Survivor空间溢出实例

jvm系列

• 垃圾回收基础

• JVM的编译策略

• GC的三大基础算法

• GC的三大高级算法

• GC策略的评价指标

• JVM信息查看

• GC通用日志解读

• jvm的card table数据结构

• Java类初始化顺序

• Java对象结构及大小计算

• Java的类加载机制

• Java对象分配简要流程

• 年老代过大有什么影响

• Survivor空间溢出实例

• 关于Object=null

• Java线程与Xss

序

本文演示survivor空间溢出的场景。

实例代码

class MemoryObject{
    private byte[] bytes;

    public MemoryObject(int objectSize){
        this.bytes = new byte[objectSize];
    }
}

/**
     * eden 13184K ,s0/s1 1600k 共16M
     * old 24m

-Xms40m -Xmx40m -Xmn16m -verbose:gc -XX:+PrintGCDetails -XX:+PrintHeapAtGC -XX:+PrintTenuringDistribution

     * @throws InterruptedException
     */
    public static void survivorNotEnough2OldSpace() throws InterruptedException {
//        Thread.sleep(50000);
        MemoryObject object = new MemoryObject(1024*1024);
        MemoryObject m2Object = new MemoryObject(1024*1024*2);
        happenMinorGC(9);
        Thread.sleep(2000);
    }

private static void happenMinorGC(int happenMinorGCIndex) throws InterruptedException {
        for(int i=0;i<happenMinorGCIndex;i++){
            if(i == happenMinorGCIndex - 1){
//                Thread.sleep(2000);
                System.out.println("minor gc should happen:"+printCurrent());
            }
            new MemoryObject(1024*1024);
            System.out.println("allocate last:"+printCurrent());
        }
    }

输出

allocate last:2014-11-04 15-43-18-829
allocate last:2014-11-04 15-43-18-830
allocate last:2014-11-04 15-43-18-830
allocate last:2014-11-04 15-43-18-831
allocate last:2014-11-04 15-43-18-831
allocate last:2014-11-04 15-43-18-832
allocate last:2014-11-04 15-43-18-833
{Heap before GC invocations=0 (full 0):
 def new generation   total 14784K, used 13182K [0x00000007f8600000, 0x00000007f9600000, 0x00000007f9600000)
  eden space 13184K,  99% used [0x00000007f8600000, 0x00000007f92df8b0, 0x00000007f92e0000)
  from space 1600K,   0% used [0x00000007f92e0000, 0x00000007f92e0000, 0x00000007f9470000)
  to   space 1600K,   0% used [0x00000007f9470000, 0x00000007f9470000, 0x00000007f9600000)
 tenured generation   total 24576K, used 0K [0x00000007f9600000, 0x00000007fae00000, 0x00000007fae00000)
   the space 24576K,   0% used [0x00000007f9600000, 0x00000007f9600000, 0x00000007f9600200, 0x00000007fae00000)
 compacting perm gen  total 21248K, used 3994K [0x00000007fae00000, 0x00000007fc2c0000, 0x0000000800000000)
   the space 21248K,  18% used [0x00000007fae00000, 0x00000007fb1e6950, 0x00000007fb1e6a00, 0x00000007fc2c0000)
No shared spaces configured.
[GC [DefNew
Desired survivor size 819200 bytes, new threshold 1 (max 15)
- age   1:    1638400 bytes（1600K）,    1638400 total
这个情况是survivor空间溢出，survivor空间被占满，然后溢出的部分，直接放到了年老代（2401K），溢出之后，计算survivor空间里头对象的年龄分布，发现年龄为1的对象大小总和超过了survivor的desired值，于是设置新的阈值为该age=1。
（每次young gc之后打印survivor区域内对象的年龄分布）
如果底下age的total大小大于Desired survivor size的大小，那么就代表了survivor空间溢出了，被填满，然后会重新计算threshold。


: 13182K->1600K(14784K), 0.0057500 secs] 13182K->4001K(39360K), 0.0057720 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
Heap after GC invocations=1 (full 0):
 def new generation   total 14784K, used 1600K [0x00000007f8600000, 0x00000007f9600000, 0x00000007f9600000)
  eden space 13184K,   0% used [0x00000007f8600000, 0x00000007f8600000, 0x00000007f92e0000)
  from space 1600K, 100% used [0x00000007f9470000, 0x00000007f9600000, 0x00000007f9600000)
  to   space 1600K,   0% used [0x00000007f92e0000, 0x00000007f92e0000, 0x00000007f9470000)
 tenured generation   total 24576K, used 2401K [0x00000007f9600000, 0x00000007fae00000, 0x00000007fae00000)
   the space 24576K,   9% used [0x00000007f9600000, 0x00000007f98586d8, 0x00000007f9858800, 0x00000007fae00000)
 compacting perm gen  total 21248K, used 3994K [0x00000007fae00000, 0x00000007fc2c0000, 0x0000000800000000)
   the space 21248K,  18% used [0x00000007fae00000, 0x00000007fb1e6950, 0x00000007fb1e6a00, 0x00000007fc2c0000)
No shared spaces configured.
}
allocate last:2014-11-04 15-43-18-839
minor gc should happen:2014-11-04 15-43-18-839
allocate last:2014-11-04 15-43-18-839
Heap
 def new generation   total 14784K, used 4202K [0x00000007f8600000, 0x00000007f9600000, 0x00000007f9600000)
  eden space 13184K,  19% used [0x00000007f8600000, 0x00000007f888a820, 0x00000007f92e0000)
  from space 1600K, 100% used [0x00000007f9470000, 0x00000007f9600000, 0x00000007f9600000)
  to   space 1600K,   0% used [0x00000007f92e0000, 0x00000007f92e0000, 0x00000007f9470000)
 tenured generation   total 24576K, used 2401K [0x00000007f9600000, 0x00000007fae00000, 0x00000007fae00000)
   the space 24576K,   9% used [0x00000007f9600000, 0x00000007f98586d8, 0x00000007f9858800, 0x00000007fae00000)
 compacting perm gen  total 21248K, used 4001K [0x00000007fae00000, 0x00000007fc2c0000, 0x0000000800000000)
   the space 21248K,  18% used [0x00000007fae00000, 0x00000007fb1e8710, 0x00000007fb1e8800, 0x00000007fc2c0000)
No shared spaces configured.

小结

并不是都得等到对象年龄达到晋升阈值才提升到年老代

如果在Survivor空间中相同年龄所有对象大小的总和>Survivor空间的一半（ -XX:TargetSurvivorRatio）时，年龄>=该年龄的对象就可以直接进入年老代

Desired survivor size = (survivor_capacity TargetSurvivorRatio) / 100 sizeof(a pointer)：survivor_capacity（一个survivor space的大小）乘以TargetSurvivorRatio

-XX:TargetSurvivorRatio

目标存活率，默认为50%
表明所有age的survivor space对象的大小如果超过Desired survivor size，则重新计算threshold，以age和MaxTenuringThreshold的最小值为准，否则以MaxTenuringThreshold为准.

-XX:TargetSurvivorRatio

目标Survivor空间占用是HotSpot尝试在MinorGC之后仍然维持的Survivor空间占用，默认值为50
是因为HotSpot研发团队对不同类型的应用程序进行了大量的负荷测试，结果表明50%的目标Survivor空间占用能适应大多数应用程序，能应对MinorGC时存活对象的急速增加。