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    <h2 id="title-%E9%81%87%E8%A7%81pod%E9%87%8D%E5%90%AFauthor-gamehudate-2024-02-18-093129tags">title: 遇见pod重启<br>
author: Gamehu<br>
date: 2024-02-18 09:31:29<br>
tags:</h2>
<div class="tag-container">
  <span class="main-tag">&#x9047;&#x89C1;&#x7CFB;&#x5217;</span>
  <span class="sub-tag">&#x7B2C;&#x4E94;&#x7BC7;</span>
</div>
<div class="article-quote"></div>
<p>现场的应用A pod一段时间就会重启，用k8s命令看pod是因为超出limits。</p>
<p>查看了gc日志。因为使用的是G1回收器</p>
<p>让我详细解释G1 GC的工作流程：</p>
<p>内存分区：</p>
<p>整个堆内存被划分为大小相等的Region（默认2048个）<br>
Region大小是2的幂次方，范围从1MB到32MB<br>
每个Region可以是Eden、Survivor、Old或Humongous区<br>
Humongous区专门用于存储大于Region大小一半的对象</p>
<p>GC周期的四个主要阶段：</p>
<p>a) 初始标记（Initial Mark）- STW：</p>
<p>标记所有直接从GC Roots可达的对象<br>
这个阶段需要Stop-The-World（STW）<br>
通常和年轻代收集同时进行<br>
时间很短</p>
<p>b) 并发标记（Concurrent Mark）：</p>
<p>遍历整个堆的对象图<br>
标记所有可达的对象<br>
与应用程序并发执行<br>
使用SATB（Snapshot-At-The-Beginning）算法<br>
记录标记期间的引用变化</p>
<p>c) 最终标记（Final Mark）- STW：</p>
<p>处理并发标记阶段遗留的SATB缓冲区<br>
完成最终的标记工作<br>
需要STW，但时间较短</p>
<p>d) 筛选回收（Live Data Counting and Evacuation）- STW：</p>
<p>计算各个Region的垃圾比例<br>
根据用户期望的停顿时间建立回收集合<br>
优先回收价值最大的Region（垃圾最多）<br>
将存活对象复制到新的Region<br>
清空旧Region</p>
<p>特殊处理：</p>
<p>a) 年轻代收集：</p>
<p>当Eden区满时触发<br>
只关注年轻代Region<br>
存活对象复制到Survivor区或老年代</p>
<p>b) 混合收集：</p>
<p>同时回收年轻代和部分老年代Region<br>
根据全局并发标记的结果<br>
老年代Region按垃圾收集价值排序</p>
<p>优化机制：</p>
<p>a) Remembered Sets（Rsets）：</p>
<p>记录跨Region的引用关系<br>
每个Region都有一个RSet<br>
避免全堆扫描</p>
<p>b) Collection Sets（CSets）：</p>
<p>需要被回收的Region集合<br>
基于垃圾占比和期望停顿时间选择</p>
<p>正常GC流程：</p>
<p>Young GC：回收年轻代Region<br>
并发标记周期：标记整个堆的存活对象<br>
混合回收：回收年轻代和部分老年代Region</p>
<p>Full GC触发条件：</p>
<p>并发标记完成之前就出现内存分配失败<br>
回收后的空间仍不足以支持下一次Young GC（空间担保失败）<br>
混合回收后仍无法满足内存需求<br>
Humongous对象分配失败</p>
<p>Full GC的特点：</p>
<p>必须Stop-The-World（STW）<br>
对整个堆进行压缩和整理<br>
单线程执行，停顿时间长<br>
会明显影响应用程序性能</p>
<p>Full GC后的处理：</p>
<p>如果Full GC后内存足够，应用继续运行<br>
如果Full GC后内存仍然不足，抛出OutOfMemoryError</p>
<p>避免Full GC的建议：</p>
<p>合理设置堆内存大小<br>
调整并发标记的触发阈值<br>
适当调整Region大小<br>
控制大对象的产生<br>
及时进行并发标记，避免内存耗尽</p>
<p>Full GC在G1中的具体执行步骤：</p>
<ol>
<li>标记阶段（Marking Phase）：</li>
</ol>
<ul>
<li>进入STW阶段，停止所有用户线程</li>
<li>从GC Roots开始进行全堆标记</li>
<li>标记所有存活对象</li>
<li>使用单线程进行标记，效率较低</li>
</ul>
<ol start="2">
<li>清理阶段（Cleanup Phase）：</li>
</ol>
<ul>
<li>清理所有未被标记的对象（垃圾对象）</li>
<li>回收空的Region</li>
<li>重置各种元数据信息（RSet等）</li>
<li>计算所有Region的存活对象信息</li>
</ul>
<ol start="3">
<li>压缩阶段（Compaction Phase）：</li>
</ol>
<ul>
<li>对存活对象进行压缩整理</li>
<li>将存活对象复制到新的Region</li>
<li>按照新的分代布局重新整理对象</li>
<li>更新对象引用关系</li>
<li>释放旧的Region</li>
<li>重建RSet等辅助数据结构</li>
</ul>
<p>特点：</p>
<ul>
<li>整个过程完全STW</li>
<li>单线程执行，性能较差</li>
<li>会对整个堆内存进行处理</li>
<li>会导致较长时间的停顿</li>
<li>会破坏原有的分代结构，需要重新构建</li>
</ul>
<p>触发Full GC通常意味着：</p>
<ul>
<li>内存分配压力过大</li>
<li>并发收集来不及回收内存</li>
<li>可能需要调整GC参数或应用程序代码</li>
</ul>

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