"-XX:+NeverTenure" and "-XX:+AlwaysTenure" not Working

Java 8 GC Tutorials - Herong's Tutorial Examples

∟"-XX:+NeverTenure" and "-XX:+AlwaysTenure" not Working

This section provides a tutorial showing '-XX:+NeverTenure' and '-XX:+AlwaysTenure' JVM options are not working with the Serial collector.

HotSpot JVM also support two hidden options you can use to control tenuring behavior:

"-XX:+NeverTenure" - The garbage collector will to not to promote live objects from Young generation to Tenured generation.
"-XX:+AlwaysTenure" - The garbage collector will promote live objects immediately from Young generation to Tenured generation. I think this is similar to "-XX:MaxTenuringThreshold=0".

First, let's try "-XX:+NeverTenure" with our test program GarbageCollection2.java:

herong> \progra~1\java\jdk1.8.0\bin\java -Xms130m -Xmx130m \
   -XX:NewSize=120m -XX:SurvivorRatio=1 \
   -XX:+NeverTenure -Xloggc:gc.log \
   -XX:+UseSerialGC -XX:+PrintGCDetails -XX:+PrintHeapAtGC \
   -XX:+PrintTenuringDistribution GarbageCollection2

{Heap before GC invocations=15 (full 0):
 def new generation   total 81920K, used 72729K [...
  eden space 40960K,  99% used [...
  from space 40960K,  78% used [...
  to   space 40960K,   0% used [...
 tenured generation   total 10240K, used 0K [...
   the space 10240K,   0% used [...
 Metaspace used 1567K, capacity 2242K, committed 2368K, reserved 4480K

[GC (Allocation Failure) [DefNew
Desired survivor size 41523608 bytes, new threshold 15 (max 15)
- age   1:   25166208 bytes,   25166208 total
- age   2:    5242960 bytes,   30409168 total
- age   3:    2097184 bytes,   32506352 total
- age   4:         72 bytes,   32506424 total
- age   7:         56 bytes,   32506480 total
: 72729K->31744K(81920K), 0.0084429 secs]
   72729K->31991K(92160K), 0.0084774 secs]

Heap after GC invocations=16 (full 0):
 def new generation   total 81920K, used 31744K [...
  eden space 40960K,   0% used [...
  from space 40960K,  77% used [...
  to   space 40960K,   0% used [...
 tenured generation   total 10240K, used 246K [...
   the space 10240K,   2% used [...
 Metaspace used 1567K, capacity 2242K, committed 2368K, reserved 4480K
}

The log message shows that some old objects were promoted to Tenured generation at Minor GC #16, even there were still some memory available in survivor space. The usage was 77%, lower than 99% given in "-XX:TargetSurvivorRatio=99" option.

So the "-XX:+NeverTenure" option is not working for Serial collector.

Now, let's try "-XX:+AlwaysTenure" with our test program GarbageCollection2.java:

herong> \progra~1\java\jdk1.8.0\bin\java -Xms130m -Xmx130m \
   -XX:NewSize=120m -XX:SurvivorRatio=1 \
   -XX:+AlwaysTenure -Xloggc:gc.log \
   -XX:+UseSerialGC -XX:+PrintGCDetails -XX:+PrintHeapAtGC \
   -XX:+PrintTenuringDistribution GarbageCollection2

...
{Heap before GC invocations=0 (full 0):
 def new generation   total 81920K, used 40551K [...
  eden space 40960K,  99% used [...
  from space 40960K,   0% used [...
  to   space 40960K,   0% used [...
 tenured generation   total 10240K, used 0K [...
   the space 10240K,   0% used [...
 Metaspace used 1567K, capacity 2242K, committed 2368K, reserved 4480K
[GC (Allocation Failure) [DefNew
Desired survivor size 41523608 bytes, new threshold 15 (max 15)
- age   1:   22273272 bytes,   22273272 total
: 40551K->21751K(81920K), 0.0172061 secs]
   40551K->21751K(92160K), 0.0172543 secs]
Heap after GC invocations=1 (full 0):
 def new generation   total 81920K, used 21751K [...
  eden space 40960K,   0% used [...
  from space 40960K,  53% used [...
  to   space 40960K,   0% used [...
 tenured generation   total 10240K, used 0K [...
   the space 10240K,   0% used [...
 Metaspace used 1567K, capacity 2242K, committed 2368K, reserved 4480K
}

The "-XX:+AlwaysTenure" option is also not working for Serial collector. The first Minor GC listed above showing 22273272B objects stayed in the survivor space. Not promoted to the Tenured generation.