Survivor空間溢出實例

jvm系列

• 垃圾回收基礎

• JVM的編譯策略

• GC的三大基礎算法

• GC的三大高級算法

• GC策略的評價指標

• JVM信息查看

• GC通用日誌解讀

• jvm的card table數據結構

• Java類初始化順序

• Java對象結構及大小計算

• Java的類加載機制

• Java對象分配簡要流程

• 年老代過大有什麼影響

• Survivor空間溢出實例

• 關於Object=null

• Java線程與Xss

序

本文演示survivor空間溢出的場景。

實例代碼

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());
        }
    }

輸出

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
這個狀況是survivor空間溢出，survivor空間被佔滿，而後溢出的部分，直接放到了年老代（2401K），溢出以後，計算survivor空間裏頭對象的年齡分佈，發現年齡爲1的對象大小總和超過了survivor的desired值，因而設置新的閾值爲該age=1。
（每次young gc以後打印survivor區域內對象的年齡分佈）
若是底下age的total大小大於Desired survivor size的大小，那麼就表明了survivor空間溢出了，被填滿，而後會從新計算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.

小結

並非都得等到對象年齡達到晉升閾值才提高到年老代

若是在Survivor空間中相同年齡全部對象大小的總和>Survivor空間的一半（ -XX:TargetSurvivorRatio）時，年齡>=該年齡的對象就能夠直接進入年老代

Desired survivor size = (survivor_capacity TargetSurvivorRatio) / 100 sizeof(a pointer)：survivor_capacity（一個survivor space的大小）乘以TargetSurvivorRatio

-XX:TargetSurvivorRatio

目標存活率，默認爲50%
代表全部age的survivor space對象的大小若是超過Desired survivor size，則從新計算threshold，以age和MaxTenuringThreshold的最小值爲準，不然以MaxTenuringThreshold爲準.

-XX:TargetSurvivorRatio

目標Survivor空間佔用是HotSpot嘗試在MinorGC以後仍然維持的Survivor空間佔用，默認值爲50是由於HotSpot研發團隊對不一樣類型的應用程序進行了大量的負荷測試，結果代表50%的目標Survivor空間佔用能適應大多數應用程序，能應對MinorGC時存活對象的急速增長。