Serial GC Tracing - Tenuring Threshold

Java 8 GC Tutorials - Herong's Tutorial Examples

∟Serial GC Tracing - Tenuring Threshold

This section provides a tutorial showing how Serial GC changes the tenuring threshold dynamically and promotes objects to 'tenured' generation earlier or later to Tenuring generation to meet the TargetSurvivorRatio setting.

In the previous tutorial, we learned that when a live object survived Minor GC multiple times and reached the tenuring threshold age, it will be promoted to Tenured generation.

But the Serial collector can also change the tenuring threshold age dynamically based on past GC statistics. This is to move object out of the survivor space quickly to avoid survivor space overflow.

Let's rerun the same test with a smaller survivor space.

herong> \progra~1\java\jdk1.8.0\bin\java -Xms360m -Xmx360m \
   -XX:NewRatio=1 -XX:SurvivorRatio=1 \
   -XX:+UseSerialGC -XX:+PrintGCDetails -XX:+PrintHeapAtGC \
   -XX:+PrintTenuringDistribution GarbageCollection2

At the first Minor GC, the survivor space usage was 42%. There is no danger of overflow, so Serial collector kept the tenuring threshold to age 15, the default age.

{Heap before GC invocations=0 (full 0):
 def new generation   total 122880K, used 60826K [...
  eden space 61440K,  99% used [...
  from space 61440K,   0% used [...
  to   space 61440K,   0% used [...
 tenured generation   total 184320K, used 0K [...
   the space 184320K,   0% used [...
 Metaspace used 1567K, capacity 2242K, committed 2368K, reserved 4480K

[GC (Allocation Failure) [DefNew
Desired survivor size 31457280 bytes, new threshold 15 (max 15)
- age   1:   26467640 bytes,   26467640 total
: 60826K->25847K(122880K), 0.0208674 secs]
   60826K->25847K(307200K), 0.0209314 secs]

Heap after GC invocations=1 (full 0):
 def new generation   total 122880K, used 25847K [...
  eden space 61440K,   0% used [...
  from space 61440K,  42% used [...
  to   space 61440K,   0% used [...
 tenured generation   total 184320K, used 0K [...
   the space 184320K,   0% used [...
 Metaspace used 1567K, capacity 2242K, committed 2368K, reserved 4480K
}

But at the second GC. 50% of survivor space was used. It reached another setting called TargetSurvivorRatio, which is defaulted to 50%. So Serial collector decided to lower the tenuring threshold age to 2. This forces all objects older than 3 to be promoted to Tenured generation at next GC.

{Heap before GC invocations=1 (full 0):
 def new generation   total 122880K, used 86444K [...
  eden space 61440K,  98% used [...
  from space 61440K,  42% used [...
  to   space 61440K,   0% used [...
 tenured generation   total 184320K, used 0K [...
   the space 184320K,   0% used [...
 Metaspace used 1567K, capacity 2242K, committed 2368K, reserved 4480K

[GC (Allocation Failure) [DefNew
Desired survivor size 31457280 bytes, new threshold 2 (max 15)
- age   1:   28311984 bytes,   28311984 total
- age   2:    3398136 bytes,   31710120 total
: 86444K->30966K(122880K), 0.0340368 secs]
   86444K->30966K(307200K), 0.0340731 secs]

Heap after GC invocations=2 (full 0):
 def new generation   total 122880K, used 30966K [...
  eden space 61440K,   0% used [...
  from space 61440K,  50% used [...
  to   space 61440K,   0% used [...
 tenured generation   total 184320K, used 0K [...
   the space 184320K,   0% used [...
 Metaspace used 1567K, capacity 2242K, committed 2368K, reserved 4480K
}

At the third GC, the survivor space usage went back to 48%. So Serial collector decided to set the tenuring threshold back to 15.

{Heap before GC invocations=2 (full 0):
 def new generation   total 122880K, used 91566K [...
  eden space 61440K,  98% used [...
  from space 61440K,  50% used [...
  to   space 61440K,   0% used [...
 tenured generation   total 184320K, used 0K [...
   the space 184320K,   0% used [...
 Metaspace used 1567K, capacity 2242K, committed 2368K, reserved 4480K

[GC (Allocation Failure) [DefNew
Desired survivor size 31457280 bytes, new threshold 15 (max 15)
- age   1:   29360576 bytes,   29360576 total
- age   2:    1048592 bytes,   30409168 total
: 91566K->29696K(122880K), 0.0151026 secs]
   91566K->31990K(307200K), 0.0151380 secs]

Heap after GC invocations=3 (full 0):
 def new generation   total 122880K, used 29696K [...
  eden space 61440K,   0% used [...
  from space 61440K,  48% used [...
  to   space 61440K,   0% used [...
 tenured generation   total 184320K, used 2294K [...
   the space 184320K,   1% used [...
 Metaspace used 1567K, capacity 2242K, committed 2368K, reserved 4480K
}

Conclusion:

By default, the Serial collector starts with a tenuring threshold of 15.
The Serial collector will reset the tenuring threshold based on survivor space usage percentage. When usage is higher than TargetSurvivorRatio, it will lower the tenuring threshold to promote objects at a younger age to Tenuring generation.