JVM调优Demo（四）合适的年轻对象存活年龄

先上代码：

生成单位大小对象的类：

public class GCDataObject {byte[] bytes = null;public GCDataObject(int i){bytes = new byte[i *1024 * 1024];}}

测试逻辑：

public class Test_MaxTenuringThreshold {public static void main(String[] args){//1-------------------------------------GCDataObject old = new GCDataObject(9);//清理掉新生代的16%内存到旧生代，不知道是什么。。。System.gc();<pre name="code" class="java">//2-------------------------------------

GCDataObject newObject1 = new GCDataObject(1);
//为了填满旧生代，触发MinorGC使上面不可回收的对象进入Survivorfor(int ii = 0 ; ii < 5 ; ii++){new GCDataObject(1);}}}

注释掉2以下的代码并使用如下的参数运行一个虚拟机 ；

-Xmn10M -Xmx20M -Xms20M -verbose:gc -XX:+PrintGCDetails -XX:+UseSerialGC -XX:SurvivorRatio=3 -XX:MaxTenuringThreshold=1

大致过程：

[Full GC (System.gc()) [Tenured: 9216K->9738K(10240K), 0.0020278 secs] 10180K->9738K(18432K), [Metaspace: 2573K->2573K(1056768K)], 0.0020877 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
Heap
 def new generation   total 8192K, used 61K [0x00000000fec00000, 0x00000000ff600000, 0x00000000ff600000)
  eden space 6144K,   1% used [0x00000000fec00000, 0x00000000fec0f738, 0x00000000ff200000)
  from space 2048K,   0% used [0x00000000ff200000, 0x00000000ff200000, 0x00000000ff400000)
  to   space 2048K,   0% used [0x00000000ff400000, 0x00000000ff400000, 0x00000000ff600000)
 tenured generation   total 10240K, used 9738K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
   the space 10240K,  95% used [0x00000000ff600000, 0x00000000fff82b30, 0x00000000fff82c00, 0x0000000100000000)
 Metaspace       used 2580K, capacity 4486K, committed 4864K, reserved 1056768K
  class space    used 286K, capacity 386K, committed 512K, reserved 1048576K

只触发了一次FullGC，是代码显式使用的System.gc触发，目的是为了让对象全部进入旧生代，如果有一个1M对象进入旧生代的话，就会触发FullGC，以此来判断设置新生代存活年龄的重要性。

取消注释，用和上面相同的参数运行一个jvm得到下面的日志：

[Full GC (System.gc()) [Tenured: 9216K->9738K(10240K), 0.0019239 secs] 10180K->9738K(18432K), [Metaspace: 2574K->2574K(1056768K)], 0.0019757 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
[GC (Allocation Failure) [DefNew: 5242K->1024K(8192K), 0.0004939 secs] 14981K->10762K(18432K), 0.0005162 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
Heap
 def new generation   total 8192K, used 2109K [0x00000000fec00000, 0x00000000ff600000, 0x00000000ff600000)
  eden space 6144K,  17% used [0x00000000fec00000, 0x00000000fed0f748, 0x00000000ff200000)
  from space 2048K,  50% used [0x00000000ff400000, 0x00000000ff500030, 0x00000000ff600000)
  to   space 2048K,   0% used [0x00000000ff200000, 0x00000000ff200000, 0x00000000ff400000)
 tenured generation   total 10240K, used 9738K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
   the space 10240K,  95% used [0x00000000ff600000, 0x00000000fff82b30, 0x00000000fff82c00, 0x0000000100000000)
 Metaspace       used 2581K, capacity 4486K, committed 4864K, reserved 1056768K
  class space    used 286K, capacity 386K, committed 512K, reserved 1048576K

可以看到from有一一半的空间（1M）被占用了，因为新对象存活年龄为1，所以发生了一次MinorGC后的那个1M的对象不仅进入旧生代。

再使用类似上面的参数，但是把-XX:MaxTenuringThreshold=0运行得到下面的日志：

[Full GC (System.gc()) [Tenured: 9216K->9738K(10240K), 0.0022147 secs] 10180K->9738K(18432K), [Metaspace: 2574K->2574K(1056768K)], 0.0022690 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
[GC (Allocation Failure) [DefNew (promotion failed) : 5242K->5242K(8192K), 0.0002177 secs][Tenured: 9738K->9738K(10240K), 0.0016011 secs] 14981K->10762K(18432K), [Metaspace: 2574K->2574K(1056768K)], 0.0018564 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
Heap
 def new generation   total 8192K, used 2109K [0x00000000fec00000, 0x00000000ff600000, 0x00000000ff600000)
  eden space 6144K,  34% used [0x00000000fec00000, 0x00000000fee0f758, 0x00000000ff200000)
  from space 2048K,   0% used [0x00000000ff400000, 0x00000000ff400000, 0x00000000ff600000)
  to   space 2048K,   0% used [0x00000000ff200000, 0x00000000ff200000, 0x00000000ff400000)
 tenured generation   total 10240K, used 9738K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
   the space 10240K,  95% used [0x00000000ff600000, 0x00000000fff82b50, 0x00000000fff82c00, 0x0000000100000000)
 Metaspace       used 2580K, capacity 4486K, committed 4864K, reserved 1056768K
  class space    used 286K, capacity 386K, committed 512K, reserved 1048576K

可见那个1M的不可回收的对象立马进入旧生代，导致旧生代触发FullGC。旧生代放不下，放在了Eden下。Eden为2M。

注意这个值只有在串行gc和并行gc的时候才可以用