1. Java;Memory
Yury Bubnov
zStore

2. Object o = new Object();

3. Object o = new Object();
java.lang.OutOfMemoryError:
Java
heap
space

4. Object o = new Object();
java.lang.OutOfMemoryError:
Java
heap
space

5. Heap

6. Heap
How many?

7. Heap
How many?
Size?

8. class Structure{
byte b;
short i;
Object o;
}

9. 1 2 4/8
class Structure{
byte b;
short i;
Object o;
}

10. class Structure{
byte b;
short i;
Object o;
}
markmarkmark klass
2 1 1 4

11. class Structure{
byte b;
short i;
Object o;
}
2
8
markmarkmark klass
2 1 1 4

12. class Structure{
byte b;
short i;
Object o;
}
markmarkmark
klassklassklass
2 1 5
888

13. class Structure{
byte b;
short i;
Object o;
}
markmarkmark
klassklassklass
2 1 5
888
4
8

14. Memory address
...01100100100111011001011000

15. Memory address
...01100100100111011001011000

16. Memory address
...01100100100111011001011000
32 bit pointer

17. Memory address
...01100100100111011001011000
32 bit pointer .... to 35 bit

18. Memory address
...01100100100111011001011000
32 bit pointer .... to 35 bit
2^35≈32GB (25-28GB Heap)

19. Memory address
...01100100100111011001011000
32 bit pointer .... to 35 bit
2^35≈32GB (25-28GB Heap)
IBM J9: Java 6 SR1- not enabled by default
Hotspot: since Java 6u14, by default since u23

20. class Structure{
byte b;
short i;
Object o;
}
markmarkmarkmark
klass 1 2 1
4 444

21. class Structure{
byte b;
short i;
Object o;
}
3
8
markmarkmarkmark
klass 1 2 1
4 444

22. klassklass flagsflags
monitormonitor 44
1 3 2 2
=24b
klassklassklassklass
flagsflags size*size*
monitormonitormonitormonitor
8888
1 3 2 2
klass flagsflagsflags
monitor 1 33
4 22 2
=40b
=24b
i386
x64
x64C
IBM J9

23. What’s the object size?
class Structure{ byte b; short i; Object o;}

24. What’s the object size?
HS 32b 16b
HS 64b before u14 32b
HS 64b after u14 (u27) 24b
J9 32b 24b
J9 64b 40b
J9 64b compressed 24b
class Structure{ byte b; short i; Object o;}

25. Mind the gap

26. Mind the gap
byte[][][] b = new byte[10][100][1000];

27. Mind the gap
byte[][][] b = new byte[1000][100][10];
byte[][][] b = new byte[10][100][1000];

28. Mind the gap
byte[][][] b = new byte[1000][100][10];
byte[][][] b = new byte[10][100][1000];
1.032.344=
1.000.000+
8.080(r)+20.220(h)+4.044(a)

29. Mind the gap
byte[][][] b = new byte[1000][100][10];
byte[][][] b = new byte[10][100][1000];
4.832.024=
1.000.000+
808.000(r)+2.020.020(h)+1.004.004(a)
1.032.344=
1.000.000+
8.080(r)+20.220(h)+4.044(a)

30. Mind the gap
byte[][][] b = new byte[1000][100][10];
byte[][][] b = new byte[10][100][1000];
4.832.024=
1.000.000+
808.000(r)+2.020.020(h)+1.004.004(a)
1.032.344=
1.000.000+
8.080(r)+20.220(h)+4.044(a)
x100

31. Mind the gap
byte[][][] b = new byte[1000][100][10];
byte[][][] b = new byte[10][100][1000];
4.832.024=
1.000.000+
808.000(r)+2.020.020(h)+1.004.004(a)
1.032.344=
1.000.000+
8.080(r)+20.220(h)+4.044(a)
x100 x100

32. Mind the gap
byte[][][] b = new byte[1000][100][10];
byte[][][] b = new byte[10][100][1000];
4.832.024=
1.000.000+
808.000(r)+2.020.020(h)+1.004.004(a)
1.032.344=
1.000.000+
8.080(r)+20.220(h)+4.044(a)
x100 x100 x250

33. Heap

34. Heap
Young/Eden
Old/Tenured

35. Heap (HS)
Young/Eden
Old/Tenured
Survivor 1 Survivor 2

36. Heap (HS)

37. Heap (HS)

38. Heap (HS)
TLAB
TLH

39. Heap (HS)
TLAB
TLH

40. Heap (HS)
TLAB
TLH

41. Heap (HS)

42. Heap (HS)

43. Heap (HS)

44. Heap (HS)
X X X

45. Heap (HS)

46. Heap (HS)

47. Heap (HS)

48. Heap (HS)
X XX XX
X

49. Heap (HS)

50. Heap (HS)

51. Heap (HS)

52. Heap (HS)
X XX XX
X

53. Heap (HS)

54. Heap (J9)
Young/Eden
Old/Tenured

55. Heap (J9)
Allocation
Old/Tenured
Survivor

56. Heap (J9)

57. Heap (J9)

58. Heap (J9)

59. Heap (J9)

60. Heap (J9)

61. Garbage Collection

62. Garbage Collection
1. Starts if allocation fails or
System.gc()* called or
condition met

63. Garbage Collection
1. Starts if allocation fails or
System.gc()* called or
condition met
2. Stop-of-the-world operation

64. Garbage Collection
1. Starts if allocation fails or
System.gc()* called or
condition met
2. Stop-of-the-world operation
3. Mark, Sweep and Compact*

65. Mark

66. Mark

67. Mark
PermGen
Stack
JNI
Stack Stack

68. Mark
PermGen
Stack
JNI
X X
X
X
Stack Stack

69. Mark
A
B C
DE

70. A B
0 0 0
C D E
0 0 0 0 0 0 0 0 0 0
Mark
A
B C
DE

71. 1
A B
0 0
C D E
0 0 0 0 0 0 0 0 0 0
A
Mark
A
B C
DE

72. 1
A B
0
C D E
0 0 0 0 0 0 0 0 0
Mark
A
B C
DE
B
C
1 1

73. 1
A B
0
C D E
0 0 0 0 0 0 0 0
Mark
A
B C
DE CE
1 1 1

74. 1
A B
0
C D E
0 0 0 0 0 0 0 0
Mark
A
B C
DE E
1 1 1

75. 1
A B
0
C D E
0 0 0 0 0 0 0 0
Mark
A
B C
DE
1 1 1

76. 1
A B C D E
0 0 0
Mark
A
B C
DE
1 1 11 1 1 1 1 1

77. 1
A B C D E
0 0 0
Mark
A
B C
DE
1 1 11 1 1 1 1 1

78. References

79. References
1. Strong - always marked
2. Soft - not marked if too old or need
memory
3. Weak - not marked if no strong
reference
4. Phantom - correct implementation of
finalize() method

80. Mark

81. MarkSerial

82. Mark

83. MarkParallel

84. Mark
PermGen
JNI
Stack Stack Stack

85. Mark
PermGen
JNI
Stack Stack Stack

86. Mark
PermGen
JNI
Stack Stack Stack

87. Mark
PermGen
JNI
Stack Stack Stack

88. Mark
PermGen
JNI
Stack Stack Stack

89. Mark
PermGen
JNI
Stack Stack Stack

90. Mark
PermGen
JNI
Stack Stack Stack

91. Mark
PermGen
JNI
Concurrent
Stack Stack Stack

92. Sweep
... 1 0 0 1 1 1 0 0 0 1 1 ...

93. Sweep
... 1 0 0 1 1 1 0 0 0 1 1 ...

94. Sweep
... 1 0 0 1 1 1 0 0 0 1 1 ...

95. Sweep
... 1 0 0 1 1 1 0 0 0 1 1 ...

96. Sweep
... 1 0 0 1 1 1 0 0 0 1 1 ...
Serial

97. Sweep
... 1 0 0 1 1 1 0 0 0 1 1 ...
... 1 0 0 1 1 1 0 0 0 1 1 ...
... 1 0 0 1 1 1 0 0 0 1 1 ...

98. Sweep
... 1 0 0 1 1 1 0 0 0 1 1 ...
... 1 0 0 1 1 1 0 0 0 1 1 ...
... 1 0 0 1 1 1 0 0 0 1 1 ...

99. Sweep
... 1 0 0 1 1 1 0 0 0 1 1 ...
Parallel
... 1 0 0 1 1 1 0 0 0 1 1 ...
... 1 0 0 1 1 1 0 0 0 1 1 ...

100. Sweep
... 1 0 0 1 1 1 0 0 0 1 1 ...

101. Sweep
... 1 0 0 1 1 1 0 0 0 1 1 ...

102. Sweep
... 1 0 0 1 1 1 0 0 0 1 1 ...

103. Sweep
... 1 0 0 1 1 1 0 0 0 1 1 ...

104. Sweep
... 1 0 0 1 1 1 0 0 0 1 1 ...

105. Sweep
... 1 0 0 1 1 1 0 0 0 1 1 ...
Concurrent

106. 1 1 0 0 1 1 1 0 0 0 1 1 ...
Compact
1 1 1 1 1 1 1 0 0 0 0 0 ...

107. Garbage Collection
Serial Parallel Concurrent
Which one to choose?

108. Heap (J9)
Allocation
Old/Tenured
Survivor

109. PermGen

110. PermGen
Bootstrap
System

111. PermGen
Bootstrap
System
Ext

112. PermGen
Bootstrap
System
Ext
Web
App

113. PermGen
Bootstrap
System
Ext
Web
App
Web
App

114. PermGen
Bootstrap
System
Ext
Web
App
Web
App
Web
App

115. PermGen
Bootstrap
System
Ext
Web
App
Web
App
Web
App
Web
App

116. PermGen
Bootstrap
System
Ext
Web
App
Web
App
Web
App
Web
App
Web
App

117. PermGen
Bootstrap
System
Ext
Web
App
Web
App
Web
App
Web
App
Web
App
java.lang.OutOfMemoryError:
PermGen
space

118. PermGen
Bootstrap
System
Ext
Web
App
Web
App
Web
App
Web
App

119. PermGen
Bootstrap
System
Ext
Web
App
Web
App
Web
App
Web
App
Heap

120. PermGen
Bootstrap
System
Ext
Web
App
Web
App
Web
App
Web
App
Stack
Heap

121. Stack

122. Stack

123. Stack

124. Stack

125. Stack

126. Stack

127. Stack
java.lang.StackOverflowError

128. Memory

129. Memory
Allocation
Old/Tenured
Survivor

130. Memory
Allocation
Old/Tenured
Survivor
PermGen

131. Memory
Allocation
Old/Tenured
Survivor
PermGen
Stack Stack Stack

132. Memory
Allocation
Old/Tenured
Survivor
PermGen GC
Stack Stack Stack

133. Memory
Allocation
Old/Tenured
Survivor
PermGen
JNI
GC
Stack Stack Stack

134. Memory
Allocation
Old/Tenured
Survivor
PermGen
JNI
GC
Net
Stack Stack Stack

135. Memory
Allocation
Old/Tenured
Survivor
PermGen
JNI
GC
Net
NIO
Stack Stack Stack

136. Memory
Allocation
Old/Tenured
Survivor
PermGen
JNI
GC
Net
NIO
JIT
Stack Stack Stack

137. Memory
Allocation
Old/Tenured
Survivor
PermGen
JNI
GC
Net
NIO
JIT
CL
Stack Stack Stack

138. Memory
Allocation
Old/Tenured
Survivor
PermGen
JNI
GC
Net
NIO
JIT
CL
Stack Stack Stack C++

139. Paging

140. Paging
1. Linux/Windows use Virtual memory
and “paging” to reference physical
2. Pages mapped to physical only when
referenced
3. OS manages pages

141. Real size of Java Process?

142. Real size of Java Process?
1. Architecture: 32 vs 64

143. Real size of Java Process?
1. Architecture: 32 vs 64
2. JVM vendor: Oracle vs IBM

144. Real size of Java Process?
1. Architecture: 32 vs 64
2. JVM vendor: Oracle vs IBM
3. JVM version incl. update

145. Real size of Java Process?
1. Architecture: 32 vs 64
2. JVM vendor: Oracle vs IBM
3. JVM version incl. update
4. JVM settings

146. Real size of Java Process?
1. Architecture: 32 vs 64
2. JVM vendor: Oracle vs IBM
3. JVM version incl. update
5. OS (above applies)
4. JVM settings

147. Real size of Java Process?
1. Architecture: 32 vs 64
2. JVM vendor: Oracle vs IBM
3. JVM version incl. update
5. OS (above applies)
+ Run under real load for long time
4. JVM settings

148. ?