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<code>Scavenger（清道夫）</code></p><p>GC（Garbage Collection，垃圾回收）是一种内存自动管理机制， 垃圾回收器（Garbage Collector）可以自动回收分配给程序的已经不再使用的内存。常见的 GC 算法有引用计数法和标记清除法等。V8（JavaScript 引擎，提供执行 JavaScript 的运行时环境）的垃圾回收器算法主要由 Mark-Compact 和 Scavenger 构成。</p><h2><a aria-hidden="true" tabindex="-1" class="anchor anchor__h2 anchorWithHideOnScrollNavbar_3R7-" id="知识点深入"></a>知识点深入<a class="hash-link" href="#知识点深入" title="Direct link to heading">#</a></h2><h3><a aria-hidden="true" tabindex="-1" class="anchor anchor__h3 anchorWithHideOnScrollNavbar_3R7-" id="1-内存泄漏"></a>1. 内存泄漏<a class="hash-link" href="#1-内存泄漏" title="Direct link to heading">#</a></h3><p>内存泄漏是指，应当被回收的对象没有被正常回收，变成常驻老生代的对象，导致内存占用越来越高。内存泄漏会导致应用程序速度变慢、高延时、崩溃等问题。</p><h4><a aria-hidden="true" tabindex="-1" class="anchor anchor__h4 anchorWithHideOnScrollNavbar_3R7-" id="11-内存生命周期"></a>1.1 内存生命周期<a class="hash-link" href="#11-内存生命周期" title="Direct link to heading">#</a></h4><ol><li>分配：按需分配内存。</li><li>使用：读写已分配的内存。</li><li>释放：释放不再需要的内存。</li></ol><h4><a aria-hidden="true" tabindex="-1" class="anchor anchor__h4 anchorWithHideOnScrollNavbar_3R7-" id="12-内存泄漏常见原因"></a>1.2 内存泄漏常见原因<a class="hash-link" href="#12-内存泄漏常见原因" title="Direct link to heading">#</a></h4><ul><li>创建全局变量，且没有手动回收。</li><li>事件监听器 / 定时器 / 闭包等未正常清理。</li><li>使用 JavaScript 对象来做缓存，且不设置过期策略和对象大小控制。</li><li>队列拥塞所带来的消费不及时问题。</li></ul><h3><a aria-hidden="true" tabindex="-1" class="anchor anchor__h3 anchorWithHideOnScrollNavbar_3R7-" id="2-reference-counting引用计数"></a>2. Reference Counting（引用计数）<a class="hash-link" href="#2-reference-counting引用计数" title="Direct link to heading">#</a></h3><p>Reference Counting 是常见的垃圾回收算法，其核心思路是：将资源（比如对象）的被引用次数保存起来，当被引用次数为零时释放。该方法的局限性：当出现<strong>循环引用</strong>时，互相引用的对象不会被回收。</p><h3><a aria-hidden="true" tabindex="-1" class="anchor anchor__h3 anchorWithHideOnScrollNavbar_3R7-" id="3-v8-垃圾回收机制"></a>3. V8 垃圾回收机制<a class="hash-link" href="#3-v8-垃圾回收机制" title="Direct link to heading">#</a></h3><p>V8 中有两个垃圾收集器。主要的 GC 使用 <strong>Mark-Compact</strong> 垃圾回收算法，从整个堆中收集垃圾。小型 GC 使用 <strong>Scavenger</strong> 垃圾回收算法，收集新生代垃圾。</p><p>两种不同的算法应对不同的场景：</p><ul><li>使用 Scavenger 算法主要处理<strong>存活周期短</strong>的对象中的可访问对象。</li><li>使用 Mark-Compact 算法主要处理<strong>存活周期长</strong>的对象中的不可访问的对象。</li></ul><p>因为新生代中存活的可访问对象占少数，老生代中的不可访问对象占少数，所以这两种回收算法配合使用十分高效。</p><h4><a aria-hidden="true" tabindex="-1" class="anchor anchor__h4 anchorWithHideOnScrollNavbar_3R7-" id="31-分代垃圾收集"></a>3.1 分代垃圾收集<a class="hash-link" href="#31-分代垃圾收集" title="Direct link to heading">#</a></h4><p>在 V8 中，所有的 JavaScript 对象都通过<strong>堆</strong>来分配。V8 将其管理的堆分成两代：新生代和老生代。其中新生代又可细分为两个子代（Nursery、Intermediate）。</p><p>即新生代中的对象为存活时间较短的对象，老生代中的对象为存活时间较长或常驻内存的对象。</p><p><img src="https://user-images.githubusercontent.com/17002181/126338733-69da76cd-33f3-4d0b-9a8e-c0e67ce0a331.png" alt="image"></p><h4><a aria-hidden="true" tabindex="-1" class="anchor anchor__h4 anchorWithHideOnScrollNavbar_3R7-" id="32-mark-compact-算法major-gc"></a>3.2 Mark-Compact 算法（Major GC）<a class="hash-link" href="#32-mark-compact-算法major-gc" title="Direct link to heading">#</a></h4><p>Mark-Compact 算法可以看作是 Mark-Sweep（标记清除）算法和 Cheney 复制算法的结合。该算法主要分为三个阶段：标记、清除、整理。</p><p><img src="https://user-images.githubusercontent.com/17002181/126341694-f3a454c4-4d88-4f7c-80b8-f323843eb6a3.png" alt="image"></p><ol><li><p>标记（Mark）</p><p>标记是找所有可访问对象的过程。GC 会从一组已知的对象指针（称为根集，包括执行堆栈和全局对象等）中，进行递归标记可访问对象。</p></li><li><p>清除（Sweep）</p><p>清除是将不可访问的对象留下的内存空间，添加到空闲链表（free list）的过程。未来为新对象分配内存时，可以从空闲链表中进行再分配。</p></li><li><p>整理（Compact）</p><p>整理是将可访问对象，往内存一端移动的过程。主要解决标记清除阶段后，内存空间出现较多内存碎片时，可能导致无法分配大对象，而提前触发垃圾回收的问题。</p></li></ol><h4><a aria-hidden="true" tabindex="-1" class="anchor anchor__h4 anchorWithHideOnScrollNavbar_3R7-" id="33-scavenger-算法minor-gc"></a>3.3 Scavenger 算法（Minor GC）<a class="hash-link" href="#33-scavenger-算法minor-gc" title="Direct link to heading">#</a></h4><p>V8 对新生代内存空间采用了 Scavenger 算法，该算法使用了 semi-space（半空间） 的设计：将堆一分为二，始终只使用一半的空间：From-Space 为使用空间，To-Space 为空闲空间。</p><p><img src="https://user-images.githubusercontent.com/17002181/126367817-b5a12c03-18ea-4eb9-8bdb-84d2fd3e8aad.png" alt="image"></p><p>新生代在 From-Space 中分配对象；在垃圾回收阶段，检查并按需复制 From-Space 中的可访问对象到 To-Space 或老生代，并释放 From-Space 中的不可访问对象占用的内存空间；最后 From-Space 和 To-Space 角色互换。</p><h2><a aria-hidden="true" tabindex="-1" class="anchor anchor__h2 anchorWithHideOnScrollNavbar_3R7-" id="参考资料"></a>参考资料<a class="hash-link" href="#参考资料" title="Direct link to heading">#</a></h2><ol><li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Memory_Management" target="_blank" rel="noopener noreferrer">Memory Management</a></li><li><a href="https://v8.dev/blog/trash-talk" target="_blank" rel="noopener noreferrer">Trash talk: the Orinoco garbage collector</a></li></ol></div></article><nav class="pagination-nav docusaurus-mt-lg" aria-label="Docs pages navigation"><div class="pagination-nav__item"><a class="pagination-nav__link" href="/awesome-interview/book2/browser-render-mechanism"><div class="pagination-nav__sublabel">Previous</div><div class="pagination-nav__label">« 浏览器：浏览器渲染机制</div></a></div><div class="pagination-nav__item pagination-nav__item--next"><a class="pagination-nav__link" href="/awesome-interview/book2/engineer-babel"><div class="pagination-nav__sublabel">Next</div><div class="pagination-nav__label">工程化：Babel 的原理 »</div></a></div></nav></div></div><div class="col col--3"><div class="tableOfContents_35-E thin-scrollbar"><ul class="table-of-contents table-of-contents__left-border"><li><a href="#相关问题" class="table-of-contents__link">相关问题</a></li><li><a href="#回答关键点" class="table-of-contents__link">回答关键点</a></li><li><a href="#知识点深入" class="table-of-contents__link">知识点深入</a><ul><li><a href="#1-内存泄漏" class="table-of-contents__link">1. 内存泄漏</a></li><li><a href="#2-reference-counting引用计数" class="table-of-contents__link">2. 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