why the gc parameter MaxTenuringThreshold doesn't work

Question

I wanna check the behavior for MaxTenuringThreshold when gc

this is my vm arguments:

-verbose:gc -Xms20M -Xmx20M -Xmn10M -XX:+PrintGCDetails -XX:+UseSerialGC -XX:SurvivorRatio=8 -XX:MaxTenuringThreshold=15 -XX:+PrintTenuringDistribution

and this is my program:

public class TestTenuringThreshold {
    private static final int _1MB = 1024 * 1024;

    public static void main(String[] args) {
        byte[] allocation1, allocation2, allocation3;

        allocation1 = new byte[_1MB / 4];
        allocation2 = new byte[4 * _1MB];
        allocation3 = new byte[4 * _1MB];
        allocation3 = null;
        allocation3 = new byte[4 * _1MB];
    }
}

as I have set MaxTenuringThreshold to 15, I think the allocation1 will be in the from space, but the result is:

[GC[DefNew
Desired survivor size 524288 bytes, new threshold 1 (max 15)
- age   1:     750416 bytes,     750416 total
: 5188K->732K(9216K), 0.0041173 secs] 5188K->4828K(19456K), 0.0041663 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
[GC[DefNew
Desired survivor size 524288 bytes, new threshold 15 (max 15)
- age   1:        136 bytes,        136 total
: 4913K->0K(9216K), 0.0018490 secs] 9009K->4827K(19456K), 0.0018711 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
Heap
 def new generation   total 9216K, used 4260K [0x00000000f9a00000, 0x00000000fa400000, 0x00000000fa400000)
  eden space 8192K,  52% used [0x00000000f9a00000, 0x00000000f9e28fd0, 0x00000000fa200000)
  from space 1024K,   0% used [0x00000000fa200000, 0x00000000fa200088, 0x00000000fa300000)
  to   space 1024K,   0% used [0x00000000fa300000, 0x00000000fa300000, 0x00000000fa400000)
 tenured generation   total 10240K, used 4827K [0x00000000fa400000, 0x00000000fae00000, 0x00000000fae00000)
   the space 10240K,  47% used [0x00000000fa400000, 0x00000000fa8b6dd8, 0x00000000fa8b6e00, 0x00000000fae00000)
 compacting perm gen  total 21248K, used 2718K [0x00000000fae00000, 0x00000000fc2c0000, 0x0000000100000000)
   the space 21248K,  12% used [0x00000000fae00000, 0x00000000fb0a78a0, 0x00000000fb0a7a00, 0x00000000fc2c0000)
No shared spaces configured.

as you see, the from space is 0% used, why? My jdk version is: java version "1.7.0_79"

Answer 1

The MaxTenuringThreshold is a maximum for the tenuring threshold. However the JVM is free to choose a much shorter threshold such as 1.

the from space is 0% used, why?

All the object you have are large so they go straight into the tenured space. Any objects in the eden generation will be created in the normal running/startup of the JVM, not due to your code directly.

https://stackoverflow.com/a/24620205/57695

I would strongly recommend moving off Java 7 to 8 or 9. Note: Java 10 will be released in 3 months.