Brief Introduction to GC of JVM

1. Garbage Collection of
Java Virtual Machine
Yongqiang Li

2. Outlines
• Recall: Runtime Data Area of JVM
• Object Live or Dead?
• Algorithm of GC
• Garbage Collectors
• Policy of Object Allocation

3. Recall: Runtime Data Areas of JVM
• Program Counter Register
• Java Heap
• Java Virtual Machine Stacks
• Native Method Stacks
• Method Area

4. Object Live or Dead?
• Reference Count

5. Object Live or Dead?
• GC Roots Tracing
• GC Roots
• Reference in stack
• Class static reference
• Final reference
• JNI reference

6. Algorithm of Garbage Collection
• Mark-Sweep
• Cons
• Low performance
• More small fragments

7. • Copying
• Cons
• Some memory is wasted.
• Need memory to handle promotion
• For extreme condition, the performance is bad.

8. • Mark-Sweep-Compact

9. • Generational Collection
• For new generation, use Copying
• For old generation, use Mark-Sweep or Mark-Sweep-Compact

10. Garbage Collectors (Hotspot JVM)
• Collectors
• Serial/Serial Old
• ParNew
• Parallel Scavenge/Parallel Old
• CMS(Concurrent Mark Sweep)

11. • Serial/Serial Old
• Algorithm: for new gen, Copying; for old gen, mark-sweep-
compact
• but it will “Stop the world”!

12. • ParNew
• Multi-threaded Serial collector

13. • Parallel Scavenge/Parallel Old
• Almost like ParNew, but it focus on the control of Throughout.
• Throughout = CPU time on user code / CPU total time.
• High throughout means more efficient.
• We can use MaxGCPauseMillis and GCTimeRatio to control this
collector.

14. • CMS (Concurrent Mark Sweep)
• Steps
• CMS initial mark
• CMS concurrent mark
• CMS remark
• CMS concurrent sweep

15. Policy of Object Allocation - Handle
Promotion
• When Survivor is too small to hold all live objects, Tenured
space will try to handle promotion.
• When minor GC is going to happen, JVM will check the
average memory amount of promoted (to tenured space)
objects.
• If it is bigger than the rest amount of tenured space. JVM will
invoke Full GC instead.
• If less, JVM will check the switch of HandlePromotionFailure.
• If switch is on, only minor GC happens.
• Else, Full GC will happen.

16. 2. Object will be allocated on Eden first.
• Where is “Eden”?
• In JVM, for new generation objects, there are 1 Eden (bigger) space
and 2 Survior (called from and to; and smaller) space.
• The default ratio between Eden and Survior is 8:1.
• Use Copying algorithm

17. -verbose:gc
-Xms20M
-Xmx20M
-Xmn10M
-XX:+PrintGCDetails
-XX:SurvivorRatio=8

18. 3. Big object will be allocated on
tenured space directly.
-verbose:gc
-Xms20M
-Xmx20M
-Xmn10M
-XX:+PrintGCDetails
-XX:SurvivorRatio=8
-XX:PretenureSizeThreshold=3145728

19. 4. Long-lived objects will enter
tenured space.
• JVM will give each object an age.
• After each minor GC (on new generation), lived object’s age
will increase by 1.
• Use MaxTenuringThreshold to control the behavior.
• Dynamic threshold
• if Memory(live objects with the same age) > Total Memory of
Survivor, then Objects with the same or old age will be promoted.
And it will ignore the setting of MaxTenuringThreshold.

20. Case: MaxTenuringThreshold
-verbose:gc
-Xms20M
-Xmx20M
-Xmn10M
-XX:+PrintGCDetails
-XX:SurvivorRatio=8
-XX:MaxTenuringThreshold=1
-XX:+PrintTenuringDistribution

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

22. Case: Dynamic Threshold
-verbose:gc
-Xms20M
-Xmx20M
-Xmn10M
-XX:+PrintGCDetails
-XX:SurvivorRatio=8
-XX:MaxTenuringThreshold=15
-XX:+PrintTenuringDistribution