5.JVM-G1垃圾回收器

一、什么是G1

二、G1的三种垃圾回收方式

region默认2048

三、YGC的过程(Step1)

3.1相关代码

public class YGC1 {/*-Xmx128M -XX:+UseG1GC -XX:+PrintGCTimeStamps -XX:+PrintGCDetails -XX:+UnlockExperimentalVMOptions -XX:G1LogLevel=finest128m5% 60%6.4M  75M*/private static final int size = 1024 * 256;public static void main(String[] args) {byte[] data = new byte[1024 * 256];for (int i = 0; i < 100; i++) {data = new byte[1024 * 256];}}
}

这里Egen区会扩容,多次垃圾回收会 25M -> 75M

0.147: [GC pause (G1 Evacuation Pause) (young), 0.0027754 secs][Parallel Time: 1.3 ms, GC Workers: 10][GC Worker Start (ms):  147.4  147.4  147.4  147.4  147.4  147.9  147.9  148.3  148.3  148.4Min: 147.4, Avg: 147.8, Max: 148.4, Diff: 1.0][Ext Root Scanning (ms):  0.4  0.3  0.2  0.2  0.2  0.0  0.0  0.0  0.0  0.0Min: 0.0, Avg: 0.1, Max: 0.4, Diff: 0.4, Sum: 1.1][Thread Roots (ms):  0.4  0.0  0.1  0.1  0.0  0.0  0.0  0.0  0.0  0.0Min: 0.0, Avg: 0.1, Max: 0.4, Diff: 0.4, Sum: 0.5][StringTable Roots (ms):  0.0  0.0  0.1  0.1  0.1  0.0  0.0  0.0  0.0  0.0Min: 0.0, Avg: 0.0, Max: 0.1, Diff: 0.1, Sum: 0.3][Universe Roots (ms):  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0][JNI Handles Roots (ms):  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0][ObjectSynchronizer Roots (ms):  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0][FlatProfiler Roots (ms):  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0][Management Roots (ms):  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0][SystemDictionary Roots (ms):  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0][CLDG Roots (ms):  0.0  0.3  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0Min: 0.0, Avg: 0.0, Max: 0.3, Diff: 0.3, Sum: 0.3][JVMTI Roots (ms):  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0][CodeCache Roots (ms):  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0][CM RefProcessor Roots (ms):  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0][Wait For Strong CLD (ms):  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0][Weak CLD Roots (ms):  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0][SATB Filtering (ms):  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0][Update RS (ms):  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0][Processed Buffers:  0  0  0  0  0  0  0  0  0  0Min: 0, Avg: 0.0, Max: 0, Diff: 0, Sum: 0][Scan RS (ms):  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0][Code Root Scanning (ms):  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0][Object Copy (ms):  0.5  0.6  0.7  0.7  0.7  0.4  0.4  0.0  0.0  0.0Min: 0.0, Avg: 0.4, Max: 0.7, Diff: 0.7, Sum: 3.9][Termination (ms):  0.1  0.1  0.1  0.1  0.1  0.1  0.2  0.0  0.0  0.0Min: 0.0, Avg: 0.1, Max: 0.2, Diff: 0.2, Sum: 0.9][Termination Attempts:  5  2  3  4  3  3  4  1  1  1Min: 1, Avg: 2.7, Max: 5, Diff: 4, Sum: 27][GC Worker Other (ms):  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.3][GC Worker Total (ms):  1.0  1.0  1.0  1.0  1.0  0.6  0.6  0.1  0.1  0.0Min: 0.0, Avg: 0.6, Max: 1.0, Diff: 1.0, Sum: 6.3][GC Worker End (ms):  148.4  148.4  148.4  148.4  148.4  148.4  148.5  148.4  148.4  148.4Min: 148.4, Avg: 148.4, Max: 148.5, Diff: 0.1][Code Root Fixup: 0.0 ms][Code Root Purge: 0.0 ms][Clear CT: 0.4 ms][Other: 1.1 ms][Choose CSet: 0.0 ms][Ref Proc: 0.6 ms][Ref Enq: 0.0 ms][Redirty Cards: 0.3 ms][Parallel Redirty:  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0][Redirtied Cards:  0  0  0  0  0  0  0  0  0  0Min: 0, Avg: 0.0, Max: 0, Diff: 0, Sum: 0][Humongous Register: 0.0 ms][Humongous Total: 0][Humongous Candidate: 0][Humongous Reclaim: 0.0 ms][Humongous Reclaimed: 0][Free CSet: 0.0 ms][Young Free CSet: 0.0 ms][Non-Young Free CSet: 0.0 ms][Eden: 24576.0K(24576.0K)->0.0B(37888.0K) Survivors: 0.0B->2048.0K Heap: 24576.0K(128.0M)->1152.0K(128.0M)][Times: user=0.00 sys=0.00, real=0.00 secs] 
Heapgarbage-first heap   total 131072K, used 13440K [0x00000000f8000000, 0x00000000f8100400, 0x0000000100000000)region size 1024K, 15 young (15360K), 2 survivors (2048K)Metaspace       used 3458K, capacity 4496K, committed 4864K, reserved 1056768Kclass space    used 370K, capacity 388K, committed 512K, reserved 1048576K

3.2停顿预测模型与垃圾回收区域选择

-XX:MaxGCPauseMillis=1 最大停顿时间1ms

Eden区域 大小从 6M 缩小到5M 尽可能满足,停顿时间

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3.2.1 如何判断先回收哪个区域

垃圾数量占Regen的百分比,优先gc百分比高的

3.2.2停顿预测模型

可以手动指定,G1会尽可能满足,默认是根据**响应时间优先算法来决定 **根据前面gc的时间整体判断这次的时间

四、MixGC的过程(Step2)

1. 初始标记阶段_1

2. 并发标记_2

3. 再标记阶段_3

4. 统计对象计数_4

整理统计每个regen里面垃圾的占比,并且排序

5. 垃圾回收_5

根据上一步算出来的gc顺序进行gc

6. 完整回收流程

6.1 为什么并发标记之前有一次YGC

YGC之后,将整理后的S区整合,清理掉不用的Eden区,根据这个S区开始作为根的一部分进行扫描

6.2 为什么混合回收会多次进行

1. 混合回收多次执行 为了符合 停顿预测模型的时间
2. 混合回收之前的YGC 为了回收的时候年轻代"干净";

垃圾占Regen 少于 15% 就不会被统计

判断GC之后 可回收空间>5% 才会执行

最多这个流程分成8次,为了符合停顿时间

五FullGC的过程(Step3)

5.1 什么情况下触发FullGC

5.2 FullGC具体过程

5.2.1FullGC如何复制对象

使用标记压缩

5.2.2 标记活跃对象(Step1)

5.2.3 计算引用对象地址(Step2)

计算gc之后存活对象所处的位置

5.2.4 更新引用对象地址(Step3)

遍历所有存活对象,告诉他们去的地址

5.2.5 复制对象(Step4)

使用标记压缩

5.2.6 复制后的处理(Step5)

5.3 大对象怎么存储呢?

默认使用多个region来存储,并且直接进入老年代,使得老年代回收变得非常慢

解决方案: 大对象不要进入老年代,不行就手动加大region大小

public class HE {/*-Xmx128M -XX:+UseG1GC -XX:+PrintGCTimeStamps -XX:+PrintGCDetails-XX:G1HeapRegionSize=4M      手动指定region大小128m5% 60%6.4M  75M*/private static final int size = 1024 * 4096;public static void main(String[] args) {byte[] data = new byte[1022 * 4096];for (int i = 0; i < 50; i++) {data = new byte[1022 * 4096];}}
}