Controlling Garbage Collection and Heap Growth to Reduce the Execution Time of Java Applications

1
Controlling Garbage Collection and Heap Growth to
Reduce the Execution Time of Java Applications

• Azhar Khan (ICS)
• ICS-599 Seminar

2
Outline

• Java
• Garbage Collection (GC)
• GC Advantages and Drawbacks
• Java GC
• GC Algorithms
• Bohem-Demers-Weiser (BDW) Collector
• Threshold-Based Algorithm
• Conclusion and Future Work

3
Java

• Object-oriented programming language developed by
  Sun Microsystems in early 1990s
• Platform independence
• Java Virtual Machine (JVM)

4
Garbage Collection (GC)

• Objects within a program's memory space that is
  unreachable from the program
• Invented by John McCarthy around 1959

5
Garbage Collection (GC)

• Removing unreferenced objects and Reclaiming
  Memory
• GC too Frequently and Infrequently
• Unnecessary overheads
• Heap Size and Paging

6
Garbage Collection (GC)

• Manual garbage-collection
• Manual tracking and freeing
• Automatic garbage-collection
• Run-time system perform tracking and freeing

7
Garbage Collection (GC)

• Manual garbage-collection
• Pascal
• C
• C
• Automatic garbage-collection
• Java
• Lisp
• Smalltalk

8
Garbage Collection (GC)

• Basic principle of Garbage Collector
• Determine what data objects in a program will not
  be accessed in the future
• Reclaim the resources used by those objects

9
GC Advantages and Drawbacks

• Automatic GC saves Programmers time
• Ensures Program Integrity
• Extra overhead on JVM in case of JAVA
• Programmers have less control of CPU Scheduling

10
Java GC

• Synchronously
• Threading
• Reclaiming Memory
• JVM uses two broad techniques
• Run garbage collector to reclaim unused or
  unneeded heap memory.
• Increase the heap size to accommodate application
  memory requirements

11
Fundamental Decisions

• When allocating memory, what algorithm should be
  used?
• If garbage collection is performed, which
  algorithm should be used?
• When should garbage collection be performed
• When should the heap be expanded, and by how much
  should it expand?
• If the heap is being compacted, when should it be
  compacted?

12
GC Algorithms

• Reference Counter
• Keeps track the live objects
• Mark-Sweep Collector
• Mark Phase
• Sweep Phase

13
GC Algorithms

• Compacting Collector
• Fragmentation
• Sliding
• Copying Collector
• Two Regions
• Stop and Copy

14
Bohem-Demers-Weiser (BDW) Collector

• Mark-Sweep Collector
• Object Types
• Atomic Objects
• Composite Objects
• Free Space Divisor (FSD) Static Variable
• Heap Size

15
Threshold-Based Algorithm

• Checks if sufficient memory available.
• Heap Grow Aggressively
• Checks if available memory becomes low.
• Avoid paging
• GC more Aggressively and Heap less Aggressively
• Tracking the amount of free memory.

16
BDW Collector Threshold-Based Algorithm

• Decision based on
• Heap Size
• Available Memory
• Threshold
• Memory Allocator
• Heap Size
• GC
• Amount of recent garbage collection

17
Threshold-Based Algorithm

• Heap Grown
• GC Collected
• Neither

18
T4
T3
F
T2
J
C
H
E
T1
D
I
G
B
R1 lt S1 T2-T1
A
R2 gt S2 T2-T1
19
Conclusion Future work

• Scheduling of GC and Heap Expansion
• Decision based on Heap Size
• Decision based on Memory Available.
• Test Threshold-based Algorithm with different
  Garbage Collectors and consider multiple
  applications,

20
Questions