Out ofmemoryerror what is the cost of java objects

OutOfMemoryError:
What is the cost of Java Objects?
Jean-Philippe BEMPEL @jpbempel
Performance Architect http://jpbempel.blogspot.com
© ULLINK 2016

Agenda
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• java.lang.Object
• CompressedOops
• Structure sizes
• Troubleshooting
• War stories
• Solutions

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java.lang.Object

java.lang.Object
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Fields 32 bits 64 bits 64 bits
CompressedOops
mark word 4 bytes 8 bytes 8 bytes
klass pointer 4 bytes 8 bytes 4 bytes
Total 8 bytes 16 bytes 12 bytes (but 16 with
padding/alignment)

java.lang.Object
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CompressedOops

• 32 bits pointer can address 4GB
• Memory addresses are aligned (4 or 8)
• Objects reside only on address 8 multiple
• Last 3 bits are not effectively used
• Use the 3 bits to increase addressable space
CompressedOops
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Example:
Address 88 in binary:
101 1000
will be encoded as (3 bits shift)
1011
Allows to encode 8 times more object
addresses then with raw 32 bits addressing
=> Maximum addressable is then 32GB
CompressedOops saves ~20-30% memory
Activated by default on 64bits JVM
JVM option: -XX:+UseCompressedOops
CompressedOops
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Padding
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With 8 bytes field alignment, padding occurs
What is the real size of this class:
class Data
{
long l;
boolean b;
int i;
char c;
String str;
}

Padding
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Heap Dump analysis with profilers can give a good
estimation
Real size depends on 32bits/64Bits/CompressedOops
A precise way: Java Object Layout (JOL)
java -XX:+UseCompressedOops -jar jol-internals.jar java.lang.Object
Running 64-bit HotSpot VM.
Using compressed references with 3-bit shift.
Objects are 8 bytes aligned.
Field sizes by type: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]
Array element sizes: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]
java.lang.Object object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 01 00 00 00 (0000 0001 0000 0000 0000 0000 0000 0000)
4 4 (object header) 00 00 00 00 (0000 0000 0000 0000 0000 0000 0000 0000)
8 4 (object header) 6d 05 88 df (0110 1101 0000 0101 1000 1000 1101 1111)
12 4 (loss due to the next object alignment)
Instance size: 16 bytes (estimated, add this JAR via -javaagent: to get accurate result)
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total

Padding
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Class Data:
java -XX:+UseCompressedOops -classpath .;jol-internals.jar org.openjdk.jol.MainObjectInternals
Data
Running 64-bit HotSpot VM.
Using compressed references with 3-bit shift.
Objects are 8 bytes aligned.
Field sizes by type: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]
Array element sizes: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]
Data object internals:
0 4 (object header) 01 00 00 00 (0000 0001 0000 0000 0000 0000 0000 0000)
4 4 (object header) 00 00 00 00 (0000 0000 0000 0000 0000 0000 0000 0000)
8 4 (object header) b6 8c 91 df (1011 0110 1000 1100 1001 0001 1101 1111)
12 4 int Data.i 0
16 8 long Data.l 0
24 2 char Data.c
26 1 boolean Data.b false
27 1 (alignment/padding gap) N/A
28 4 String Data.str null

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Structure sizes

Arrays
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Arrays have additional int for size
Header for 64 bits + CompressedOops: 16 bytes
byte[32] => header + 1*32 bytes = 48 bytes
short[32] => header + 2*32 bytes = 80 bytes
char[32] => header + 2*32 bytes = 80 bytes
int[32] => header + 4*32 bytes = 144 bytes
long[32] => header + 8*32 bytes = 272 bytes
double[32] => header + 8*32 bytes = 272 bytes
Object[32] => header + RefSize*32 bytes = 144
bytes

java.lang.String
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String class fields
1.6.0_45 (32 bytes)
• char[] value
• int hash
• int count
• int offset
1.7.0_55 (24 bytes)
• char[] value
• int hash
• int hash32
1.8.0_60 (24 bytes)
• char[] value
• int hash
String.subString()
● <1.7.0_06 => shared char[]
● >= 1.7.0_06 => make copy of char[]

java.lang.String
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Class String + char[]
example for string foo
Class String Size + char array header + 2*3 bytes = 24 + 16 + 6 = 46
bytes
overhead = 1433%
for string of 100 characters:
Class String Size + char array header + 2*100 bytes = 24 + 16 + 200 = 240
bytes
overhead = 140%

java.util.LinkedList
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Class LinkedList: 32 bytes
Class Node: 24 bytes
Example for 100 elements: 32 + 100*24 = 2432 bytes

java.util.ArrayList
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Class ArrayList: 24 bytes
Object[]: 16 + n*4 bytes
Example for 100 elements: 24 + 16 + 100*4 = 440 bytes

java.util.HashMap
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class HashMap: 48 bytes
Entry[]: 16 + n*4 bytes
class Entry: 32 bytes
Example for 100 key/value pairs:
48 + 16 + 256*4 + 100*32 = 4288 bytes

java.util.HashMap
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● entrySet() called => add EntrySet instance (16 bytes)
● keySet called => add KeySet instance (16 bytes)
● values() called => add Values instance (16 bytes)
For those inner classes this is object header + outer ref
java.util.HashMap.Values object internals:
0 12 (object header) N/A
12 4 HashMap Values.this$0 N/A
Instance size: 16 bytes (estimated, the sample instance is not available)

java.util.HashSet
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class HashSet: 16 bytes
example for 100 elements: 16 + HashMap cost = 4304 bytes

java.util.TreeMap
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class TreeMap: 48 bytes
class Entry: 40 bytes (double penalty)
example for 100 elements:
48 + 100*40 = 4048 bytes

java.util.TreeMap
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● entrySet() called => add EntrySet instance (16 bytes)
● keySet called => add KeySet instance (16 bytes)
● values() called => add Values instance (16 bytes)
● navigableKeySet called => add KeySet instance (16 bytes)
● descendingMap called => add DescendingSubMap instance
(56 bytes)

java.util.concurrent.ConcurrentHashMap (1.8)
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class ConcurrentHashMap: 64 bytes
class Node: 32 bytes
Example for 100 elements:
64 + 16 + 256*4 + 100*32 = 4304 bytes

java.util.concurrent.ConcurrentHashMap (1.7)
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class ConcurrentHashMap: 48 bytes
Segment[]: 16+16*4 = 80 bytes
class Segment: 40 bytes + class Sync: 32 bytes
HashEntry[]: 16 + n*4 bytes
class HashEntry: 32 bytes
example for 100 elements:
48 + 80 + 16*(40 + 32 + 16 + 16*4) + 32*100 = 5760 bytes

Summary
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Collections Overhead for 100
ArrayList 440 bytes
LinkedList 2432 bytes
TreeMap 4048 bytes
HashMap 4288 bytes
HashSet 4304 bytes
ConcurrentHashMap (1.8) 4304 bytes
ConcurrentHashMap (1.7) 5760 bytes

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Troubleshooting

Troubleshooting: Class histogram
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jmap -histo <pid>
num #instances #bytes class name
----------------------------------------------
1: 73903 5978960 [C
2: 4309 4085624 [I
3: 2790 1730968 [B
4: 49641 1191384 java.lang.String
5: 22080 706560 java.util.HashMap$Node
6: 5420 571328 java.lang.Class
7: 7408 431080 [Ljava.lang.Object;
8: 2908 331584 [Ljava.util.HashMap$Node;
9: 1339 289224 sun.java2d.SunGraphics2D
10: 2882 253616 java.lang.reflect.Method
11: 6586 227248 [Ljava.lang.Class;
12: 1632 223768 [Ljava.lang.String;
13: 4973 198920 java.security.AccessControlContext
14: 3880 186240 java.util.HashMap
15: 4890 156480 java.util.Hashtable$Entry
16: 5376 129024 java.lang.StringBuilder
17: 3293 105376 java.lang.ref.WeakReference
18: 1424 102528 java.awt.geom.AffineTransform
19: 3186 101952 java.awt.Rectangle
20: 3005 96160 java.util.concurrent.ConcurrentHashMap$Node
JVM Option: -XX:+PrintClassHistogramBeforeFullGC
Includes class histogram in GC logs

Troubleshooting: Heap dump
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To generate it:
● jmap -dump:live,format=b,file=heap.hprof <pid>
● JVM Options:
○ -XX:+HeapDumpOnOutOfMemoryError
○ -XX:HeapDumpPath=../logs
● JMX: com.sun.management:type=HotSpotDiagnostic.dumpHeap(filename,
liveonly)
To exploit heap dump:
● visualVM
● YourKit
● Eclipse MAT

War stories
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Overhead structures example:
251614.001: [GC 251614.001: [ParNew (promotion failed): 943744K->940573K(943744K), 0.9248570 secs]251614.926:
[Class Histogram:
----------------------------------------------
1: 421751083 20244051984 java.util.concurrent.ConcurrentHashMap$HashEntry
2: 23327688 12130397760 com.ullink.ulmonitoring.api.model.SimpleBMOrder
3: 23428922 6694953456 [Ljava.util.concurrent.ConcurrentHashMap$HashEntry;
4: 11574497 5007170576 [C
5: 178973 2246627208 [B
6: 23357783 1681760376 java.util.concurrent.ConcurrentHashMap
7: 26115162 1253527776 java.util.HashMap$Entry
8: 23386843 1122568464 java.util.concurrent.locks.ReentrantLock$NonfairSync
9: 23381591 1122316368 java.util.concurrent.ConcurrentHashMap$Segment
10: 27363594 1094543760 java.lang.String
11: 22350856 894034240 com.ullink.ulmonitoring.api.model.IndentedHelper
12: 22350849 894033960 com.ullink.ulmonitoring.api.model.OrderLine
13: 23357784 747639528 [Ljava.util.concurrent.ConcurrentHashMap$Segment;
14: 22350792 715225344 com.ullink.ulmonitoring.extension.view.HierarchicalOrderLine
15: 23356168 560548032 com.ullink.ulmonitoring.api.model.property.PropertyHolderImpl
...
817: 40 4480 com.ullink.ulmonitoring.extension.view.HierarchicalViewDataContainer

War stories
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Client class (104 bytes)
com.ullink.oms.model.Client object internals:
0 4 (object header) 01 00 00 00 (0000 0001 0000 0000 0000 0000 0000 0000)
4 4 (object header) 00 00 00 00 (0000 0000 0000 0000 0000 0000 0000 0000)
8 4 (object header) 3b de 91 df (0011 1011 1101 1110 1001 0001 1101 1111)
12 4 String OdisysObject.id null
16 4 String Client.type null
20 4 Capacity Client.capacity null
24 4 String Client.description null
28 4 String Client.currency null
32 4 String Client.parentClientId null
36 4 Collection Client.contact null
40 4 String Client.data null
44 4 Contact Client.office null
48 4 String Client.originCountry null
52 4 Boolean Client.allowUnknownAccount null
56 4 String Client.connectionDetails null
60 4 String Client.backoffice null
64 4 Policy Client.policy null
68 4 ConfirmationSchedule Client.confirmationSchedule null
72 4 Collection Client.stampExemptCountries null
76 4 Map Client.customProperties null
80 4 Delivery Client.delivery null
84 4 Warehousing Client.warehousing null
88 4 Map Client.externalIds null
92 4 String Client.feeId null
96 4 Boolean Client.active null
100 4 (loss due to the next object alignment)

War stories
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----------------------------------------------
1: 116925840 18595529736 [C
2: 100508918 10452927472 com.ullink.oms.model.Client
3: 116908590 2805806160 java.lang.String
4: 39690 773638968 [B
5: 77391 596989120 [Ljava.lang.Object;
6: 6116590 195730880 java.util.HashMap$Entry
7: 2370792 149697448 [Ljava.util.HashMap$Entry;
8: 2406042 115490016 java.util.HashMap
9: 149661 20530168 <constMethodKlass>
10: 149661 19166816 <methodKlass>
Event user.update
1 user with 32 560 Clients
3796 events = 245M Client instances => 25GB + Strings
Persistence delete operation keep objects in a Collection

War stories
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Instrument class:
● 62 ref fields
● String, BigDecimal, Boolean, Double, Collection,
Custom Classes (AlternateId (56B), Time (80B),
Enums (24B)...)
Size: 264 bytes

War stories
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Instrument Referential: 3M
Only Instrument instances: 792MB
Add 8 chars for instrumentId: +168MB (56*3M)
Add 4 chars for symbol: + 312MB (48*3M + 56*3M)
Add settlement date: + 408MB (56*3M + 80*3M)
Add 3 chars for currency: + 138MB (46*3M)
Total 1818MB
Other representation:
for each instrument : Map<String, MDRecord>

References
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Java Object Layout:
http://openjdk.java.net/projects/code-tools/jol/
What Heap Dumps Are Lying To You About:
http://shipilev.net/blog/2014/heapdump-is-a-lie/
From Java code to Java heap:
http://www.ibm.com/developerworks/library/j-codetoheap/
Building Memory-efficient Java Applications: Practices and
Challenges:
http://www.cs.virginia.edu/kim/publicity/pldi09tutorials/memory-
efficient-java-tutorial.pdf

Q & A
Jean-Philippe BEMPEL @jpbempel
Performance Architect http://jpbempel.blogspot.com
© ULLINK 2016

Out ofmemoryerror what is the cost of java objects

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