GC 回收实战

ゞ浴缸里的玫瑰 2022-08-29 09:22 107阅读 0赞

# 一 代码 #

/**
    * 在 jdk7 和 jdk8 中分别执行
    * -Xms20M -Xmx20M -Xmn10M -XX:+PrintGCDetails -XX:SurvivorRatio=8 -XX:+UseSerialGC
    */
    public class GCLogTest1 {
        private static final int _1MB = 1024 * 1024;
    
        public static void testAllocation() {
            byte[] allocation1, allocation2, allocation3, allocation4;
            allocation1 = new byte[2 * _1MB];
            allocation2 = new byte[2 * _1MB];
            allocation3 = new byte[2 * _1MB];
            allocation4 = new byte[4 * _1MB];
        }
    
        public static void main(String[] agrs) {
            testAllocation();
        }
    }

## 二 JDK 7 测试—— -Xms20M -Xmx20M -Xmn10M -XX:+PrintGCDetails -XX:SurvivorRatio=8 -XX:+UseSerialGC ##

## 1 测试结果 ##

> D:\\ProgramFiles\\Java\\jdk1.7.0\_80\\bin\\java.exe -Xms20M -Xmx20M -Xmn10M -XX:+PrintGCDetails -XX:SurvivorRatio=8 -XX:+UseSerialGC -javaagent:D:\\ProgramFiles\\JetBrains\\IDEA\\lib\\idea\_rt.jar=52005:D:\\ProgramFiles\\JetBrains\\IDEA\\bin -Dfile.encoding=UTF-8 -classpath D:\\ProgramFiles\\Java\\jdk1.7.0\_80\\lib\\ant-javafx.jar;D:\\ProgramFiles\\Java\\jdk1.7.0\_80\\lib\\dt.jar;D:\\ProgramFiles\\Java\\jdk1.7.0\_80\\lib\\javafx-doclet.jar;D:\\ProgramFiles\\Java\\jdk1.7.0\_80\\lib\\javafx-mx.jar;D:\\ProgramFiles\\Java\\jdk1.7.0\_80\\lib\\jconsole.jar;D:\\ProgramFiles\\Java\\jdk1.7.0\_80\\lib\\sa-jdi.jar;D:\\ProgramFiles\\Java\\jdk1.7.0\_80\\lib\\tools.jar;E:\\JVMDemo\\out\\production\\chapter17 com.atguigu.java.GCLogTest1
> 
> \[GC\[DefNew: 7649K->510K(9216K), 0.0048118 secs\] 7649K->6654K(19456K), 0.0048629 secs\] \[Times: user=0.00 sys=0.03, real=0.01 secs\]
> 
> Heap
> 
> def new generation   total 9216K, used 4855K \[0x00000000f9a00000, 0x00000000fa400000, 0x00000000fa400000)
> 
>   eden space 8192K,  53% used \[0x00000000f9a00000, 0x00000000f9e3e600, 0x00000000fa200000)
> 
>   from space 1024K,  49% used \[0x00000000fa300000, 0x00000000fa37f8e0, 0x00000000fa400000)
> 
>   to   space 1024K,   0% used \[0x00000000fa200000, 0x00000000fa200000, 0x00000000fa300000)
> 
> tenured generation   total 10240K, used 6144K \[0x00000000fa400000, 0x00000000fae00000, 0x00000000fae00000)
> 
>    the space 10240K,  60% used \[0x00000000fa400000, 0x00000000faa00030, 0x00000000faa00200, 0x00000000fae00000)
> 
> compacting perm gen  total 21248K, used 2925K \[0x00000000fae00000, 0x00000000fc2c0000, 0x0000000100000000)
> 
>    the space 21248K,  13% used \[0x00000000fae00000, 0x00000000fb0db568, 0x00000000fb0db600, 0x00000000fc2c0000)
> 
> No shared spaces configured.

## 2 图解 ##

首先会将3个 2M 的数组存放到 Eden 区，然后后面 4M 的数组来了后，将无法存储，因为Eden 区只剩下 2M 的剩余空间了，那么将会进行一次 Young GC 操作，将原来 Eden 区的内容，存放到 Survivor 区，但是 Survivor 区也存放不下，那么就会直接晋级存入Old 区。

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZW5ncWl1bWluZw_size_16_color_FFFFFF_t_70][]

然后将 4M 对象存入到 Eden 区中

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZW5ncWl1bWluZw_size_16_color_FFFFFF_t_70 1][]

# 三 JDK 8 测试—— -Xms20M -Xmx20M -Xmn10M -XX:+PrintGCDetails -XX:SurvivorRatio=8 -XX:+UseSerialGC #

测试结果

> \[GC (Allocation Failure) \[DefNew: 7964K->595K(9216K), 0.0050876 secs\] 7964K->6739K(19456K), 0.0051507 secs\] \[Times: user=0.00 sys=0.00, real=0.01 secs\]
> 
> Heap
> 
> def new generation   total 9216K, used 4857K \[0x00000000fec00000, 0x00000000ff600000, 0x00000000ff600000)
> 
>   eden space 8192K,  52% used \[0x00000000fec00000, 0x00000000ff0297b0, 0x00000000ff400000)
> 
>   from space 1024K,  58% used \[0x00000000ff500000, 0x00000000ff594c80, 0x00000000ff600000)
> 
>   to   space 1024K,   0% used \[0x00000000ff400000, 0x00000000ff400000, 0x00000000ff500000)
> 
> tenured generation   total 10240K, used 6144K \[0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
> 
>    the space 10240K,  60% used \[0x00000000ff600000, 0x00000000ffc00030, 0x00000000ffc00200, 0x0000000100000000)
> 
> Metaspace       used 3135K, capacity 4496K, committed 4864K, reserved 1056768K
> 
>   class space    used 341K, capacity 388K, committed 512K, reserved 1048576K

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