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第二季：8.G1垃圾收集器【Java面试题】

2023-08-23 60
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简介： 第二季：8.G1垃圾收集器【Java面试题】

前言

2022 10/13 15:38

路漫漫其修远兮,吾将上下而求索

本文是根据尚硅谷学习所做笔记

仅供学习交流使用，转载注明出处

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尚硅谷Java大厂面试题第2季，面试必刷，跳槽大厂神器

第二季大佬总结

8.G1垃圾收集器

说明

本文目录前是相关视频的名字和具体视频中思维导图的名字

题目

8.G1垃圾收集器

以前收集器特点

  • 年轻代和老年代是各自独立且连续的内存块
  • 年轻代收集使用单eden + S0 + S1 进行复制算法
  • 老年代收集必须扫描珍整个老年代区域
  • 都是以尽可能少而快速地执行GC为设计原则

G1是什么

G1：Garbage-First 收集器，是一款面向服务端应用的收集器

我们知道G1：Garbage-First 收集器，是一款面向服务端应用的收集器，应用在多处理器和大容量内存环境中，在实现高吞吐量的同时，尽可能满足垃圾收集暂停时间的要求。另外，它还具有一下特征：


像CMS收集器一样，能与应用程序并发执行

整理空闲空间更快

需要更多的时间来预测GC停顿时间

不希望牺牲大量的吞吐量性能

不需要更大的Java Heap

G1收集器设计目标是取代CMS收集器，它同CMS相比，在以下方面表现的更出色

G1是一个有整理内存过程的垃圾收集器，不会产生很多内存碎片。
G1的Stop The World（STW）更可控，G1在停顿时间上添加了预测机制，用户可以指定期望停顿时间。


CMS垃圾收集器虽然减少了暂停应用程序的运行时间，但是它还存在着内存碎片问题。于是，为了去除内存碎片问题，同时又保留CMS垃圾收集器低暂停时间的优点，JAVA7发布了一个新的垃圾收集器-G1垃圾收集器

G1是在2012才在JDK1.7中可用，Oracle官方计划在JDK9中将G1变成默认的垃圾收集器以替代CMS，它是一款面向服务端应用的收集器，主要应用在多CPU和大内存服务器环境下，极大减少垃圾收集的停顿时间，全面提升服务器的性能，逐步替换Java8以前的CMS收集器

主要改变时：Eden，Survivor和Tenured等内存区域不再是连续了，而是变成一个个大小一样的region，

每个region从1M到32M不等。一个region有可能属于Eden，Survivor或者Tenured内存区域。

特点

1.G1能充分利用多CPU，多核环境硬件优势，尽量缩短STW

2.G1整体上采用标记-整理算法，局部是通过复制算法，不会产生内存碎片

3.宏观上看G1之中不再区分年轻代和老年代。把内存划分成多个独立的子区域（Region），可以近似理解为一个围棋的棋盘

4.G1收集器里面将整个内存区域都混合在一起了，但其本身依然在小范围内要进行年轻代和老年代的区分，保留了新生代和老年代，但他们不再是物理隔离的，而是通过一部分Region的集合且不需要Region是连续的，也就是说依然会采取不同的GC方式来处理不同的区域

5.G1虽然也是分代收集器，但整个内存分区不存在物理上的年轻代和老年代的区别，也不需要完全独立的Survivor（to space）堆做复制准备，G1 只有逻辑上的分代概念 ，或者说每个分区都可能随G1的运行在不同代之间前后切换。

底层原理

Region区域化垃圾收集器

Region区域化垃圾收集器，化整为零，打破了原来新生区和老年区的壁垒，避免了全内存扫描，只需要按照区域来进行扫描即可。


区域化内存划片Region，整体遍为了一些列不连续的内存区域，避免了全内存区的GC操作。


核心思想是将整个堆内存区域分成大小相同的子区域（Region），在JVM启动时会自动设置子区域大小

在堆的使用上，G1并不要求对象的存储一定是物理上连续的，只要逻辑上连续即可，每个分区也不会固定地为某个代服务，可以按需在年轻代和老年代之间切换。启动时可以通过参数-XX:G1HeapRegionSize=n 可指定分区大小（1MB~32MB，且必须是2的幂），默认将整堆划分为2048个分区。

大小范围在1MB~32MB，最多能设置2048个区域，也即能够支持的最大内存为：32MB*2048 = 64G内存

Region区域化垃圾收集器

G1将新生代、老年代的物理空间划分取消了

同时对内存进行了区域划分


G1算法将堆划分为若干个区域（Reign），它仍然属于分代收集器，

这些Region的一部分包含新生代，新生代的垃圾收集依然采用暂停所有应用线程的方式，将存活对象拷贝到老年代或者Survivor空间
这些Region的一部分包含老年代，G1收集器通过将对象从一个区域复制到另外一个区域，完成了清理工作。这就意味着，在正常的处理过程中，G1完成了堆的压缩（至少是部分堆的压缩），这样也就不会有CMS内存碎片的问题存在了。

在G1中，还有一种特殊的区域，叫做Humongous（巨大的）区域

如果一个对象占用了空间超过了分区容量50%以上，G1收集器就认为这是一个巨型对象，这些巨型对象默认直接分配在老年代，但是如果他是一个短期存在的巨型对象，就会对垃圾收集器造成负面影响，为了解决这个问题，G1划分了一个Humongous区，它用来专门存放巨型对象。如果一个H区装不下一个巨型对象，那么G1会寻找连续的H区来存储，为了能找到连续的H区，有时候不得不启动Full GC。

回收步骤

针对Eden区进行收集，Eden区耗尽后会被触发，主要是小区域收集 + 形成连续的内存块，避免内碎片


Eden区的数据移动到Survivor区，加入出现Survivor区空间不够，Eden区数据会晋升到Old区

Survivor区的数据移动到新的Survivor区，部分数据晋升到Old区

最后Eden区收拾干净了，GC结束，用户的应用程序继续执行


回收完成后


小区域收集 + 形成连续的内存块，最后在收集完成后，就会形成连续的内存空间，这样就解决了内存碎片的问题

4步过程

  • 初始标记：只标记GC Roots能直接关联到的对象
  • 并发标记：进行GC Roots Tracing（链路扫描）的过程
  • 最终标记：修正并发标记期间，因为程序运行导致标记发生变化的那一部分对象
  • 筛选回收：根据时间来进行价值最大化回收



case案例

package g1gc8;
import java.util.Random;
/**
 * @author CSDN@日星月云
 * @date 2022/10/13 15:42
 *
 * 6
 * -Xms10m -Xmx10m -XX:+PrintGCDetails -XX:+PrintCommandLineFlags -XX:+UseG1GC            后面单独讲解G1
 *
 *
 */
public class GCDemo {
    public static void main(String[] args) {
        System.out.println("*******GCDemo hello") ;
        try{
            String str = "atguigu";
            while(true){
                str += str + new Random().nextInt(77777777)+new Random().nextInt(88888888);
                str.intern();
            }
        }catch (Throwable e){
            e .printStackTrace();
        }
    }
}

-Xms10m -Xmx10m -XX:+PrintGCDetails -XX:+PrintCommandLineFlags -XX:+UseG1GC         

-XX:InitialHeapSize=10485760 -XX:MaxHeapSize=10485760 -XX:+PrintCommandLineFlags -XX:+PrintGCDetails -XX:+UseCompressedClassPointers -XX:+UseCompressedOops -XX:+UseG1GC -XX:-UseLargePagesIndividualAllocation 
*******GCDemo hello
[GC pause (G1 Humongous Allocation) (young) (initial-mark) (to-space exhausted), 0.0040149 secs]
   [Parallel Time: 3.0 ms, GC Workers: 6]
      [GC Worker Start (ms): Min: 164.2, Avg: 164.4, Max: 164.5, Diff: 0.3]
      [Ext Root Scanning (ms): Min: 2.5, Avg: 2.6, Max: 2.8, Diff: 0.3, Sum: 15.8]
      [Update RS (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
         [Processed Buffers: Min: 0, Avg: 0.0, Max: 0, Diff: 0, Sum: 0]
      [Scan RS (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [Code Root Scanning (ms): Min: 0.0, Avg: 0.0, Max: 0.1, Diff: 0.1, Sum: 0.1]
      [Object Copy (ms): Min: 0.0, Avg: 0.1, Max: 0.2, Diff: 0.2, Sum: 0.6]
      [Termination (ms): Min: 0.0, Avg: 0.0, Max: 0.1, Diff: 0.1, Sum: 0.2]
         [Termination Attempts: Min: 1, Avg: 1.0, Max: 1, Diff: 0, Sum: 6]
      [GC Worker Other (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.2]
      [GC Worker Total (ms): Min: 2.7, Avg: 2.8, Max: 3.0, Diff: 0.3, Sum: 16.9]
      [GC Worker End (ms): Min: 167.2, Avg: 167.2, Max: 167.2, Diff: 0.0]
   [Code Root Fixup: 0.0 ms]
   [Code Root Purge: 0.0 ms]
   [Clear CT: 0.1 ms]
   [Other: 0.9 ms]
      [Evacuation Failure: 0.7 ms]
      [Choose CSet: 0.0 ms]
      [Ref Proc: 0.1 ms]
      [Ref Enq: 0.0 ms]
      [Redirty Cards: 0.1 ms]
      [Humongous Register: 0.0 ms]
      [Humongous Reclaim: 0.0 ms]
      [Free CSet: 0.0 ms]
   [Eden: 6144.0K(6144.0K)->0.0B(1024.0K) Survivors: 0.0B->0.0B Heap: 8385.6K(10.0M)->6571.4K(10.0M)]
 [Times: user=0.00 sys=0.00, real=0.00 secs] 
[GC concurrent-root-region-scan-start]
[GC concurrent-root-region-scan-end, 0.0000181 secs]
[GC concurrent-mark-start]
[GC concurrent-mark-end, 0.0006601 secs]
[GC remark [Finalize Marking, 0.0001423 secs] [GC ref-proc, 0.0000974 secs] [Unloading, 0.0009640 secs], 0.0013821 secs]
 [Times: user=0.00 sys=0.00, real=0.00 secs] 
[GC cleanup 8043K->8043K(10M), 0.0002541 secs]
 [Times: user=0.00 sys=0.00, real=0.00 secs] 
[GC pause (G1 Humongous Allocation) (young) (to-space exhausted), 0.0007920 secs]
   [Parallel Time: 0.5 ms, GC Workers: 6]
      [GC Worker Start (ms): Min: 172.1, Avg: 172.3, Max: 172.6, Diff: 0.4]
      [Ext Root Scanning (ms): Min: 0.0, Avg: 0.1, Max: 0.2, Diff: 0.2, Sum: 0.8]
      [Update RS (ms): Min: 0.0, Avg: 0.1, Max: 0.2, Diff: 0.2, Sum: 0.9]
         [Processed Buffers: Min: 0, Avg: 1.0, Max: 2, Diff: 2, Sum: 6]
      [Scan RS (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [Code Root Scanning (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [Object Copy (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [Termination (ms): Min: 0.0, Avg: 0.0, Max: 0.1, Diff: 0.1, Sum: 0.3]
         [Termination Attempts: Min: 1, Avg: 1.0, Max: 1, Diff: 0, Sum: 6]
      [GC Worker Other (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [GC Worker Total (ms): Min: 0.0, Avg: 0.3, Max: 0.4, Diff: 0.4, Sum: 2.0]
      [GC Worker End (ms): Min: 172.6, Avg: 172.6, Max: 172.6, Diff: 0.0]
   [Code Root Fixup: 0.0 ms]
   [Code Root Purge: 0.0 ms]
   [Clear CT: 0.1 ms]
   [Other: 0.3 ms]
      [Evacuation Failure: 0.1 ms]
      [Choose CSet: 0.0 ms]
      [Ref Proc: 0.1 ms]
      [Ref Enq: 0.0 ms]
      [Redirty Cards: 0.1 ms]
      [Humongous Register: 0.0 ms]
      [Humongous Reclaim: 0.0 ms]
      [Free CSet: 0.0 ms]
   [Eden: 1024.0K(1024.0K)->0.0B(1024.0K) Survivors: 0.0B->0.0B Heap: 8043.4K(10.0M)->8043.4K(10.0M)]
 [Times: user=0.00 sys=0.00, real=0.00 secs] 
[Full GC (Allocation Failure)  8043K->2088K(10M), 0.0038437 secs]
   [Eden: 0.0B(1024.0K)->0.0B(5120.0K) Survivors: 0.0B->0.0B Heap: 8043.4K(10.0M)->2089.0K(10.0M)], [Metaspace: 3280K->3280K(1056768K)]
 [Times: user=0.00 sys=0.00, real=0.00 secs] 
[GC pause (G1 Humongous Allocation) (young) (initial-mark), 0.0008725 secs]
   [Parallel Time: 0.6 ms, GC Workers: 6]
      [GC Worker Start (ms): Min: 178.0, Avg: 178.2, Max: 178.4, Diff: 0.4]
      [Ext Root Scanning (ms): Min: 0.0, Avg: 0.2, Max: 0.4, Diff: 0.4, Sum: 1.4]
      [Update RS (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
         [Processed Buffers: Min: 0, Avg: 0.3, Max: 1, Diff: 1, Sum: 2]
      [Scan RS (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [Code Root Scanning (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [Object Copy (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [Termination (ms): Min: 0.0, Avg: 0.1, Max: 0.2, Diff: 0.2, Sum: 0.4]
         [Termination Attempts: Min: 1, Avg: 1.0, Max: 1, Diff: 0, Sum: 6]
      [GC Worker Other (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [GC Worker Total (ms): Min: 0.2, Avg: 0.3, Max: 0.6, Diff: 0.4, Sum: 2.0]
      [GC Worker End (ms): Min: 178.5, Avg: 178.5, Max: 178.5, Diff: 0.0]
   [Code Root Fixup: 0.0 ms]
   [Code Root Purge: 0.0 ms]
   [Clear CT: 0.1 ms]
   [Other: 0.2 ms]
      [Choose CSet: 0.0 ms]
      [Ref Proc: 0.1 ms]
      [Ref Enq: 0.0 ms]
      [Redirty Cards: 0.1 ms]
      [Humongous Register: 0.0 ms]
      [Humongous Reclaim: 0.0 ms]
      [Free CSet: 0.0 ms]
   [Eden: 1024.0K(5120.0K)->0.0B(4096.0K) Survivors: 0.0B->0.0B Heap: 3642.9K(10.0M)->2814.7K(10.0M)]
 [Times: user=0.05 sys=0.00, real=0.00 secs] 
[GC concurrent-root-region-scan-start]
[GC concurrent-root-region-scan-end, 0.0000116 secs]
[GC concurrent-mark-start]
[GC concurrent-mark-end, 0.0018901 secs]
[GC pause (G1 Humongous Allocation) (young), 0.0007752 secs]
   [Parallel Time: 0.5 ms, GC Workers: 6]
      [GC Worker Start (ms): Min: 181.0, Avg: 181.2, Max: 181.4, Diff: 0.4]
      [Ext Root Scanning (ms): Min: 0.0, Avg: 0.1, Max: 0.4, Diff: 0.4, Sum: 0.8]
      [Update RS (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
         [Processed Buffers: Min: 0, Avg: 0.5, Max: 2, Diff: 2, Sum: 3]
      [Scan RS (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [Code Root Scanning (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [Object Copy (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [Termination (ms): Min: 0.0, Avg: 0.0, Max: 0.1, Diff: 0.1, Sum: 0.2]
         [Termination Attempts: Min: 1, Avg: 1.0, Max: 1, Diff: 0, Sum: 6]
      [GC Worker Other (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [GC Worker Total (ms): Min: 0.0, Avg: 0.2, Max: 0.4, Diff: 0.4, Sum: 1.1]
      [GC Worker End (ms): Min: 181.4, Avg: 181.4, Max: 181.4, Diff: 0.0]
   [Code Root Fixup: 0.0 ms]
   [Code Root Purge: 0.0 ms]
   [Clear CT: 0.1 ms]
   [Other: 0.2 ms]
      [Choose CSet: 0.0 ms]
      [Ref Proc: 0.1 ms]
      [Ref Enq: 0.0 ms]
      [Redirty Cards: 0.0 ms]
      [Humongous Register: 0.0 ms]
      [Humongous Reclaim: 0.0 ms]
      [Free CSet: 0.0 ms]
   [Eden: 1024.0K(4096.0K)->0.0B(2048.0K) Survivors: 0.0B->1024.0K Heap: 5799.5K(10.0M)->4268.1K(10.0M)]
 [Times: user=0.00 sys=0.00, real=0.00 secs] 
[GC remark [Finalize Marking, 0.0001215 secs] [GC ref-proc, 0.0000982 secs] [Unloading, 0.0007132 secs], 0.0010118 secs]
 [Times: user=0.00 sys=0.00, real=0.00 secs] 
[GC cleanup 5719K->5719K(10M), 0.0002569 secs]
 [Times: user=0.00 sys=0.00, real=0.00 secs] 
[GC pause (G1 Humongous Allocation) (young), 0.0008609 secs]
   [Parallel Time: 0.6 ms, GC Workers: 6]
      [GC Worker Start (ms): Min: 186.0, Avg: 186.3, Max: 186.5, Diff: 0.5]
      [Ext Root Scanning (ms): Min: 0.0, Avg: 0.2, Max: 0.4, Diff: 0.4, Sum: 0.9]
      [Update RS (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
         [Processed Buffers: Min: 0, Avg: 0.3, Max: 1, Diff: 1, Sum: 2]
      [Scan RS (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [Code Root Scanning (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [Object Copy (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [Termination (ms): Min: 0.0, Avg: 0.0, Max: 0.1, Diff: 0.1, Sum: 0.2]
         [Termination Attempts: Min: 1, Avg: 1.0, Max: 1, Diff: 0, Sum: 6]
      [GC Worker Other (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [GC Worker Total (ms): Min: 0.0, Avg: 0.2, Max: 0.5, Diff: 0.5, Sum: 1.2]
      [GC Worker End (ms): Min: 186.5, Avg: 186.5, Max: 186.5, Diff: 0.0]
   [Code Root Fixup: 0.0 ms]
   [Code Root Purge: 0.0 ms]
   [Clear CT: 0.1 ms]
   [Other: 0.3 ms]
      [Choose CSet: 0.0 ms]
      [Ref Proc: 0.1 ms]
      [Ref Enq: 0.0 ms]
      [Redirty Cards: 0.1 ms]
      [Humongous Register: 0.0 ms]
      [Humongous Reclaim: 0.0 ms]
      [Free CSet: 0.0 ms]
   [Eden: 1024.0K(2048.0K)->0.0B(1024.0K) Survivors: 1024.0K->1024.0K Heap: 5781.1K(10.0M)->5721.6K(10.0M)]
 [Times: user=0.00 sys=0.00, real=0.00 secs] 
[GC pause (G1 Humongous Allocation) (young) (initial-mark) (to-space exhausted), 0.0007797 secs]
   [Parallel Time: 0.4 ms, GC Workers: 6]
      [GC Worker Start (ms): Min: 187.5, Avg: 187.7, Max: 187.8, Diff: 0.3]
      [Ext Root Scanning (ms): Min: 0.0, Avg: 0.2, Max: 0.4, Diff: 0.3, Sum: 1.3]
      [Update RS (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
         [Processed Buffers: Min: 0, Avg: 0.3, Max: 1, Diff: 1, Sum: 2]
      [Scan RS (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [Code Root Scanning (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [Object Copy (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [Termination (ms): Min: 0.0, Avg: 0.0, Max: 0.1, Diff: 0.1, Sum: 0.2]
         [Termination Attempts: Min: 1, Avg: 1.0, Max: 1, Diff: 0, Sum: 6]
      [GC Worker Other (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [GC Worker Total (ms): Min: 0.1, Avg: 0.3, Max: 0.4, Diff: 0.3, Sum: 1.6]
      [GC Worker End (ms): Min: 187.9, Avg: 187.9, Max: 187.9, Diff: 0.0]
   [Code Root Fixup: 0.0 ms]
   [Code Root Purge: 0.0 ms]
   [Clear CT: 0.1 ms]
   [Other: 0.3 ms]
      [Evacuation Failure: 0.1 ms]
      [Choose CSet: 0.0 ms]
      [Ref Proc: 0.1 ms]
      [Ref Enq: 0.0 ms]
      [Redirty Cards: 0.1 ms]
      [Humongous Register: 0.0 ms]
      [Humongous Reclaim: 0.0 ms]
      [Free CSet: 0.0 ms]
   [Eden: 0.0B(1024.0K)->0.0B(1024.0K) Survivors: 1024.0K->0.0B Heap: 7173.1K(10.0M)->7173.1K(10.0M)]
 [Times: user=0.00 sys=0.00, real=0.00 secs] 
[GC concurrent-root-region-scan-start]
[GC concurrent-root-region-scan-end, 0.0000097 secs]
[GC concurrent-mark-start]
[GC pause (G1 Humongous Allocation) (young), 0.0005939 secs]
   [Parallel Time: 0.4 ms, GC Workers: 6]
      [GC Worker Start (ms): Min: 188.5, Avg: 188.6, Max: 188.8, Diff: 0.3]
      [Ext Root Scanning (ms): Min: 0.0, Avg: 0.1, Max: 0.2, Diff: 0.2, Sum: 0.6]
      [Update RS (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
         [Processed Buffers: Min: 0, Avg: 0.5, Max: 2, Diff: 2, Sum: 3]
      [Scan RS (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [Code Root Scanning (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [Object Copy (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [Termination (ms): Min: 0.0, Avg: 0.1, Max: 0.1, Diff: 0.1, Sum: 0.3]
         [Termination Attempts: Min: 1, Avg: 1.0, Max: 1, Diff: 0, Sum: 6]
      [GC Worker Other (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [GC Worker Total (ms): Min: 0.0, Avg: 0.2, Max: 0.3, Diff: 0.3, Sum: 1.0]
      [GC Worker End (ms): Min: 188.8, Avg: 188.8, Max: 188.8, Diff: 0.0]
   [Code Root Fixup: 0.0 ms]
   [Code Root Purge: 0.0 ms]
   [Clear CT: 0.1 ms]
   [Other: 0.2 ms]
      [Choose CSet: 0.0 ms]
      [Ref Proc: 0.1 ms]
      [Ref Enq: 0.0 ms]
      [Redirty Cards: 0.1 ms]
      [Humongous Register: 0.0 ms]
      [Humongous Reclaim: 0.0 ms]
      [Free CSet: 0.0 ms]
   [Eden: 0.0B(1024.0K)->0.0B(1024.0K) Survivors: 0.0B->0.0B Heap: 7173.1K(10.0M)->7173.1K(10.0M)]
 [Times: user=0.00 sys=0.00, real=0.00 secs] 
[Full GC (Allocation Failure)  7173K->3536K(10M), 0.0026271 secs]
   [Eden: 0.0B(1024.0K)->0.0B(3072.0K) Survivors: 0.0B->0.0B Heap: 7173.1K(10.0M)->3536.6K(10.0M)], [Metaspace: 3280K->3280K(1056768K)]
 [Times: user=0.00 sys=0.00, real=0.00 secs] 
[GC concurrent-mark-abort]
[GC pause (G1 Humongous Allocation) (young) (initial-mark), 0.0009548 secs]
   [Parallel Time: 0.6 ms, GC Workers: 6]
      [GC Worker Start (ms): Min: 193.2, Avg: 193.5, Max: 193.7, Diff: 0.5]
      [Ext Root Scanning (ms): Min: 0.0, Avg: 0.2, Max: 0.5, Diff: 0.5, Sum: 1.3]
      [Update RS (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
         [Processed Buffers: Min: 0, Avg: 0.2, Max: 1, Diff: 1, Sum: 1]
      [Scan RS (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [Code Root Scanning (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [Object Copy (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [Termination (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.1]
         [Termination Attempts: Min: 1, Avg: 1.0, Max: 1, Diff: 0, Sum: 6]
      [GC Worker Other (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [GC Worker Total (ms): Min: 0.1, Avg: 0.3, Max: 0.6, Diff: 0.5, Sum: 1.5]
      [GC Worker End (ms): Min: 193.7, Avg: 193.7, Max: 193.7, Diff: 0.0]
   [Code Root Fixup: 0.0 ms]
   [Code Root Purge: 0.0 ms]
   [Clear CT: 0.1 ms]
   [Other: 0.2 ms]
      [Choose CSet: 0.0 ms]
      [Ref Proc: 0.1 ms]
      [Ref Enq: 0.0 ms]
      [Redirty Cards: 0.1 ms]
      [Humongous Register: 0.0 ms]
      [Humongous Reclaim: 0.0 ms]
      [Free CSet: 0.0 ms]
   [Eden: 1024.0K(3072.0K)->0.0B(2048.0K) Survivors: 0.0B->0.0B Heap: 6501.0K(10.0M)->4988.1K(10.0M)]
 [Times: user=0.02 sys=0.00, real=0.00 secs] 
[GC concurrent-root-region-scan-start]
[GC concurrent-root-region-scan-end, 0.0000046 secs]
[GC concurrent-mark-start]
[GC pause (G1 Humongous Allocation) (young), 0.0007066 secs]
   [Parallel Time: 0.3 ms, GC Workers: 6]
      [GC Worker Start (ms): Min: 194.3, Avg: 194.5, Max: 194.6, Diff: 0.3]
      [Ext Root Scanning (ms): Min: 0.0, Avg: 0.1, Max: 0.2, Diff: 0.2, Sum: 0.4]
      [Update RS (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
         [Processed Buffers: Min: 0, Avg: 0.3, Max: 1, Diff: 1, Sum: 2]
      [Scan RS (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [Code Root Scanning (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [Object Copy (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [Termination (ms): Min: 0.0, Avg: 0.0, Max: 0.1, Diff: 0.1, Sum: 0.2]
         [Termination Attempts: Min: 1, Avg: 1.0, Max: 1, Diff: 0, Sum: 6]
      [GC Worker Other (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [GC Worker Total (ms): Min: 0.0, Avg: 0.1, Max: 0.3, Diff: 0.3, Sum: 0.7]
      [GC Worker End (ms): Min: 194.6, Avg: 194.6, Max: 194.6, Diff: 0.0]
   [Code Root Fixup: 0.0 ms]
   [Code Root Purge: 0.0 ms]
   [Clear CT: 0.1 ms]
   [Other: 0.3 ms]
      [Choose CSet: 0.0 ms]
      [Ref Proc: 0.1 ms]
      [Ref Enq: 0.0 ms]
      [Redirty Cards: 0.2 ms]
      [Humongous Register: 0.0 ms]
      [Humongous Reclaim: 0.0 ms]
      [Free CSet: 0.0 ms]
   [Eden: 0.0B(2048.0K)->0.0B(2048.0K) Survivors: 0.0B->0.0B Heap: 4988.1K(10.0M)->4988.1K(10.0M)]
 [Times: user=0.00 sys=0.00, real=0.00 secs] 
[Full GC (Allocation Failure)  4988K->4988K(10M), 0.0028774 secs]
   [Eden: 0.0B(2048.0K)->0.0B(2048.0K) Survivors: 0.0B->0.0B Heap: 4988.1K(10.0M)->4988.1K(10.0M)], [Metaspace: 3280K->3280K(1056768K)]
 [Times: user=0.00 sys=0.00, real=0.00 secs] 
[Full GC (Allocation Failure)  4988K->4968K(10M), 0.0025809 secs]
   [Eden: 0.0B(2048.0K)->0.0B(2048.0K) Survivors: 0.0B->0.0B Heap: 4988.1K(10.0M)->4968.3K(10.0M)], [Metaspace: 3280K->3280K(1056768K)]
 [Times: user=0.00 sys=0.00, real=0.00 secs] 
[GC concurrent-mark-abort]
java.lang.OutOfMemoryError: Java heap space
  at java.util.Arrays.copyOf(Arrays.java:3332)
  at java.lang.AbstractStringBuilder.expandCapacity(AbstractStringBuilder.java:137)
  at java.lang.AbstractStringBuilder.ensureCapacityInternal(AbstractStringBuilder.java:121)
  at java.lang.AbstractStringBuilder.append(AbstractStringBuilder.java:647)
  at java.lang.StringBuilder.append(StringBuilder.java:208)
  at g1gc8.GCDemo.main(GCDemo.java:53)
Heap
 garbage-first heap   total 10240K, used 4968K [0x00000000ff600000, 0x00000000ff700050, 0x0000000100000000)
  region size 1024K, 1 young (1024K), 0 survivors (0K)
 Metaspace       used 3311K, capacity 4496K, committed 4864K, reserved 1056768K
  class space    used 361K, capacity 388K, committed 512K, reserved 1048576K
Process finished with exit code 0

Heap
garbage-first heap   横跨新老区
Metaspace

常用配置参数(了解)

开发人员仅仅需要申明以下参数即可

三步归纳：开始G1+设置最大内存+设置最大停顿时间


-XX:+UseG1GC -Xmx32G -XX:MaxGCPauseMillis=100


-XX:MaxGCPauseMillis=n：最大GC停顿时间单位毫秒，这是个软目标，JVM尽可能停顿小于这个时间


XX:+UseG1GC


XX:G1HeapRegionSize=n:设置的G1区域的大小。值是2的幂，范围是1MB到32MB。目标是根据最小的Java堆大小划分出约2048个区域。


XX:MaxGCPauseMillis=n:最大GC停顿时间，这是个软目标，JVM将尽可能（但不保证停顿小于这个时间）


XX:InitiatingHeapOccupancyPercent=n:堆占用了多少的时候就触发Gc，默认为45


Xx:ConcGCThreads=n:并发GC使用的线程数


XX:G1ReservePercent=n;设置作为空闲空间的预留内存百分比，以降低目标空间溢出的风险，默认值是10%

和CMS相比的优势

两个优势

  • G1不会产生内碎片
  • 是可以精准控制停顿。该收集器是把整个堆（新生代、老年代）划分成多个固定大小的区域，每次根据允许停顿的时间去收集垃圾最多的区域。

小总结



最后

2022 10/13 16：15

p101~p103

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