Save Java memory

1
● Jan Kotek
– jan@kotek.net
– @JanKotek
– MapDB author (database engine)
– Java developer for past 12 years
– Last 3 years independent consultant for MapDB
● Slides and code at
– https://github.com/jankotek/talk-save-java-memory
Intro

2
Intro
● This talk is about:
– Reducing memory footprint
– Object overhead
– Primitive variables and arrays
– Alternative Java Collections
● Not about:
– Garbage Collection
– Off-heap
– Performance
– Memory leaks

3
Measure memory consumption
● Runtime.getRuntime().freeMemory() difference
● Allocate until OutOfMemoryError
● Profiler
● Class or heap dump analyze such as Java Object Layout

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Runtime.getRuntime().freeMemory()
● Get allocated memory stats
● Create new objects
● Call GC many times
● Get allocated memory difference
● Not very precise, but easy
● May not always work,
some JVMs ignore System.gc()
NanoBench nanoBench = NanoBench.create();
nanoBench.memoryOnly().warmUps(2).measurements(10)
.measure(String.format("Measure memory for %d keys", i),
new Runnable() {
@Override public void run() {
Random setRandom = new Random(4532);
ref = new HashSet<Integer>(cnt);
w hile (ref.size() < cnt) {
ref.add(setRandom.nextInt());
}
}
});
nanoBench.getMemoryBytes()

5
OutOfMemoryError
● Memory limit (-Xmx1G)
● Add entries to array in loop
● Print number of items
● JVM eventually crashes
try{
//add new entry in loop
while(true)
array[arrayPos++] = new HashMap();
}catch(OutOfMemoryError e){
memoryConsumption =
Runtime.getRuntime().maxMemory()
/ counter;
return;
}

6
Profiler
● Shows memory consumed
by single class
– (byte[], Object[])
● VisualVM is easy to use
● Flight Recorder is best
– Free outside production env.

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Java Object Layout
● Command line tool to analyze memory layout (and space usage)
● Most advanced
● http://openjdk.java.net/projects/code-tools/jol/
● Written by Aleksey Shipilev
● Example:
$ java -jar jol-cli/target/jol-cli.jar internals java.util.HashMap
Instance size: 48 bytes (reported by Instrumentation API)
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total

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Instance overhead in Java
● Pointer overhead
– 4 to 8 bytes
– depends on JVM and amount of memory
– 64bit JVM 8 uses compressed pointer when possible
● Class info and object header
– 12 bytes
● Object alligment padding
– Between 0 and 15
35 bytes worst case scenario

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java.lang.Integer overhead
● One object + one reference
● Total overhead 16 bytes +
reference (4 to 8 bytes)
JOL output for java.lang.Integer
OFFSET SIZE TYPE DESC
0 12 (object header)
12 4 int Integer.value
total size: 16 bytes

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java.lang.String overhead
● Two objects and two references
● One int field (String.hash)
● Two bytes per char (UTF16)
● Other problems:
– Weak hash (many collisions)
– Defensive char[] copies
– Substring create copies etc..
JOL output for java.lang.String
OFFSET SIZE TYPE DESC
0 4 (object header)
4 4 (object header)
8 4 (object header)
12 4 char[] String.value
16 4 int String.hash
20 4 (alignment loss)
total size: 24 bytes

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String.intern()
●
Reduces number of instances
● Each string creates new weak ref (huge GC overhead)
● Long.valueOf(11) is similar
name.toLowerCase().intern()
==
name.toLowerCase().intern()

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Represent String as char[]
● Field is char[] or byte[]
● Get/set takes string
● GC overhead, but short lived
objects are fast to collect
● Reduced memory with many
objects (large Maps etc)
public class Person{
private char[] name;
public String getName(){
return new String(name);
}
public void setName(String name){
this.name = name.toCharArray();
}
}

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char[] as key in Map
● char[] has no equals() and hashCode() methods
●
TreeMap key can use Comparator<char[]>
– com.google.common.primitives.Chars.lexicographicalComparator()
● java.util.HashMap can not change hash strategy
– Object2ObjectOpenCustomHashMap from FastUtil has Hash Strategies
– String.hashCode() is broken, many collisions
● Part of spec, can not be changed :-(
● ConcurrentHashMap Java8 has sorted lists to handle hash duplicates :-)

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char[] as value in Map
● Works (mostly)
● Concurrent Compare-And-Swap operations broken
– char[].equals() uses identity
– ConcurrentMap.replace(K key, V oldValue, V newValue);
– MapDB Maps allow custom equality

15
Bitwise
class Address{
String street;
int houseNumber;
}
● Replace street with number from dictionary
●
Combine streetId and houseNumber into single long number
– 5 bytes for streetId, 3 bytes for house number and flags
● Adress become primitive number, no objects or references

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Primitive key / value in collections
● HashMap<Long,Long> uses Object[] for keys and values
● ~40 bytes overhead for each key-value pair
● Use long[] or int[] instead
● Data locality → CPU cache friendly
– Binary search much faster
– Faster fetch of values
● Almost zero Garbage Collection
– entire HashMap uses two objects
– No objects allocated for new key or value

17
FastUtil
● http://fastutil.di.unimi.it/
● Autogenerated classes with
primitive keys and values
● Int2IntMap, Int2BooleanMap, Int2ObjectMap<V> ….
● Many classes, 17MB jar

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FastUtil
// create new map
Int2ObjectMap<String> map =
new Int2ObjectOpenHashMap();
// put/get has primitive arg, no unboxing!
map.put(1, "aa");
map.get(1);
// also implements full mapdb interface
Map<Integer, String> map2 = map;
● Parallel interface hierarchy
● Get / set has primitive arg
version, no unboxing!
● Implements Map interface

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Chronicle Map
● ConcurrentHashMap
● Keys and values are serialized
outside heap
● Supports any serializable data
type
● No objects or references on
heap, memory efficient
● http://chronicle.software/product
s/chronicle-map/
Map<Long, long[]> graph =
ChronicleMap
.of(Long.class, long[].class)
.entries(1_000_000_000L)
.averageValue(new long[150])
.create();

20
Conclusion
● Avoid too many object instances in long lived objects
● Short lived objects are fine, when traded for smaller memory
● Email: jan@kotek.net
● Slides and code at
– https://github.com/jankotek/talk-save-java-memory

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