The document discusses garbage collection techniques in Java. It begins with an overview of manual memory management and the problems it poses, such as memory leaks and dangling pointers. It then covers automatic memory management techniques used in Java, including garbage collection algorithms like reference counting and tracing. Specific algorithms used in OpenJDK's HotSpot virtual machine are examined, such as the G1, Shenandoah, and Z garbage collectors. Key aspects of garbage collection like heap layout, generations, and concurrent versus stop-the-world collection are summarized.

1. Java Team
Introduction to Garbage
Collection
OpenJDK: Automatic Memory Management
Monica Beckwith
JVM Performance
java-performance@Microsoft
@mon_beck

2. Agenda
What is Garbage Collection (GC)?
Allocation, Reclamation == Memory Management
Techniques of Automatic Memory Management
A Few GC Algorithms in OpenJDK HotSpot VM
July 1st , 2020
What We Will Try To Cover Today

3. Garbage Collection aka
Memory Management
==
Allocation
+
Reclamation

4. Allocation: The Manual Way …
Allocation Space

5. Allocations Happen …
O O O O O O O O O O
O O O O O O O
O O O O O O O O

6. Space Gets Filled Up
O O O O O O O O O O
O O O O O O O O O O
O O O O O O O O O O
O O O O O O O O O
O O O O O O O O O
O O O O O O O O O

7. Its Marking Time!
Root Set
O
O
O
O
O
O O O O O O O O O O
O O O O O O O O O O
O O O O O O O O O O
O O O O O O O O O
O O O O O O O O O
O O O O O O O O O
Its Marking Time! – Tracing from the Root Set

8. Its Marking Time! – Creating Live Object Graph
Root Set
O
O
O
O
O
O O O O O O O O O O
O O O O O O O O O O
O O O O O O O O O O
O O O O O O O O O
O O O O O O O O O
O O O O O O O O O

9. Ready to Reclaim?
Root Set
O
O
O
O
O
O O O O O
O O O O O O O O
O O O O O O O O
O O O O O O O O O
O O O O O O O O O
O O O O O O O O O
O O O O O O O O O O
O O O O O O O O O O
O O O O O O O O O O
Ready to Reclaim? – Are We Done?

10. Manual Reclamation – Problem!
Root Set
O
O
O
O
O
O O O O O
O O O O O O O O
O O O O O O O
O O O O O O O O O
O O O O O O O O O
O O O O O O O O O
This can leadto a
memory leak!!
Manual Reclamation – Problem: Memory Leak

11. Root Set
O
O
O
O
O
O O O O O
O O O O O O O O
O O O O O O O
O O O O O O O O O
O O O O O O O O O
O O O O O O O O O
Manual Reclamation – Problem!

12. Root Set
O
O
O
O
O
O O O O O
O O O O O O O O
O O O O O O O
O O O O O O O O O
O O O O O O O O O
O O O O O O O O
Two dangling
pointers!!
Manual Reclamation – Problem: Dangling Pointers

13. So What Can We Do?
Stack Based Allocation:
- Push and pop will take care of the issues
- E.g. Java bytecode interpreter
Hybrid Allocation:
- These typically combine region-based allocation with reference
counting or tracing garbage collection
- E.g. OpenJDK GCs using Thread Local Allocation Buffers (TLABs)

14. Reference Counting vs Tracing
“The Continuous Tracker” Active When Full Or When Threshold Crossed

15. Basic Considerations of Automatic Memory
Management
Collection Efficiency
- It maybe more efficient to leave some garbage around
- E.g. G1 GC’s incremental compaction / mixed collection
Performance Trifecta
- GC designs are based around responsiveness, footprint considerations
and throughput efficiency
- E.g. OpenJDK’s Serial GC for low footprint, Parallel GC for throughput
efficiency

16. Further Considerations …
Generational Hypothesis
- Most objects die young
- E.g. OpenJDK GCs other than ZGC and Shenandoah GC
Age Threshold
- Promote only the long-lived objects (including transients)
- E.g. OpenJDK’s generational collectors with survivor space

17. Groundwork : Heap
layout - regions and
generations

18. Heap Layout
Heap
Z GC
Shenandoah GC
Young Generation
G1 GC
Old Generation

19. Commonalities :
Copying collector –
from and to spaces From To

20. HeapFrom Space To Space
O O O O O O O O O O
O O O O O O O O O O
O O O O O O O O O O
GC ROOTS
THREAD
1 STACK
THREAD
N STACK
STATIC
VARIABLES
ANY JNI
REFERENCES
Copying Collector aka Compacting Collector aka
Evacuation

21. O O O O O O O O O O
O O O O O O O O O O
O O O O O O O O O O
GC ROOTS
THREAD
1 STACK
THREAD
N STACK
O O
O O
O
STATIC
VARIABLES
ANY JNI
REFERENCES
O
OO
O
O
O O O O O O O O O O
O O O O O O O O O O
O O O O O O O O O O
Copying Collector aka Compacting Collector aka
Evacuation

22. Copying Collector aka Compacting Collector aka
Evacuation
O O O O O O O O
O O O O O O O
O O O O O O O
O O O
O O O
O O

23. Commonalities : Regions
– occupied and free

24. Occupied and Free Regions
O O O O O O O O O O
O O O O O O O O O O
O O O O O O O O O O
O O O O
O O O O
O O O O
• List of free regions
• In case of generational heap (like G1), the occupied regions could be young, old or
humongous

25. Commonalities :
Collection set and
priority

26. Collection Priority and Collection Set
O O O O O O O O O O
O O O O O O O O O O
O O O O O O O O O O
O O O O
O O O O
O O O O
OO
OO
OO
OO
OO
OO
OO
OO
OO
O
OO
OO
OO
OO
OO
OO
OO
OO
OO
O
OOOO
• Priority is to reclaim regions with most garbage
• The candidate regions for collection/reclamation/relocation are said to be in a collection set
• There are threshold based on how expensive a region can get and maximum regions to
collect
• Efficiency: Incremental collection aka incremental compaction or partial compaction
• Usually needs a threshold that triggers the compaction
• Stops after the desired reclamation threshold or free-ness threshold is reached
• Doesn’t need to be stop-the-world

27. Introduction : G1,
Shenandoah & Z -
Algorithms

28. Algorithm and Other Considerations
Garbage Collectors G1 GC Shenandoah GC Z GC
Regionalized? Yes Yes Yes
Generational? Yes No No
Compaction? Yes, STW, Forwarding
address in header
Yes, Concurrent,
Forwarding Pointer
Yes, Concurrent,
Colored Pointers
Target Pause Times? 200ms 10ms 10ms
Concurrent Marking
Algorithm?
SATB SATB Striped

29. Further Reading
https://www.youtube.com/watch?v=VCeHkcwfF9Q
https://www.usenix.org/legacy/events/vee05/full_papers/p46-click.pdf
http://mail.openjdk.java.net/pipermail/zgc-dev/2017-December/000047.html
http://hg.openjdk.java.net/zgc/zgc/file/ffab403eaf14/src/hotspot/share/gc/z/zB
arrier.cpp
https://wiki.openjdk.java.net/display/zgc/Main
https://www.azul.com/files/c4_paper_acm1.pdf

30. © Copyright Microsoft Corporation. All rights reserved.

31. Stop-the-world aka STW GC
Application Threads
GC Threads
Application Threads
Safepoint
Requested
GC
Completed
Application Threads GC Threads Application Threads
Safepoint
Requested
GC
Completed
Handshakes
Thread local handshakes vs Global
Time To Safepoint
(TTSP)

32. Concurrent GC
Application Threads
GC Threads

33. © Copyright Microsoft Corporation. All rights reserved.