用法: -XX:MaxTenuringThreshold=15
該參數主要是控制新生代需要經歷多少次GC晉升到老年代中的最大閾值。在JVM中用4個bit存儲（放在對象頭中），（1111）所以其最大值是15。
但並非意味着，對象必須要經歷15次YGC才會晉升到老年代中。例如，當survivor區空間不夠時，便會提前進入到老年代中，但這個次數一定不大於設置的最大閾值。
那么JVM到底是如何來計算S區對象晉升到Old區的呢？
首先介紹另一個重要的JVM參數：-XX:TargetSurvivorRatio：一個計算期望s區存活大小(Desired survivor size)的參數。默認值為50，即50%。
當一個S區中所有的age對象的大小如果大於等於Desired survivor size，則重新計算threshold，以age和MaxTenuringThreshold兩者的最小值為准。
以一個Demo為例:
//-Xmx200M -Xmn50m -XX:TargetSurvivorRatio=60 -XX:+PrintGCDetails -XX:+PrintGCDateStamps -XX:+UseConcMarkSweepGC -XX:+UseParNewGC -XX:MaxTenuringThreshold=3 //最小堆為50M，默認SurvivorRatio為8，那么可以知道Eden區為40M，S0和S1為5M public class App { public static void main(String[] args) throws InterruptedException { // main方法作為主線程，變量不會被回收 byte[] byte1 = new byte[1 * 1024 * 1024]; byte[] byte2 = new byte[1 * 1024 * 1024]; YGC(40); Thread.sleep(3000); YGC(40); Thread.sleep(3000); YGC(40); Thread.sleep(3000); // 這次再ygc時, 由於byte1和byte2的年齡經過3次ygc后已經達到3(-XX:MaxTenuringThreshold=3), 所以會晉升到old YGC(40); // ygc后, s0(from)/s1(to)的空間為0 Thread.sleep(3000); // 達到TargetSurvivorRatio這個比例指定的值,即 5M(S區)*60%(TargetSurvivorRatio)=3M(Desired survivor size) byte[] byte4 = new byte[1 * 1024 * 1024]; byte[] byte5 = new byte[1 * 1024 * 1024]; byte[] byte6 = new byte[1 * 1024 * 1024]; // 這次ygc時, 由於s區已經占用達到了60%(-XX:TargetSurvivorRatio=60), // 所以會重新計算對象晉升的min(age, MaxTenuringThreshold) = 1 YGC(40); Thread.sleep(3000); // 由於前一次ygc時算出age=1, 所以這一次再ygc時, byte4, byte5, byte6就要晉升到Old, // 而不需要等MaxTenuringThreshold這么多次, 此次ygc后, s0(from)/s1(to)的空間再次為0, 對象全部晉升到old YGC(40); Thread.sleep(3000); System.out.println("GC end!"); } //塞滿Eden區，局部變量會被回收,作為觸發GC的小工具 private static void YGC(int edenSize){ for (int i = 0 ; i < edenSize ; i ++) { byte[] byte1m = new byte[1 * 1024 * 1024]; } } }
可以看到結果
//第一次YGC 2017-07-22T17:43:50.615-0800: [GC (Allocation Failure) 2017-07-22T17:43:50.615-0800: [ParNew: 39936K->2812K(46080K), 0.0126581 secs] 39936K->2812K(125952K), 0.0127387 secs] [Times: user=0.00 sys=0.00, real=0.01 secs] //第二次YGC 2017-07-22T17:43:53.653-0800: [GC (Allocation Failure) 2017-07-22T17:43:53.653-0800: [ParNew: 43542K->2805K(46080K), 0.0144079 secs] 43542K->2805K(125952K), 0.0144607 secs] [Times: user=0.01 sys=0.00, real=0.01 secs] //第三次YGC 2017-07-22T17:43:56.679-0800: [GC (Allocation Failure) 2017-07-22T17:43:56.679-0800: [ParNew: 43329K->2877K(46080K), 0.0010447 secs] 43329K->2877K(125952K), 0.0010784 secs] [Times: user=0.01 sys=0.00, real=0.01 secs] //三次YGC后，此時age達到MaxTenuringThreshold閾值3，再次YGC時，會晉升到Old區，可以看到此時新生代空間為0 2017-07-22T17:43:59.691-0800: [GC (Allocation Failure) 2017-07-22T17:43:59.691-0800: [ParNew: 43604K->0K(46080K), 0.0065182 secs] 43604K->2675K(125952K), 0.0065664 secs] [Times: user=0.01 sys=0.01, real=0.00 secs] //分配3M不回收的對象，經歷一次YGC，此時age=1 2017-07-22T17:44:02.708-0800: [GC (Allocation Failure) 2017-07-22T17:44:02.708-0800: [ParNew: 40731K->3072K(46080K), 0.0050035 secs] 43406K->5747K(125952K), 0.0050444 secs] [Times: user=0.00 sys=0.00, real=0.01 secs] //此時可以看到，3M的對象也被晉升到了Old區，這可以證明，如果只靠MaxTenuringThreshold來決定多少次YGC才晉升到Old區的話，此時age=1，並沒有達到閾值。 //因為此時3m達到TargetSurvivorRatio=60（5M*60=3m）的要求，那么前一次YGC，會重新計算對象晉升的threshold=min(age, MaxTenuringThreshold) = min(1,3) = 1次，所以此時晉升到了Old區 2017-07-22T17:44:05.722-0800: [GC (Allocation Failure) 2017-07-22T17:44:05.722-0800: [ParNew: 43806K->0K(46080K), 0.0060735 secs] 46481K->5747K(125952K), 0.0061215 secs] [Times: user=0.01 sys=0.00, real=0.00 secs] GC end! Heap par new generation total 46080K, used 18003K [0x00000007b3800000, 0x00000007b6a00000, 0x00000007b6a00000) eden space 40960K, 43% used [0x00000007b3800000, 0x00000007b4994d88, 0x00000007b6000000) from space 5120K, 0% used [0x00000007b6000000, 0x00000007b6000000, 0x00000007b6500000) to space 5120K, 0% used [0x00000007b6500000, 0x00000007b6500000, 0x00000007b6a00000) concurrent mark-sweep generation total 79872K, used 5747K [0x00000007b6a00000, 0x00000007bb800000, 0x00000007c0000000) Metaspace used 3267K, capacity 4494K, committed 4864K, reserved 1056768K class space used 354K, capacity 386K, committed 512K, reserved 1048576K
另外你可以在JVM啟動參數中加上-XX:+PrintTenuringDistribution，該參數可以輸出age的額外信息。
2017-07-22T18:07:51.401-0800: [GC (Allocation Failure) 2017-07-22T18:07:51.401-0800: [ParNew Desired survivor size 3145728 bytes, new threshold 3 (max 3) - age 1: 2854440 bytes, 2854440 total : 39936K->2819K(46080K), 0.0136187 secs] 39936K->2819K(125952K), 0.0138705 secs] [Times: user=0.01 sys=0.01, real=0.01 secs] 2017-07-22T18:07:54.452-0800: [GC (Allocation Failure) 2017-07-22T18:07:54.452-0800: [ParNew Desired survivor size 3145728 bytes, new threshold 3 (max 3) - age 2: 2682504 bytes, 2682504 total : 43549K->2918K(46080K), 0.0172463 secs] 43549K->2918K(125952K), 0.0173506 secs] [Times: user=0.00 sys=0.00, real=0.02 secs] 2017-07-22T18:07:57.484-0800: [GC (Allocation Failure) 2017-07-22T18:07:57.484-0800: [ParNew Desired survivor size 3145728 bytes, new threshold 3 (max 3) - age 3: 2680152 bytes, 2680152 total : 43442K->2723K(46080K), 0.0115410 secs] 43442K->2723K(125952K), 0.0116179 secs] [Times: user=0.00 sys=0.00, real=0.01 secs] 2017-07-22T18:08:00.511-0800: [GC (Allocation Failure) 2017-07-22T18:08:00.511-0800: [ParNew Desired survivor size 3145728 bytes, new threshold 3 (max 3) : 43449K->0K(46080K), 0.0226608 secs] 43449K->2673K(125952K), 0.0228187 secs] [Times: user=0.03 sys=0.00, real=0.02 secs] 2017-07-22T18:08:03.547-0800: [GC (Allocation Failure) 2017-07-22T18:08:03.547-0800: [ParNew Desired survivor size 3145728 bytes, new threshold 1 (max 3) - age 1: 3145776 bytes, 3145776 total : 40731K->3072K(46080K), 0.0055686 secs] 43404K->5745K(125952K), 0.0056352 secs] [Times: user=0.01 sys=0.00, real=0.01 secs] 2017-07-22T18:08:06.572-0800: [GC (Allocation Failure) 2017-07-22T18:08:06.572-0800: [ParNew Desired survivor size 3145728 bytes, new threshold 3 (max 3) : 43806K->0K(46080K), 0.0068945 secs] 46479K->5745K(125952K), 0.0070480 secs] [Times: user=0.00 sys=0.01, real=0.00 secs] GC end! Heap par new generation total 46080K, used 18003K [0x00000007b3800000, 0x00000007b6a00000, 0x00000007b6a00000) eden space 40960K, 43% used [0x00000007b3800000, 0x00000007b4994d88, 0x00000007b6000000) from space 5120K, 0% used [0x00000007b6000000, 0x00000007b6000000, 0x00000007b6500000) to space 5120K, 0% used [0x00000007b6500000, 0x00000007b6500000, 0x00000007b6a00000) concurrent mark-sweep generation total 79872K, used 5745K [0x00000007b6a00000, 0x00000007bb800000, 0x00000007c0000000) Metaspace used 3273K, capacity 4494K, committed 4864K, reserved 1056768K class space used 354K, capacity 386K, committed 512K, reserved 1048576K
https://www.jianshu.com/p/2e69aa552b01