands
|
|
* implementations that should instead rely on lookupKeyRead(),
|
|
* lookupKeyWrite() and lookupKeyReadWithFlags(). */
|
|
robj *lookupKey(redisDb *db, robj *key, int flags) {
|
|
dictEntry *de = dictFind(db->dict,key->ptr);
|
|
if (de) {
|
|
robj *val = dictGetVal(de);
|
|
|
|
/* Update the access time for the ageing algorithm.
|
|
* Don't do it if we have a saving child, as this will trigger
|
|
* a copy on write madness. */
|
|
if (!hasActiveChildProcess() && !(flags & LOOKUP_NOTOUCH)){
|
|
if (server.maxmemory_policy & MAXMEMORY_FLAG_LFU) {
|
|
// 这个是后面一节的内容
|
|
updateLFU(val);
|
|
} else {
|
|
// 对于这个分支，访问时就会去更新 lru 值
|
|
val->lru = LRU_CLOCK();
|
|
}
|
|
}
|
|
return val;
|
|
} else {
|
|
return NULL;
|
|
}
|
|
}
|
|
/* This function is used to obtain the current LRU clock.
|
|
* If the current resolution is lower than the frequency we refresh the
|
|
* LRU clock (as it should be in production servers) we return the
|
|
* precomputed value, otherwise we need to resort to a system call. */
|
|
unsigned int LRU_CLOCK(void) {
|
|
unsigned int lruclock;
|
|
if (1000/server.hz <= LRU_CLOCK_RESOLUTION) {
|
|
// 如果服务器的频率server.hz大于 1 时就是用系统预设的 lruclock
|
|
lruclock = server.lruclock;
|
|
} else {
|
|
lruclock = getLRUClock();
|
|
}
|
|
return lruclock;
|
|
}
|
|
/* Return the LRU clock, based on the clock resolution. This is a time
|
|
* in a reduced-bits format that can be used to set and check the
|
|
* object->lru field of redisObject structures. */
|
|
unsigned int getLRUClock(void) {
|
|
return (mstime()/LRU_CLOCK_RESOLUTION) & LRU_CLOCK_MAX;
|
|
}<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre><p>redis 处理命令是在这里<a target="_blank" rel="noopener" href="https://github.com/antirez/redis/blob/unstable/src/server.c#L3355"><code>processCommand</code></a></p><pre class="line-numbers language-C" data-language="C"><code class="language-C">/* If this function gets called we already read a whole
|
|
* command, arguments are in the client argv/argc fields.
|
|
* processCommand() execute the command or prepare the
|
|
* server for a bulk read from the client.
|
|
*
|
|
* If C_OK is returned the client is still alive and valid and
|
|
* other operations can be performed by the caller. Otherwise
|
|
* if C_ERR is returned the client was destroyed (i.e. after QUIT). */
|
|
int processCommand(client *c) {
|
|
moduleCallCommandFilters(c);
|
|
|
|
|
|
|
|
/* Handle the maxmemory directive.
|
|
*
|
|
* Note that we do not want to reclaim memory if we are here re-entering
|
|
* the event loop since there is a busy Lua script running in timeout
|
|
* condition, to avoid mixing the propagation of scripts with the
|
|
* propagation of DELs due to eviction. */
|
|
if (server.maxmemory && !server.lua_timedout) {
|
|
int out_of_memory = freeMemoryIfNeededAndSafe() == C_ERR;
|
|
/* freeMemoryIfNeeded may flush slave output buffers. This may result
|
|
* into a slave, that may be the active client, to be freed. */
|
|
if (server.current_client == NULL) return C_ERR;
|
|
|
|
/* It was impossible to free enough memory, and the command the client
|
|
* is trying to execute is denied during OOM conditions or the client
|
|
* is in MULTI/EXEC context? Error. */
|
|
if (out_of_memory &&
|
|
(c->cmd->flags & CMD_DENYOOM ||
|
|
(c->flags & CLIENT_MULTI &&
|
|
c->cmd->proc != execCommand &&
|
|
c->cmd->proc != discardCommand)))
|
|
{
|
|
flagTransaction(c);
|
|
addReply(c, shared.oomerr);
|
|
return C_OK;
|
|
}
|
|
}
|
|
}<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre><p>这里只摘了部分，当需要清理内存时就会调用, 然后调用了<a target="_blank" rel="noopener" href="https://github.com/antirez/redis/blob/unstable/src/evict.c#L631"><code>freeMemoryIfNeededAndSafe</code></a></p><pre class="line-numbers language-C" data-language="C"><code class="language-C">/* This is a wrapper for freeMemoryIfNeeded() that only really calls the
|
|
* function if right now there are the conditions to do so safely:
|
|
*
|
|
* - There must be no script in timeout condition.
|
|
* - Nor we are loading data right now.
|
|
*
|
|
*/
|
|
int freeMemoryIfNeededAndSafe(void) {
|
|
if (server.lua_timedout || server.loading) return C_OK;
|
|
return freeMemoryIfNeeded();
|
|
}
|
|
/* This function is periodically called to see if there is memory to free
|
|
* according to the current "maxmemory" settings. In case we are over the
|
|
* memory limit, the function will try to free some memory to return back
|
|
* under the limit.
|
|
*
|
|
* The function returns C_OK if we are under the memory limit or if we
|
|
* were over the limit, but the attempt to free memory was successful.
|
|
* Otehrwise if we are over the memory limit, but not enough memory
|
|
* was freed to return back under the limit, the function returns C_ERR. */
|
|
int freeMemoryIfNeeded(void) {
|
|
int keys_freed = 0;
|
|
/* By default replicas should ignore maxmemory
|
|
* and just be masters exact copies. */
|
|
if (server.masterhost && server.repl_slave_ignore_maxmemory) return C_OK;
|
|
|
|
size_t mem_reported, mem_tofree, mem_freed;
|
|
mstime_t latency, eviction_latency;
|
|
long long delta;
|
|
int slaves = listLength(server.slaves);
|
|
|
|
/* When clients are paused the dataset should be static not just from the
|
|
* POV of clients not being able to write, but also from the POV of
|
|
* expires and evictions of keys not being performed. */
|
|
if (clientsArePaused()) return C_OK;
|
|
if (getMaxmemoryState(&mem_reported,NULL,&mem_tofree,NULL) == C_OK)
|
|
return C_OK;
|
|
|
|
mem_freed = 0;
|
|
|
|
if (server.maxmemory_policy == MAXMEMORY_NO_EVICTION)
|
|
goto cant_free; /* We need to free memory, but policy forbids. */
|
|
|
|
latencyStartMonitor(latency);
|
|
while (mem_freed < mem_tofree) {
|
|
int j, k, i;
|
|
static unsigned int next_db = 0;
|
|
sds bestkey = NULL;
|
|
int bestdbid;
|
|
redisDb *db;
|
|
dict *dict;
|
|
dictEntry *de;
|
|
|
|
if (server.maxmemory_policy & (MAXMEMORY_FLAG_LRU|MAXMEMORY_FLAG_LFU) ||
|
|
server.maxmemory_policy == MAXMEMORY_VOLATILE_TTL)
|
|
{
|
|
struct evictionPoolEntry *pool = EvictionPoolLRU;
|
|
|
|
while(bestkey == NULL) {
|
|
unsigned long total_keys = 0, keys;
|
|
|
|
/* We don't want to make local-db choices when expiring keys,
|
|
* so to start populate the eviction pool sampling keys from
|
|
* every DB. */
|
|
for (i = 0; i < server.dbnum; i++) {
|
|
db = server.db+i;
|
|
dict = (server.maxmemory_policy & MAXMEMORY_FLAG_ALLKEYS) ?
|
|
db->dict : db->expires;
|
|
if ((keys = dictSize(dict)) != 0) {
|
|
evictionPoolPopulate(i, dict, db->dict, pool);
|
|
total_keys += keys;
|
|
}
|
|
}
|
|
if (!total_keys) break; /* No keys to evict. */
|
|
|
|
/* Go backward from best to worst element to evict. */
|
|
for (k = EVPOOL_SIZE-1; k >= 0; k--) {
|
|
if (pool[k].key == NULL) continue;
|
|
bestdbid = pool[k].dbid;
|
|
|
|
if (server.maxmemory_policy & MAXMEMORY_FLAG_ALLKEYS) {
|
|
de = dictFind(server.db[pool[k].dbid].dict,
|
|
pool[k].key);
|
|
} else {
|
|
de = dictFind(server.db[pool[k].dbid].expires,
|
|
pool[k].key);
|
|
}
|
|
|
|
/* Remove the entry from the pool. */
|
|
if (pool[k].key != pool[k].cached)
|
|
sdsfree(pool[k].key);
|
|
pool[k].key = NULL;
|
|
pool[k].idle = 0;
|
|
|
|
/* If the key exists, is our pick. Otherwise it is
|
|
* a ghost and we need to try the next element. */
|
|
if (de) {
|
|
bestkey = dictGetKey(de);
|
|
break;
|
|
} else {
|
|
/* Ghost... Iterate again. */
|
|
}
|
|
}
|
|
}
|
|
}
|
|
|
|
/* volatile-random and allkeys-random policy */
|
|
else if (server.maxmemory_policy == MAXMEMORY_ALLKEYS_RANDOM ||
|
|
server.maxmemory_policy == MAXMEMORY_VOLATILE_RANDOM)
|
|
{
|
|
/* When evicting a random key, we try to evict a key for
|
|
* each DB, so we use the static 'next_db' variable to
|
|
* incrementally visit all DBs. */
|
|
for (i = 0; i < server.dbnum; i++) {
|
|
j = (++next_db) % server.dbnum;
|
|
db = server.db+j;
|
|
dict = (server.maxmemory_policy == MAXMEMORY_ALLKEYS_RANDOM) ?
|
|
db->dict : db->expires;
|
|
if (dictSize(dict) != 0) {
|
|
de = dictGetRandomKey(dict);
|
|
bestkey = dictGetKey(de);
|
|
bestdbid = j;
|
|
break;
|
|
}
|
|
}
|
|
}
|
|
|
|
/* Finally remove the selected key. */
|
|
if (bestkey) {
|
|
db = server.db+bestdbid;
|
|
robj *keyobj = createStringObject(bestkey,sdslen(bestkey));
|
|
propagateExpire(db,keyobj,server.lazyfree_lazy_eviction);
|
|
/* We compute the amount of memory freed by db*Delete() alone.
|
|
* It is possible that actually the memory needed to propagate
|
|
* the DEL in AOF and replication link is greater than the one
|
|
* we are freeing removing the key, but we can't account for
|
|
* that otherwise we would never exit the loop.
|
|
*
|
|
* AOF and Output buffer memory will be freed eventually so
|
|
* we only care about memory used by the key space. */
|
|
delta = (long long) zmalloc_used_memory();
|
|
latencyStartMonitor(eviction_latency);
|
|
if (server.lazyfree_lazy_eviction)
|
|
dbAsyncDelete(db,keyobj);
|
|
else
|
|
dbSyncDelete(db,keyobj);
|
|
latencyEndMonitor(eviction_latency);
|
|
latencyAddSampleIfNeeded("eviction-del",eviction_latency);
|
|
latencyRemoveNestedEvent(latency,eviction_latency);
|
|
delta -= (long long) zmalloc_used_memory();
|
|
mem_freed += delta;
|
|
server.stat_evictedkeys++;
|
|
notifyKeyspaceEvent(NOTIFY_EVICTED, "evicted",
|
|
keyobj, db->id);
|
|
decrRefCount(keyobj);
|
|
keys_freed++;
|
|
|
|
/* When the memory to free starts to be big enough, we may
|
|
* start spending so much time here that is impossible to
|
|
* deliver data to the slaves fast enough, so we force the
|
|
* transmission here inside the loop. */
|
|
if (slaves) flushSlavesOutputBuffers();
|
|
|
|
/* Normally our stop condition is the ability to release
|
|
* a fixed, pre-computed amount of memory. However when we
|
|
* are deleting objects in another thread, it's better to
|
|
* check, from time to time, if we already reached our target
|
|
* memory, since the "mem_freed" amount is computed only
|
|
* across the dbAsyncDelete() call, while the thread can
|
|
* release the memory all the time. */
|
|
if (server.lazyfree_lazy_eviction && !(keys_freed % 16)) {
|
|
if (getMaxmemoryState(NULL,NULL,NULL,NULL) == C_OK) {
|
|
/* Let's satisfy our stop condition. */
|
|
mem_freed = mem_tofree;
|
|
}
|
|
}
|
|
} else {
|
|
latencyEndMonitor(latency);
|
|
latencyAddSampleIfNeeded("eviction-cycle",latency);
|
|
goto cant_free; /* nothing to free... */
|
|
}
|
|
}
|
|
latencyEndMonitor(latency);
|
|
latencyAddSampleIfNeeded("eviction-cycle",latency);
|
|
return C_OK;
|
|
|
|
cant_free:
|
|
/* We are here if we are not able to reclaim memory. There is only one
|
|
* last thing we can try: check if the lazyfree thread has jobs in queue
|
|
* and wait... */
|
|
while(bioPendingJobsOfType(BIO_LAZY_FREE)) {
|
|
if (((mem_reported - zmalloc_used_memory()) + mem_freed) >= mem_tofree)
|
|
break;
|
|
usleep(1000);
|
|
}
|
|
return C_ERR;
|
|
}<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre><p>这里就是根据具体策略去淘汰 key，首先是要往 pool 更新 key，更新key 的方法是<a target="_blank" rel="noopener" href="https://github.com/antirez/redis/blob/unstable/src/evict.c#L162"><code>evictionPoolPopulate</code></a></p><pre class="line-numbers language-C" data-language="C"><code class="language-C">void evictionPoolPopulate(int dbid, dict *sampledict, dict *keydict, struct evictionPoolEntry *pool) {
|
|
int j, k, count;
|
|
dictEntry *samples[server.maxmemory_samples];
|
|
|
|
count = dictGetSomeKeys(sampledict,samples,server.maxmemory_samples);
|
|
for (j = 0; j < count; j++) {
|
|
unsigned long long idle;
|
|
sds key;
|
|
robj *o;
|
|
dictEntry *de;
|
|
|
|
de = samples[j];
|
|
key = dictGetKey(de);
|
|
|
|
/* If the dictionary we are sampling from is not the main
|
|
* dictionary (but the expires one) we need to lookup the key
|
|
* again in the key dictionary to obtain the value object. */
|
|
if (server.maxmemory_policy != MAXMEMORY_VOLATILE_TTL) {
|
|
if (sampledict != keydict) de = dictFind(keydict, key);
|
|
o = dictGetVal(de);
|
|
}
|
|
|
|
/* Calculate the idle time according to the policy. This is called
|
|
* idle just because the code initially handled LRU, but is in fact
|
|
* just a score where an higher score means better candidate. */
|
|
if (server.maxmemory_policy & MAXMEMORY_FLAG_LRU) {
|
|
idle = estimateObjectIdleTime(o);
|
|
} else if (server.maxmemory_policy & MAXMEMORY_FLAG_LFU) {
|
|
/* When we use an LRU policy, we sort the keys by idle time
|
|
* so that we expire keys starting from greater idle time.
|
|
* However when the policy is an LFU one, we have a frequency
|
|
* estimation, and we want to evict keys with lower frequency
|
|
* first. So inside the pool we put objects using the inverted
|
|
* frequency subtracting the actual frequency to the maximum
|
|
* frequency of 255. */
|
|
idle = 255-LFUDecrAndReturn(o);
|
|
} else if (server.maxmemory_policy == MAXMEMORY_VOLATILE_TTL) {
|
|
/* In this case the sooner the expire the better. */
|
|
idle = ULLONG_MAX - (long)dictGetVal(de);
|
|
} else {
|
|
serverPanic("Unknown eviction policy in evictionPoolPopulate()");
|
|
}
|
|
|
|
/* Insert the element inside the pool.
|
|
* First, find the first empty bucket or the first populated
|
|
* bucket that has an idle time smaller than our idle time. */
|
|
k = 0;
|
|
while (k < EVPOOL_SIZE &&
|
|
pool[k].key &&
|
|
pool[k].idle < idle) k++;
|
|
if (k == 0 && pool[EVPOOL_SIZE-1].key != NULL) {
|
|
/* Can't insert if the element is < the worst element we have
|
|
* and there are no empty buckets. */
|
|
continue;
|
|
} else if (k < EVPOOL_SIZE && pool[k].key == NULL) {
|
|
/* Inserting into empty position. No setup needed before insert. */
|
|
} else {
|
|
/* Inserting in the middle. Now k points to the first element
|
|
* greater than the element to insert. */
|
|
if (pool[EVPOOL_SIZE-1].key == NULL) {
|
|
/* Free space on the right? Insert at k shifting
|
|
* all the elements from k to end to the right. */
|
|
|
|
/* Save SDS before overwriting. */
|
|
sds cached = pool[EVPOOL_SIZE-1].cached;
|
|
memmove(pool+k+1,pool+k,
|
|
sizeof(pool[0])*(EVPOOL_SIZE-k-1));
|
|
pool[k].cached = cached;
|
|
} else {
|
|
/* No free space on right? Insert at k-1 */
|
|
k--;
|
|
/* Shift all elements on the left of k (included) to the
|
|
* left, so we discard the element with smaller idle time. */
|
|
sds cached = pool[0].cached; /* Save SDS before overwriting. */
|
|
if (pool[0].key != pool[0].cached) sdsfree(pool[0].key);
|
|
memmove(pool,pool+1,sizeof(pool[0])*k);
|
|
pool[k].cached = cached;
|
|
}
|
|
}
|
|
|
|
/* Try to reuse the cached SDS string allocated in the pool entry,
|
|
* because allocating and deallocating this object is costly
|
|
* (according to the profiler, not my fantasy. Remember:
|
|
* premature optimizbla bla bla bla. */
|
|
int klen = sdslen(key);
|
|
if (klen > EVPOOL_CACHED_SDS_SIZE) {
|
|
pool[k].key = sdsdup(key);
|
|
} else {
|
|
memcpy(pool[k].cached,key,klen+1);
|
|
sdssetlen(pool[k].cached,klen);
|
|
pool[k].key = pool[k].cached;
|
|
}
|
|
pool[k].idle = idle;
|
|
pool[k].dbid = dbid;
|
|
}
|
|
}<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre><p><code>Redis</code>随机选择<code>maxmemory_samples</code>数量的key，然后计算这些<code>key</code>的空闲时间<code>idle time</code>，当满足条件时(比pool中的某些键的空闲时间还大)就可以进<code>pool</code>。<code>pool</code>更新之后，就淘汰<code>pool</code>中空闲时间最大的键。</p><p><a target="_blank" rel="noopener" href="https://github.com/antirez/redis/blob/unstable/src/evict.c#L90"><code>estimateObjectIdleTime</code></a>用来计算Redis对象的空闲时间：</p><pre class="line-numbers language-C" data-language="C"><code class="language-C">/* Given an object returns the min number of milliseconds the object was never
|
|
* requested, using an approximated LRU algorithm. */
|
|
unsigned long long estimateObjectIdleTime(robj *o) {
|
|
unsigned long long lruclock = LRU_CLOCK();
|
|
if (lruclock >= o->lru) {
|
|
return (lruclock - o->lru) * LRU_CLOCK_RESOLUTION;
|
|
} else {
|
|
return (lruclock + (LRU_CLOCK_MAX - o->lru)) *
|
|
LRU_CLOCK_RESOLUTION;
|
|
}
|
|
}<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre><p>空闲时间第一种是 lurclock 大于对象的 lru，那么就是减一下乘以精度，因为 lruclock 有可能是已经预生成的，所以会可能走下面这个</p><h3 id="LFU"><a href="#LFU" class="headerlink" title="LFU"></a>LFU</h3><p>上面介绍了LRU 的算法，但是考虑一种场景</p><pre class="line-numbers language-none"><code class="language-none">~~~~~A~~~~~A~~~~~A~~~~A~~~~~A~~~~~A~~|
|
|
~~B~~B~~B~~B~~B~~B~~B~~B~~B~~B~~B~~B~|
|
|
~~~~~~~~~~C~~~~~~~~~C~~~~~~~~~C~~~~~~|
|
|
~~~~~D~~~~~~~~~~D~~~~~~~~~D~~~~~~~~~D|<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span></span></code></pre><p>可以发现，当采用 lru 的淘汰策略的时候，D 是最新的，会被认为是最值得保留的，但是事实上还不如 A 跟 B，然后 antirez 大神就想到了LFU (Least Frequently Used) 这个算法, 显然对于上面的四个 key 的访问频率，保留优先级应该是 B > A > C = D<br>那要怎么来实现这个 LFU 算法呢，其实像LRU，理想的情况就是维护个链表，把最新访问的放到头上去，但是这个会影响访问速度，注意到前面代码的应该可以看到，redisObject 的 lru 字段其实是两用的，当策略是 LFU 时，这个字段就另作他用了，它的 24 位长度被分成两部分</p><pre class="line-numbers language-none"><code class="language-none"> 16 bits 8 bits
|
|
+----------------+--------+
|
|
+ Last decr time | LOG_C |
|
|
+----------------+--------+<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span></span></code></pre><p>前16位字段是最后一次递减时间，因此Redis知道 上一次计数器递减，后8位是 计数器 counter。<br>LFU 的主体策略就是当这个 key 被访问的次数越多频率越高他就越容易被保留下来，并且是最近被访问的频率越高。这其实有两个事情要做，一个是在访问的时候增加计数值，在一定长时间不访问时进行衰减，所以这里用了两个值，前 16 位记录上一次衰减的时间，后 8 位记录具体的计数值。<br>Redis4.0之后为maxmemory_policy淘汰策略添加了两个LFU模式：</p><p><code>volatile-lfu</code>：对有过期时间的key采用LFU淘汰策略<br><code>allkeys-lfu</code>：对全部key采用LFU淘汰策略<br>还有2个配置可以调整LFU算法：</p><pre class="line-numbers language-none"><code class="language-none">lfu-log-factor 10
|
|
lfu-decay-time 1
|
|
```
|
|
`lfu-log-factor` 可以调整计数器counter的增长速度，lfu-log-factor越大，counter增长的越慢。
|
|
|
|
`lfu-decay-time`是一个以分钟为单位的数值，可以调整counter的减少速度
|
|
这里有个问题是 8 位大小够计么，访问一次加 1 的话的确不够，不过大神就是大神，才不会这么简单的加一。往下看代码
|
|
```C
|
|
/* Low level key lookup API, not actually called directly from commands
|
|
* implementations that should instead rely on lookupKeyRead(),
|
|
* lookupKeyWrite() and lookupKeyReadWithFlags(). */
|
|
robj *lookupKey(redisDb *db, robj *key, int flags) {
|
|
dictEntry *de = dictFind(db->dict,key->ptr);
|
|
if (de) {
|
|
robj *val = dictGetVal(de);
|
|
|
|
/* Update the access time for the ageing algorithm.
|
|
* Don't do it if we have a saving child, as this will trigger
|
|
* a copy on write madness. */
|
|
if (!hasActiveChildProcess() && !(flags & LOOKUP_NOTOUCH)){
|
|
if (server.maxmemory_policy & MAXMEMORY_FLAG_LFU) {
|
|
// 当淘汰策略是 LFU 时，就会调用这个updateLFU
|
|
updateLFU(val);
|
|
} else {
|
|
val->lru = LRU_CLOCK();
|
|
}
|
|
}
|
|
return val;
|
|
} else {
|
|
return NULL;
|
|
}
|
|
}<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre><p><a target="_blank" rel="noopener" href="https://github.com/antirez/redis/blob/unstable/src/db.c#L46"><code>updateLFU</code></a> 这个其实个入口，调用了两个重要的方法</p><pre class="line-numbers language-C" data-language="C"><code class="language-C">/* Update LFU when an object is accessed.
|
|
* Firstly, decrement the counter if the decrement time is reached.
|
|
* Then logarithmically increment the counter, and update the access time. */
|
|
void updateLFU(robj *val) {
|
|
unsigned long counter = LFUDecrAndReturn(val);
|
|
counter = LFULogIncr(counter);
|
|
val->lru = (LFUGetTimeInMinutes()<<8) | counter;
|
|
}<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre><p>首先来看看<a target="_blank" rel="noopener" href="https://github.com/antirez/redis/blob/unstable/src/evict.c#L335"><code>LFUDecrAndReturn</code></a>，这个方法的作用是根据上一次衰减时间和系统配置的 <code>lfu-decay-time</code> 参数来确定需要将 counter 减去多少</p><pre class="line-numbers language-C" data-language="C"><code class="language-C">/* If the object decrement time is reached decrement the LFU counter but
|
|
* do not update LFU fields of the object, we update the access time
|
|
* and counter in an explicit way when the object is really accessed.
|
|
* And we will times halve the counter according to the times of
|
|
* elapsed time than server.lfu_decay_time.
|
|
* Return the object frequency counter.
|
|
*
|
|
* This function is used in order to scan the dataset for the best object
|
|
* to fit: as we check for the candidate, we incrementally decrement the
|
|
* counter of the scanned objects if needed. */
|
|
unsigned long LFUDecrAndReturn(robj *o) {
|
|
// 右移 8 位，拿到上次衰减时间
|
|
unsigned long ldt = o->lru >> 8;
|
|
// 对 255 做与操作，拿到 counter 值
|
|
unsigned long counter = o->lru & 255;
|
|
// 根据lfu_decay_time来算出过了多少个衰减周期
|
|
unsigned long num_periods = server.lfu_decay_time ? LFUTimeElapsed(ldt) / server.lfu_decay_time : 0;
|
|
if (num_periods)
|
|
counter = (num_periods > counter) ? 0 : counter - num_periods;
|
|
return counter;
|
|
}<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre><p>然后是加，调用了<a target="_blank" rel="noopener" href="https://github.com/antirez/redis/blob/unstable/src/evict.c#L315"><code>LFULogIncr</code></a></p><pre class="line-numbers language-C" data-language="C"><code class="language-C">/* Logarithmically increment a counter. The greater is the current counter value
|
|
* the less likely is that it gets really implemented. Saturate it at 255. */
|
|
uint8_t LFULogIncr(uint8_t counter) {
|
|
// 最大值就是 255，到顶了就不加了
|
|
if (counter == 255) return 255;
|
|
// 生成个随机小数
|
|
double r = (double)rand()/RAND_MAX;
|
|
// 减去个基础值，LFU_INIT_VAL = 5，防止刚进来就被逐出
|
|
double baseval = counter - LFU_INIT_VAL;
|
|
// 如果是小于 0，
|
|
if (baseval < 0) baseval = 0;
|
|
// 如果 baseval 是 0，那么 p 就是 1了，后面 counter 直接加一，如果不是的话，得看系统参数lfu_log_factor，这个越大，除出来的 p 越小，那么 counter++的可能性也越小，这样子就把前面的疑问给解决了，不是直接+1 的
|
|
double p = 1.0/(baseval*server.lfu_log_factor+1);
|
|
if (r < p) counter++;
|
|
return counter;
|
|
}<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre><p>大概的变化速度可以参考</p><pre class="line-numbers language-none"><code class="language-none">+--------+------------+------------+------------+------------+------------+
|
|
| factor | 100 hits | 1000 hits | 100K hits | 1M hits | 10M hits |
|
|
+--------+------------+------------+------------+------------+------------+
|
|
| 0 | 104 | 255 | 255 | 255 | 255 |
|
|
+--------+------------+------------+------------+------------+------------+
|
|
| 1 | 18 | 49 | 255 | 255 | 255 |
|
|
+--------+------------+------------+------------+------------+------------+
|
|
| 10 | 10 | 18 | 142 | 255 | 255 |
|
|
+--------+------------+------------+------------+------------+------------+
|
|
| 100 | 8 | 11 | 49 | 143 | 255 |
|
|
+--------+------------+------------+------------+------------+------------+<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre><p>简而言之就是 lfu_log_factor 越大变化的越慢</p><h3 id="总结"><a href="#总结" class="headerlink" title="总结"></a>总结</h3><p>总结一下，redis 实现了近似的 lru 淘汰策略，通过增加了淘汰 key 的池子(pool)，并且增大每次抽样的 key 的数量来将淘汰效果更进一步地接近于 lru，这是 lru 策略，但是对于前面举的一个例子，其实 lru 并不能保证 key 的淘汰就如我们预期，所以在后期又引入了 lfu 的策略，lfu的策略比较巧妙，复用了 redis 对象的 lru 字段，并且使用了factor 参数来控制计数器递增的速度，防止 8 位的计数器太早溢出。</p></div><footer class="post-footer"><div class="reward-container"><div>Buy me a coffee</div><button>赞赏</button><div class="post-reward"><div><img src="https://i.loli.net/2020/01/12/NUlWanT31E8LQGx.png" alt="Nicksxs 微信"> <span>微信</span></div><div><img src="https://ooo.0o0.ooo/2016/08/17/57b421e773057.jpg" alt="Nicksxs 支付宝"> <span>支付宝</span></div></div></div><div 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