JVM参数:MaxTenuringThreshold和TargetSurvivorRatio说明

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-XX:MaxTenuringThreshold
在新生代中对象存活次数(经过Minor GC的次数)后仍然存活，就会晋升到旧生代。

-XX:TargetSurvivorRatio
一个计算期望存活大小Desired survivor size的参数.

计算公式： (survivor_capacity * TargetSurvivorRatio) / 100 * sizeof(a pointer)：survivor_capacity（一个survivor space的大小）乘以TargetSurvivorRatio，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.

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public class GCTenuringThreshold {      public static void main(String[] args) throws Exception {          GCMemoryObject object1 = new GCMemoryObject(2);          GCMemoryObject object2 = new GCMemoryObject(8);          GCMemoryObject object3 = new GCMemoryObject(8);          GCMemoryObject object4 = new GCMemoryObject(8);          object2 = null;          object3 = null;          GCMemoryObject object5 = new GCMemoryObject(8);          Thread.sleep(4000);          object2 = new GCMemoryObject(8);          object3 = new GCMemoryObject(8);          object2 = null;          object3 = null;          object5 = null;          GCMemoryObject object6 = new GCMemoryObject(8);          Thread.sleep(5000);           GCMemoryObject object7 = new GCMemoryObject(8);          GCMemoryObject object8 = new GCMemoryObject(8);          GCMemoryObject object9 = new GCMemoryObject(8);          GCMemoryObject object10 = new GCMemoryObject(8);          //object6 = null;          Thread.sleep(5000);          System.out.println("ok");      }  }

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class GCMemoryObject {      private byte[] bytes = null;      public GCMemoryObject(int multi) {           bytes = new byte[1024 * 256 * multi];        }  }

以-Xms20M –Xmx20M –Xmn10M –XX:+UseSerialGC参数执行以上代码，
输出如下：

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[GC [DefNewDesired survivor size 524288 bytes, new threshold 1 (max 15)- age   1:     677968 bytes,     677968 total: 6999K->662K(9216K), 0.0028753 secs] 6999K->2710K(19456K), 0.0029048 secs] [Times: user=0.00 sys=0.00, real=0.00 secs][GC [DefNewDesired survivor size 524288 bytes, new threshold 15 (max 15)- age   1:         16 bytes,         16 total: 6891K->0K(9216K), 0.0025312 secs] 8939K->2709K(19456K), 0.0026029 secs] [Times: user=0.00 sys=0.00, real=0.00 secs][GC [DefNewDesired survivor size 524288 bytes, new threshold 15 (max 15)- age   1:         48 bytes,         48 total- age   2:         16 bytes,         64 total: 6200K->0K(9216K), 0.0095025 secs] 8910K->8853K(19456K), 0.0095770 secs] [Times: user=0.00 sys=0.00, real=0.01 secs]okHeap def new generation   total 9216K, used 4297K [0x32310000, 0x32d10000, 0x32d10000)  eden space 8192K,  52% used [0x32310000, 0x32742508, 0x32b10000)  from space 1024K,   0% used [0x32c10000, 0x32c10040, 0x32d10000)  to   space 1024K,   0% used [0x32b10000, 0x32b10000, 0x32c10000) tenured generation   total 10240K, used 8853K [0x32d10000, 0x33710000, 0x33710000)   the space 10240K,  86% used [0x32d10000, 0x335b57a8, 0x335b5800, 0x33710000) compacting perm gen  total 12288K, used 369K [0x33710000, 0x34310000, 0x37710000)   the space 12288K,   3% used [0x33710000, 0x3376c438, 0x3376c600, 0x34310000)    ro space 10240K,  51% used [0x37710000, 0x37c3b700, 0x37c3b800, 0x38110000)    rw space 12288K,  54% used [0x38110000, 0x387a76c0, 0x387a7800, 0x38d10000)

可以看出，每次都重新计算threshhold,或者说从survivor space晋升到旧生代后，重新计算threshold。
SurvivorRatio过大，都应该是age为准，因为触发gc的条件没有达到，但survivor space/2的条件优先达到。
所以调小SurvivorRatio，可以达到MaxTenuringThreshold次数还存活的对象。代码如下：

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public class GCTenuringThreshold1 {       //private  GCMemoryObject1 ng = new GCMemoryObject1(15);       public static void main(String[] args) throws Exception {          GCMemoryObject1 object2 = new GCMemoryObject1(0.1f);          GCMemoryObject1 object1 = new GCMemoryObject1(2f);          Thread.sleep(4000);           object1 = null;          GCMemoryObject1 object4 = new GCMemoryObject1(0.1f);          GCMemoryObject1 object3 = new GCMemoryObject1(2f);           Thread.sleep(4000);          object3 = null;          GCMemoryObject1 object6 = new GCMemoryObject1(0.3f);          GCMemoryObject1 object5 = new GCMemoryObject1(2f);            Thread.sleep(4000);          object5 = null;          GCMemoryObject1 object8 = new GCMemoryObject1(0.3f);          GCMemoryObject1 object7 = new GCMemoryObject1(2f);             Thread.sleep(4000);          object7 = null;          GCMemoryObject1 object10 = new GCMemoryObject1(0.3f);          GCMemoryObject1 object9 = new GCMemoryObject1(2f);            System.out.println("ok");      }  }   class GCMemoryObject1 {      private byte[] bytes = null;      public GCMemoryObject1(float multi) {           bytes = new byte[(int)(1024 * 1024*multi)];        }  }

以-Xms20M -Xmx20M -Xmn10M -XX:SurvivorRatio=1 -XX:+UseSerialGC -XX:MaxTenuringThreshold=3 -XX:+PrintTenuringDistribution -XX:+PrintGCDetails
运行上述代码。

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 [GC [DefNewDesired survivor size 1736704 bytes, new threshold 3 (max 3)- age   1:     363408 bytes,     363408 total: 2529K->354K(6848K), 0.0017645 secs] 2529K->354K(17088K), 0.0018275 secs] [Times: user=0.00 sys=0.00, real=0.00 secs][GC [DefNewDesired survivor size 1736704 bytes, new threshold 3 (max 3)- age   1:     314616 bytes,     314616 total- age   2:     363176 bytes,     677792 total: 2748K->661K(6848K), 0.0009114 secs] 2748K->661K(17088K), 0.0009399 secs] [Times: user=0.00 sys=0.00, real=0.00 secs][GC [DefNewDesired survivor size 1736704 bytes, new threshold 3 (max 3)- age   1:     314616 bytes,     314616 total- age   2:     314600 bytes,     629216 total- age   3:     363176 bytes,     992392 total: 3042K->969K(6848K), 0.0010587 secs] 3042K->969K(17088K), 0.0010872 secs] [Times: user=0.00 sys=0.00, real=0.00 secs][GC [DefNewDesired survivor size 1736704 bytes, new threshold 3 (max 3)- age   1:     314616 bytes,     314616 total- age   2:     314600 bytes,     629216 total- age   3:     314600 bytes,     943816 total: 3341K->921K(6848K), 0.0012151 secs] 3341K->1276K(17088K), 0.0012443 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]okHeap def new generation   total 6848K, used 3049K [0x32310000, 0x32d10000, 0x32d10000)  eden space 3456K,  61% used [0x32310000, 0x32524128, 0x32670000)  from space 3392K,  27% used [0x32670000, 0x327566c8, 0x329c0000)  to   space 3392K,   0% used [0x329c0000, 0x329c0000, 0x32d10000) tenured generation   total 10240K, used 354K [0x32d10000, 0x33710000, 0x33710000)   the space 10240K,   3% used [0x32d10000, 0x32d68aa8, 0x32d68c00, 0x33710000) compacting perm gen  total 12288K, used 369K [0x33710000, 0x34310000, 0x37710000)   the space 12288K,   3% used [0x33710000, 0x3376c430, 0x3376c600, 0x34310000)    ro space 10240K,  51% used [0x37710000, 0x37c3b700, 0x37c3b800, 0x38110000)    rw space 12288K,  54% used [0x38110000, 0x387a76c0, 0x387a7800, 0x38d10000)

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