Serial GC Tracing - Tenuring Threshold Controlled

Java 8 GC Tutorials - Herong's Tutorial Examples

∟Serial GC Tracing - Tenuring Threshold Controlled

This section provides a tutorial showing how to control Serial collector Tenuring threshold with InitialTenuringThreshold and MaxTenuringThreshold JVM options.

In previous tutorials, we learned that the Serial collector controls objects tenuring using the tenuring threshold (default is 15 age old). When a live object reaches the tenuring threshold age, it is considered as matured to be promoted to tenured generation.

But if the survivor space usage reaches the TargetSurvivorRatio (default is 50% used), all remaining live objects will be promoted to tenured generation regardless of their ages.

The Serial collector also dynamically adjust the tenuring threshold based on current survivor space usage.

If we give enough survivor space, live objects may stay in Young generation for a long time surviving up to 15 times of Minor GCs. This may have performance impact on your application. An object of age 15 means that it has been copied 15 times between the two survivor spaces ("from" and "to" spaces).

If we want to change the default behavior, we can use InitialTenuringThreshold and MaxTenuringThreshold JVM options:

"-XX:InitialTenuringThreshold=3" - Set the initial value of tenuring threshold to 3. The Serial collector can still change it.
"-XX:MaxTenuringThreshold=3" - Set the maximum value of tenuring threshold to 3. This ensures than any objects older than "age 3" will be promoted to Tenured generation.

Let's try it with our test program GarbageCollection2.java:

herong> \progra~1\java\jdk1.8.0\bin\java -Xms105m -Xmx105m \
   -XX:NewSize=102m -XX:SurvivorRatio=1 \
   -XX:TargetSurvivorRatio=99 \
   -XX:InitialTenuringThreshold=3 -XX:MaxTenuringThreshold=3 \
   -XX:+UseSerialGC -XX:+PrintGCDetails -XX:+PrintHeapAtGC \
   -XX:+PrintTenuringDistribution GarbageCollection2

...
{Heap before GC invocations=3 (full 0):
 def new generation   total 69632K, used 65443K [...
  eden space 34816K,  99% used [...
  from space 34816K,  88% used [...
  to   space 34816K,   0% used [...
 tenured generation   total 4096K, used 0K [...
   the space 4096K,   0% used [...
 Metaspace used 1567K, capacity 2242K, committed 2368K, reserved 4480K

[GC (Allocation Failure) [DefNew
Desired survivor size 35295068 bytes, new threshold 3 (max 3)
- age   1:   22020432 bytes,   22020432 total
- age   2:    7340144 bytes,   29360576 total
- age   3:    1048592 bytes,   30409168 total
: 65443K->29696K(69632K), 0.0114630 secs]
   65443K->31990K(73728K), 0.0114985 secs]

Heap after GC invocations=4 (full 0):
 def new generation   total 69632K, used 29696K [...
  eden space 34816K,   0% used [...
  from space 34816K,  85% used [...
  to   space 34816K,   0% used [...
 tenured generation   total 4096K, used 2294K [...
   the space 4096K,  56% used [...
 Metaspace used 1567K, capacity 2242K, committed 2368K, reserved 4480K
}

As you can see from the output, Minor GC #4 promoted 2294K objects to tenured generation, because the maximum tenuring threshold was set to 3. Those objects would grow to age 4, if left in the survivor space.