The document discusses memory usage of Java objects. It explains that the memory needed to store objects goes beyond just the data and includes an object header overhead as well as memory for things like references to other objects. Wrapping primitives in their object equivalents like Integer can significantly increase memory usage compared to using the primitives directly. Other collections like ArrayList can also use 5x more memory than just storing the raw data. Special collections like Trove can reduce these overheads.

1. Nikita Salnikov-Tarnovski
HOW MUCH MEMORY DO
YOUR OBJECTS REALLY
NEED?
Monday, April 1, 13

2. WHO AM I
Master Developer at Plumbr
We solve memory leaks ... for now
Giving you the exact location of the leak with enough
information to fix it
The foundation is based on machine learning
Ongoing effort
Monday, April 1, 13

3. MORE PLUMBR
trained on 500,000 memory snapshots
From 3,000 different applications
Finding 88% of the existing leaks.
20,000 monthly unique visitors in our site
400 monthly downloads
1700+ leaks discovered
Monday, April 1, 13

4. AGENDA
What’s the deal?
How to measure
Primitives
References
Collections
Monday, April 1, 13

5. INTRO
How much space is needed to store 10M integers in a set
in Java?
Hint: 4 bytes * 10 000 000 = 40 MB
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6. SHALLOW VS DEEP
You can measure shallow size of the object
Or deep size of the subgraph starting with the object
Or retained size of the subgraph dominated by the object
Monday, April 1, 13

7. RETAINED SIZE
r(O1)=O1+O2+O3+O4 r(O3)=O3+O4
r(O2)=O2 r(O4)=O4
Monday, April 1, 13

8. SIZE OF AN OBJECT
An overhead of being an Object
Call it object header
constant on a given JVM
Data
primitives
arrays
pointers to other objects
plus all this from superclasses
Monday, April 1, 13

9. HOW TO MEASURE
Manually, based on JLS/JVM spec
Memory measurer
SizeofAgent
http://sourceforge.net/projects/sizeof/
Javaspecialists.eu issue 142
https://github.com/shipilev/java-object-layout
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10. HANDS ON
•
Lets measure java.lang.Object
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11. ALIGNMENT
8 byte alignment
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12. JAVA.LANG.OBJECT
8 bytes @ 32 bit JVM
12 bytes @ 64 bit JVM
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13. PRIMITIVES
Type Java Language Spec (JLS)
byte 1 byte
short 2 bytes
int 4 bytes
long 8 bytes
char 2 bytes
float 4 bytes
double 8 bytes
boolean 1 bit
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14. HANDS ON
Lets measure primitives
Monday, April 1, 13

15. PRIMITIVES
Type JLS JVM cost
byte 1 byte 1..8 bytes
short 2 bytes 2..8 bytes
int 4 bytes 4..8 bytes
long 8 bytes 8 bytes
char 2 bytes 2..8 bytes
float 4 bytes 4..8 bytes
double 8 bytes 8 bytes
boolean 1 bit 1..8 bytes
Monday, April 1, 13

16. WRAPPER OBJECTS
Type JLS JVM cost Wrapper*
byte 1 byte 1..8 bytes 16 bytes
short 2 bytes 2..8 bytes 16 bytes
int 4 bytes 4..8 bytes 16 bytes
long 8 bytes 8 bytes 24 bytes
char 2 bytes 2..8 bytes 16 bytes
float 4 bytes 4..8 bytes 16 bytes
double 8 bytes 8 bytes 24 bytes
boolean 1 bit 1..8 bytes 16 bytes
* 64 bit JVM objects, thus adding 12 bytes
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17. INTERMISSION
10 000 000 ints as Integer objects =
10 000 000 * 16 = 160 000 000 bytes
160 MB of stuff worth of 40 MB of data!
Monday, April 1, 13

18. OBJECT REFERENCES:
QUIZ
What is the size of an instance of this class:
class With2Members {
Object x = null;
Object y = null
}
16 bytes?
24 bytes?
32 bytes?
Monday, April 1, 13

19. QUIZ: ANSWER
32 bit
8 + 4 + 4 = 16
64 bit +CompressedOOPs (Xmx < 32g)
12 + 4 + 4 = 20 (align) → 24
64 bit –CompressedOOPs (Xmx > 32g)
12 + 8 + 8 = 28 (align) → 32
Monday, April 1, 13

20. FLYWEIGHTS
Flyweight pattern
<PrimitiveWrapper>.valueOf()
Byte
Short
Integer
Only 1 byte is
cached
Long
Character
String.intern()
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21. BACK TO COLLECTIONS
Collection (10 M ints) Overhead
Pure data 0
int[] ~0
Integer[] 5x (200M)
Integer[] (valueOf) <5x (200M)
Lets run intro again.
ArrayList<Integer>(10M) <5x (200M)
ArrayList<Integer>() 5.15x (205M)
HashSet<Integer>() 13.7x (547M)
HashSet<Integer>(10M) 13.7x (547M)
Monday, April 1, 13

22. TROVE
Collection (10 M ints) Size
Pure data 40 000 000 (40M)
TIntArrayList ~1.05x (42M)
TIntArrayList(10M) ~0 (40M)
TIntHashSet ~3.3x (131M)
TIntHashSet(10M) ~2.6x (105M)
Monday, April 1, 13

23. MORE
Collection (10 M ints) Size
Pure data 40 000 000 (40M)
fastutil IntOpenHashSet ~2.1x (83M)
org.a.c.c.p. ArrayIntList ~1.4x (55M)
hppc.IntIntOpenHashMap ~3.8x (150M)
cern.colt.map.OpenIntIntHashMap ~6.5x (260M)
Monday, April 1, 13