Java Performance Tuning Study Memo

Wonderful blogs from http://java-performance.info/!!! List of articles http://java-performance.com/

Java type memory usage

byte, boolean	1 byte
short, char	2 bytes
int, float	4 bytes
long, double	8 bytes

Byte, Boolean	16 bytes
Short, Character	16 bytes
Integer, Float	16 bytes
Long, Double	24 bytes

EnumSet, BitSet	1 bit per value
EnumMap	4 bytes (for value, nothing for key)
ArrayList	4 bytes (but may be more if ArrayList capacity is seriously more than its size)
LinkedList	24 bytes (fixed)
ArrayDeque	4 to 8 bytes, 6 bytes on average

JDK collection	Size	Possible Trove substitution	Size
HashMap	32 * SIZE + 4 * CAPACITY bytes	THashMap	8 * CAPACITY bytes
HashSet	32 * SIZE + 4 * CAPACITY bytes	THashSet	4 * CAPACITY bytes
LinkedHashMap	40 * SIZE + 4 * CAPACITY bytes	None
LinkedHashSet	32 * SIZE + 4 * CAPACITY bytes	TLinkedHashSet	8 * CAPACITY bytes
TreeMap, TreeSet	40 * SIZE bytes	None
PriorityQueue	4 * CAPACITY bytes	None

All Java objects start with 8 bytes containing service information like object class and its identity hash code (returned by System.identityHashCode method). Arrays have 4 more bytes (one int field) containing array length. It looks like all user-written (not JDK classes) have another reference to object Class. These fields are followed by all declared fields. All objects are aligned by 8 bytes boundary. All primitive fields must be aligned by their size (for example, chars should be aligned by 2 bytes boundary).Object reference (including any arrays) occupy 4 bytes. What does it mean for us? In order to get most use of available memory, all object fields must occupy N*8+4 bytes (4, 12, 20, 28 and so on). In this case 100% memory will contain useful data.

Java Boxing Type Caching

Byte, Short, Long	Character	Integer	Float, Double
From -128 to 127	From 0 to 127	From -128 to java.lang.Integer.IntegerCache.high or 127, whichever is bigger	No caching

Java Performance Tips

Never use exceptions as return code replacement or for any likely to happen events (especially in not IO-bound methods!). Throwing an exception is too expensive - you may experience 100 times slowdown for simple methods.

Throwing an exception in Java is a very slow operation. Expect that throwing an exception costs you something between 100 and 1000 ticks in most cases.

Case Study

http://java-performance.info/use-case-optimizing-memory-footprint-of-read-only-csv-file-trove-unsafe-bytebuffer-data-compression/