3장. Garbage Collection

Java Performance
artdb@ex-em.com | performeister.tistory.com | twitter @novathinker

Garbage Collection

Java Performance
2


1) Garbage Collection
2) Garbage Collection Algorithm
3) Hotspot JVM Garbage Collection

4) IBM JVM Garbage Collection

Java Performance
3
Java Performance Fundamental | twitter @novathinker
artdb@ex-em.com | performeister.tistory.com
Garbage Collection

• Garbage Collection
– Garbage Collector
– Unreferenced Memory
–
– Fragmentation

“Heap storage for objects is reclaimed by an automatic
storage management system (typically a garbage
collector); objects are never explicitly deallocated.”
- Java Virtual Machine Speculation, Section 3.5.3 [JVMS2
1999]

Java Performance
4
Garbage Collection

!
Garbage
Collector

Java Performance
4
Garbage Collection

! object
Garbage
Collector
Heap memory
.

Java Performance
4
Garbage Collection

! object
Garbage
Collector
Heap memory
.

Garbage Collection
Memory Recycling Heap
Fragmentation .

Java Performance
4
Garbage Collection

! object
Garbage
Collector
Heap memory
.

Garbage Collection
Memory Recycling Heap
Fragmentation .

• JVM heap new, newarray, anewarray, multianewarray instruction
instruction
• JVM Spec Garbage Collection Algorithm

Java Performance
5
Garbage Collection

!
Garbage
Collector

Java Performance
5
Garbage Collection

Memory

! Memory System
Garbage
Collector
crash
.

Java Performance
5
Garbage Collection

Memory

! Memory System
Garbage
Collector
crash
.

Program

CPU Time Scheduling

.

Java Performance
6
Garbage Collection

– Garbage Collection

Java Performance
7
Garbage Collection

Java Performance
7
Garbage Collection

ROOT SET

Java Performance
7
Garbage Collection

ROOT SET

Local Variable,
Operand Stack

Java Performance
7
Garbage Collection

ROOT SET

Local Variable,
Operand Stack

Class
Constant Pool
Reference

Java Performance
7
Garbage Collection

ROOT SET

Local Variable, Unreleased Native
Operand Stack Method
Object
Reference
Class
Constant Pool
Reference

Java Performance
8
Garbage Colletion

• Root Set

Java Performance
9
Garbage Colletion

• Memory Leak
class Main{
public static void main (String args[]) {
Leak lk = new Leak();
for(int a=0; a<9000000; a++) {
lk.addList(a);
lk.removeStr(a);
}
}
}

class Leak {
ArrayList lst = new ArrayList();
public void addList(int a) {
lst.add(" "+a);
}
public void removeStr(int i) {
Object obj = lst.get(i);
obj = null;
}
}

Java Performance
9
Garbage Colletion

• Memory Leak
class Main{
public static void main (String args[]) {
Leak lk = new Leak();
for(int a=0; a<9000000; a++) {
lk.addList(a);
lk.removeStr(a);
}
} Why leak?
}

class Leak {
ArrayList lst = new ArrayList();
public void addList(int a) {
lst.add(" "+a);
}
public void removeStr(int i) {
Object obj = lst.get(i);
obj = null;
}
}

Java Performance
10

Garbage Collection
Algorithm

Java Performance
11


Garbage Collection Algorithm
1) Reference Counting Algorithm
2) Mark-and-Sweep Algorithm
3) Mark-and-Compacting Algorithm
4) Copying Algorithm
5) Generational Algorithm
6) Train Algorithm
7) Adaptive Algorithm

Java Performance
12

• Garbage Collection Algorithm

Java Performance
12


Garbage Object Detection
• Live Object : Root Set
• Garbage Object

Java Performance
12


Garbage Object Detection
• Live Object : Root Set
• Garbage Object

Garbage Object
• Heap Memory Reclaim

Java Performance
13

• Reference Counting Algorithm
• Mark-and-Sweep Algorithm
• Copying Algorithm
• Mark-and-Compacting Algorithm
• Generational Algorithm
• Train Algorithm
• Adaptive Algorithm

Java Performance
14

– GC
– Object Reference count
– Reference Count 0 GC
– Object GC Object
Object Object Count
Count

Java Performance
15

Integer

RefCnt 00

Integer

RefCnt 00

Integer

RefCnt 00

Java Performance
15

Integer
Object a = new Ingeter(1);

RefCnt 00

Integer

RefCnt 00

Integer

RefCnt 00

Java Performance
15

Integer
Object a = new Ingeter(1); a 1
RefCnt 01
0

Integer

RefCnt 00

Integer

RefCnt 00

Java Performance
15

Integer
Object b = a; RefCnt 01
0

Integer

RefCnt 00

Integer

RefCnt 00

Java Performance
15

Integer
Object b = a; b RefCnt 01
0
2

Integer

RefCnt 00

Integer

RefCnt 00

Java Performance
15

Integer
0
2

Integer

Object a = new Ingeter(1); RefCnt 00

Integer

RefCnt 00

Java Performance
15

Integer
0
2

Integer

a 1
Object a = new Ingeter(1); 0
RefCnt 01
Integer

RefCnt 00

Java Performance
15

Integer
0
2

Integer

a 1
RefCnt 01
Object b = new Ingeter(2); Integer

RefCnt 00

Java Performance
15

Integer
0
2

Integer

a 1
RefCnt 01
2
b RefCnt 01
0

Java Performance
15

Integer
0
2

Integer

a 1
RefCnt 01
a = b; 2
b RefCnt 01
0

Java Performance
15

Integer
0
2

Integer

a 1
RefCnt 01
a = b; 2
b RefCnt 01
0
2

Java Performance
16

Java Performance
16

Object a = new TwoIngeter(new Integer(1), new Integer(2));

Java Performance
16


Integer
1
TwoInteger
RefCnt 00
1
ref
a
RefCnt 00
1 Integer
2
RefCnt 01
0

Java Performance
16


a = null;

Integer
1
TwoInteger
RefCnt 00
1
ref
a
RefCnt 00
1 Integer
2
RefCnt 01
0

Java Performance
16


a = null;

Integer
1
TwoInteger
RefCnt 00
1
ref
a GC
RefCnt 00
1 Integer
2
RefCnt 01
0

Java Performance
16


a = null;

Integer
1
RefCnt 00
1
a
Integer
2
RefCnt 01
0
0

Java Performance
17


• Garbage Object
• Garbage
• Garbage Collector ( )

• Reference Count
• Linked List Reference Count 0
Leak

Java Performance
18

a

Linked List
next ref
RefCnt 02

Linked List
next ref
RefCnt 11

Linked List
next ref
RefCnt 01

Java Performance
18

a

Linked List
next ref
2
RefCnt 01

Linked List
next ref
RefCnt 11

Linked List
next ref
RefCnt 01

Java Performance
19

– Root Set Reference
– Tracing Algorithm
– Mark Sweep
• Mark : Live Object Mark
(Object ﬂag or bitmap table )
• Sweep : Heap Mark

Java Performance
20

R ref ref ref
 flag  flag  flag

ref ref ref

ref ref ref

Java Performance
20

Mark Phase
R ref ref ref

ref ref ref

ref ref ref

Java Performance
20

Mark Phase
R ref ref ref

 flag 
 flag 
 flag

ref ref ref
 flag  flag 
 flag

ref ref ref
 flag 
 flag 
 flag

Java Performance
20

Sweep Phase
R ref ref ref

 flag 
 flag 
 flag

ref ref ref
 flag  flag 
 flag

ref ref ref
 flag 
 flag 
 flag

Java Performance
20

Sweep Phase
R ref ref ref

 flag 
 flag 
 flag

ref

 flag

ref ref

 flag 
 flag

Java Performance
20

R ref ref ref

 flag 
 flag 
 flag

ref

 flag

ref ref

 flag 
 flag

Java Performance
20

R ref ref ref

ref
 flag

ref ref
 flag  flag

Java Performance
21


• Reference
• Reference Overhead

• Suspend
• Fragmentation
 Memory

Java Performance
22

– Fragmentation Memory
Algorithm
– Mark Compaction
• Mark Phase : Live Object Mark
• Compaction : Live Object Memory

– Handle
• Mark Phase : Handle Marking
• Compaction : Object Handle Update

Java Performance
23

– Compaction
Arbitrary Worst

4 1 2 3
1 2 3 4

Linear

1 3 4 2

• Arbitrary : Sliding
• Linear : Pointer
1 2 3 4
• Sliding : Allocation
Best

Java Performance
24

2

Handle
0  flag
1  flag T22 A 5 Z 1
2  flag T11
3  flag T12
B 9 W 7
4  flag
5  flag T21
6  flag Q 3 C 2
7  flag T31
8  flag
9  flag T32

Java Performance
24

2 Mark Phase
Handle
0  flag
1  flag T22 A 5 Z 1
2  flag T11
3  flag T12
B 9 W 7
4  flag
5  flag T21
6  flag Q 3 C 2
7  flag T31
8  flag
9  flag T32

Java Performance
24

2 Mark Phase
Handle
0  flag
1  flag T22 A 5 Z 1

2  flag T11
3  flag T12
B 9 W 7
4  flag

5  flag T21
6  flag Q 3 C 2
7  flag T31
8  flag

9  flag T32

Java Performance
24

2 Compation Phase
Handle
0  flag
1  flag T22 A 5 Z 1

2  flag T11
3  flag T12
B 9 W 7
4  flag

5  flag T21
6  flag Q 3 C 2
7  flag T31
8  flag

9  flag T32

Java Performance
24

2 Compation Phase
Handle
0  flag
1  flag T22 A 5

2  flag T11
3  flag T12
B 9
4  flag

5  flag T21
6  flag C 2
7  flag T31
8  flag

9  flag T32

Java Performance
24

2 Compation Phase
Handle
0  flag
1  flag T22 A 5 B 9

2  flag T11
3  flag T12
4  flag

5  flag T12
T21
6  flag C 2
7  flag T31
8  flag

9  flag T32

Java Performance
24

2 Compation Phase
Handle
0  flag
1  flag T22 A 5 B 9

2  flag T11
3  flag T12
C 2
4  flag

5  flag T12
T21
6  flag
7  flag T31
8  flag

9  flag T21
T32

Java Performance
24

2

Handle
0  flag
1  flag T22 A 5 B 9
2  flag T11
3  flag T12
C 2
4  flag
5  flag T12
T21
6  flag
7  flag T31
8  flag
9  flag T21
T32

Java Performance
25


• Fragmentation
• Memory

• Reference Object Access Overhead
• Suspend

Java Performance
26

– Stop-the-Copy Algorithm
– Heap Active Inactive

– Active Object
– Active Live Object Inactive
Copy

Java Performance
27

Active Area

A C

B

R

Inactive Area

Java Performance
27

Active Area

A C
null null

B
null

R

Inactive Area

Java Performance
27

Active Area

A C
null

B
null

R
A’
null

Inactive Area

Java Performance
27

Active Area

A C
null

B

R
A’ B’
null null

Inactive Area

Java Performance
27

Active Area

A C

B

R
A’ B’ C’
null null null

Inactive Area

Java Performance
27

Inactive Area

R
A’ B’ C’
null null null

Active Area

Java Performance
28
Garbage Collection Algorithm artdb@ex-em.com | performeister.tistory.com


• Fragmentation
 Inactive Copy

• GC Suspend
• Copy Overhead
•

Java Performance
29

– Copying Algorithm
• Copy
• Object

•
Object
• Long Lived Object
– Heap age sub heap
Youngest Generation Sub heap GC
– Object Age

Java Performance
30

Youngest Generation Sub Heap

Matured Live Live Dead Dead

Live Matured Dead Live Dead

Dead Live Dead Dead Matured

Tenured Generation Sub Heap

Java Performance
30


Live Live Dead Dead

Live Dead Live Dead

Dead Live Dead Dead

Tenured Generation Sub Heap GC

Matured Matured Matured

Java Performance
30


Live Live

Live Live

Live

Tenured Generation Sub Heap GC


Java Performance
30


Matured Matured Dead
Live

Matured Dead

Live Matured Live

Tenured Generation Sub Heap


Java Performance
31
Garbage Collection Algorithm artdb@ex-em.com | performeister.tistory.com


• Sub Heap Mark-and-Sweep Copying Algorithm

Fragmentation, Memory , Copy

• JVM

Java Performance
32

• Train Algorithm
– Incremental Algorithm

– Memory
Mark-and-Copy GC

– GC Suspend
Idea

Java Performance
33

• Train Algorithm

Car : Memory Block , Fixed Size

RememberSet RememberSet
Car root
A B C A’ B’ C’ set

Train : Car( ) Car

Java Performance
34

Car 1-1 Car 1-2
Train 1 {R1,E}

A B C D E F

R1 Car 2-1
Train 2 {R2,B}
R2
G

Java Performance
34

Free Car Car 1-1 Car 1-2
Train 1 {C}

D E F C

R1 Car 2-1
Train 2 {R2,B}
R2
G

Car 3-1
Train 3 {R1}

A B

Java Performance
34

Free Car Free Car Free Car
Free Train

R1 Car 2-1
Train 2 {R2,B}
R2
G

Car 3-1
Train 3 {R1}

A B

Java Performance
35

• Train Algorithm

• Suspend

• Suspend
• Fragmentation

Java Performance
36

• Adaptive Algorithm
– Algorithm Trade Off
 Algorithm
– Adaptive Algorithm Algorithm

– Heap
Algorithm
– Hotspot Ergonomic , IBM Tilt

– Application

Java Performance

Hotspot JVM

Java Performance
38


Hotspot JVM Garbage Collection
1) Garbage Collection of Hotspot JVM
2) Heap of Hotspot JVM
3) Garbage Collector

4) Serial Collector
5) Incremental Collector
6) Parallel Collector
7) Parallel Compacting Collector
8) CMS Collector
9) Garbage First Collector

Java Performance
39
Hotspot JVM

• Garbage Collection of Hotspot JVM
– Weak Generational Hypothesis
• High Infant Mortality
• Few References from Older to Younger Objects
Exists
– Young Generation GC Algorithm
• Speed – Fast Allocation & TLAB
– Old Generation GC Algorithm
• – Card Table & Write Barrier

Java Performance
40
Hotspot JVM

• Fast Allocation & TLAB
T T T T T Synchronization
1 2 3 4 5 Wait
Allocation

T T T T T
1 2 3 4 5
TLAB Allocation Allocation Allocation Allocation Allocation

Java Performance
41
Hotspot JVM

• Fast Allocation & TLAB
– Memory
– Bump-the-Pointer
– Multi Thread
– Hotspot JVM TLAB
(Thread Local Allocation Buffer)
• Thread
• TLAB
• Allocation code
– 10 native instructions

Java Performance
42
Hotspot JVM

• Card Table Write Barrier
– 1 Byte Card / 512 Byte of Old Generation
– Old to Young Reference
– Write Barrier
• Bytecode Interpreter
• 2 native instructions
Young Generation

Old Generation

Card Table

Java Performance
43
Hotspot JVM

• Garbage Collection of Hotspot JVM
– Garbage Collection
• Minor Collection : Young Generation
• Major Collection : Old Generation
– Full Collection
– Method Area GC Major Full

Java Performance
44
Hotspot JVM

• Heap of Hotspot JVM

Survivor 1

Survivor 2
Eden Tenured Permanent

Young Generation Old Generation Method Area

GC

Java Performance
45
Hotspot JVM

• JVM Option
– Standard Option
: JVM Option
:-
– Non-Standard Option
• JVM
• , Parameter
• -X Option : Macro
• -XX Option : Micro

Java Performance
46
Hotspot JVM

• Automatic Selection
– Java5 Garbage Collector,
Heap Size
– Hardware Resource OS
– Sever Class Client Class

Java Performance
47
Hotspot JVM

– Server Class
• :2 CPU, 2GB Physical Mem.
• 32bit Windows
• Default Garbage Collector Parallel Collector
• Initial Heap Size
– 1GB 1/64 * Physical Memory
– 1GB 32MB
• Max Heap Size : 1GB ¼ * Physical Mem.
• Server Runtime Compiler

Java Performance
48
Hotspot JVM

– Client Class
• Server Class
• Default Garbage Collector Serial Collector
• Initial Heap Size : 4MB
• Max Heap Size : 64MB
• Client Runtime Compiler

Java Performance
49
Hotspot JVM

– Option
• -server : Server Class
• -client : Client Class

Java Performance
50
Hotspot JVM

• Garbage Collector
– Serial Collector
• Hotspot JVM Default Garbage Collector
• Default Collector
• Young Generation : Generational Algorithm
• Old Generation : Mark-and-Compacting
Algorithm

Java Performance
51
Hotspot JVM

– Parallel Collector
• (Throughput) Garbage Collector
• Throughput Garbage Collector
• Young Generation
• Young Generation : Parallel Copy Algorithm
• Old Generation : Mark-and-Compacting
Algorithm

Java Performance
52
Hotspot JVM

– Parallel Compacting Collector
• Parallel Collector Old Generation

• Java SE 5.0 update 6
• Old Generation : Parallel Compacting
Algorithm

Java Performance
53
Hotspot JVM

– CMS Collector
• CMS : Concurrent Mark-Sweep
•
• Old Generation Pause Time
(Low Pause Garbage Collector)
• Old Generation : Concurrent Mark-and-
Sweep
Algorithm

Java Performance
54
Hotspot JVM

– Incremental Collector
• Low Pause Garbage Collector
• Train Collector
• Young Generation : Generational Algorithm
• Old Generation : Train Algorithm

Java Performance
55
Hotspot JVM

– Garbage First Collector
• Java SE 6 Update 14
• Train Algorithm
• Generation
• Heap Region
Young , Old Area
• Realtime Low Pause

Java Performance
56
Hotspot JVM

• Serial Collector
– Young, Old Generation Serial
(Single CPU Use)
– Suspend during Collecting
– Client Class Collector
–

Java Performance
57
Hotspot JVM

– Young Generation Collection :
Generational

Young Eden

A

Survivor1 - From Survivor2 - To

B C Empty

Old

Java Performance
57
Hotspot JVM

Generational
Minor GC
Young Eden

A


B C A’ B’ C’

Old

Java Performance
57
Hotspot JVM

Generational

Young Eden

Empty


Empty A’ B’ C’

Old

Java Performance
58
Hotspot JVM


Young Eden

A

Survivor1 - To Survivor2 - From

Empty B M

Old

Java Performance
58
Hotspot JVM


Minor GC
Young Eden

Empty

Survivor1 - To Survivor2 - From

A’ B’ Empty

Old
M’

Java Performance
59
Hotspot JVM

– Old Generation Collection
: Mark-and-Compacting

Old

Java Performance
59
Hotspot JVM


Mark

Old
√ √ √

Java Performance
59
Hotspot JVM


Sweep

Old
√ √ √

Java Performance
59
Hotspot JVM


Sliding Compaction

Old
√ √ √

Sliding Compaction : Fragmentation

Java Performance
60
Hotspot JVM

– JVM Option
• -XX:+UseSerialGC
: J2SE5.0, update 6 Serial Collector

• -XX:MaxTenuringThreshold=<value>
: Aging , default 31
• -XX:PretenureSizeThreshold=<value>
: Old Generation directly Allocation
• -XX:+PrintTenuringDistribution
: Promotion

Java Performance
61
Hotspot JVM

• Parallel Collector
– Memory, Multi-CPU

– GC Multi Thread
– Large Young Generation
– Server Class Default Garbage Collector
(CPU 1 )
: Serial Collector

Java Performance
62
Hotspot JVM

– Young Generation Collection : Parallel
Copy

Serial Collector Parallel Collector

Stop & Copy

Java Performance
63
Hotspot JVM

– Promotion Buffer
• Parallel Local Allocation Buffer(PLAB)
• Thread
• Promotion Thread Promotion
Buffer
• Fragmentation
– GC Thread
– Old Generation Size

Java Performance
64
Hotspot JVM

– Promotion Buffer

T1 T2 T1 T2

Promotion Buffer

Java Performance
65
Hotspot JVM

– Option
• -XX:+UseParallelGC : Parallel Collector
(CMS Collector )
• -XX:ParallelGCThreads=<value>
: GC Thread
default CPU

Java Performance
66
Hotspot JVM

– Ergonomics Option
• -XX:MaxGCPauseMills=<value>
(1.5+)
: Maximum Pause Time Goal
Pause Time
• -XX:GCTimeRatio=<value>
(1.5+)
: Throughput Goal , default 1%
GC Time
• -XX:+UseAdaptiveSizePolicy
(1.4.1+)
: Young/Old Sizing

Java Performance
67
Hotspot JVM

• -XX:YoungGenerationSizeIncrement=<value>
: Young Growing Percent, Default = 20
(1.5+)
•-
XX:TenuredGenerationSizeIncrement=<value>
: Old Growing Percent, Default = 20 (1.5+)
•-
XX:AdaptiveSizeDecrementScaleFactor=<value
>
: Growing Shrink , Default = 4 (1.5+)
• -XX:+AggressiveHeap
: Physical Memory Max Heap
Hardware Resource

Java Performance
68
Hotspot JVM

• -XX:GCHeapFreeLimit=<value>
(1.4.1+)
: GC Free Space
Heap (default 5)
-XX:+UseParallelGC GC
OOME
• -XX:GCTimeLimit=<value>
(1.4.1+)
: GC
(default 90)
-XX:+UseParallelGC GC
OOME

Java Performance
69
Hotspot JVM

• Parallel Compacting Collector
– Parallel Collector Old Generation
Collection Algorithm
– Parallel Collector
– Multi CPU
– Young Generation Collection
• Parallel Copy Algorithm

Java Performance
70
Hotspot JVM

: Parallel Compaction

Parallel Compacting Collector

Mark Phase

Summary Phase

Compaction Phase

Java Performance
71
Hotspot JVM

– Mark Phase
• Generation Region
• Live Object Marking
• Parallel Work

Java Performance
71
Hotspot JVM

– Mark Phase
• Generation Region
• Live Object Marking
• Parallel Work

      

Java Performance
72
Hotspot JVM

– Summary Phase
• Region Operation
• Single Work
• Density
• Dense Preﬁx
• Dense Preﬁx GC

      

Java Performance
72
Hotspot JVM

– Summary Phase
• Region Operation
• Single Work
• Density
• Dense Prefix
• Dense Prefix GC

      

Dense Prefix

Java Performance
73
Hotspot JVM

– Compaction Phase
• Thread Region Collecting
• Source, Destination
• Live Object Destination
Compaction

      

Java Performance
73
Hotspot JVM

Compaction

Destination Source

      

Java Performance
73
Hotspot JVM

Compaction

Destination Source

      

T1 T2

Java Performance
73
Hotspot JVM

Compaction

Destination Source

      

T1

Java Performance
74
Hotspot JVM

– Option
• -XX:+UseParallelOldGC (+Java6)
: Parallel Compaction Collector
• -XX:+UseParallelOldGCCompacting
(+Java6)
: Parallel Compaction , Default True
• -XX:+UseParallelDensePreﬁxUpdate
(+Java6)
: Dense Preﬁx , Default True

Java Performance
75
Hotspot JVM

• CMS Collector
– Fast Elapsed Time
– Old
Generation
– Low Latency Collector
–
– GC Pause Time

– Young Generation Collection

Java Performance
76
Hotspot JVM

• CMS Collector
: Concurrent Mark-Sweep
Concurrent Mark-Sweep Collector

Java Performance
76
Hotspot JVM

• CMS Collector

Initial Mark Phase

Java Performance
76
Hotspot JVM

• CMS Collector

Initial Mark Phase

Concurrent Mark Phase

Java Performance
76
Hotspot JVM

• CMS Collector

Initial Mark Phase

Concurrent Mark Phase

Remark Phase

3장. Garbage Collection

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3장. Garbage Collection