JVM Internals - Garbage Collection & Runtime Optimizations

JVM InternalsDouglas Q. Hawkins

Sunday, August 15, 2010

JVM Internals

Bytecode

Garbage Collection

Optimizations

Compile Time

Run Time

Java Bytecode

Stack Based

Local Variable Space

Local Variables

Operand Stack

Java Bytecode

Stack Based

Local Variables

Operand Stack

Java Bytecode

Stack Based

Local Variables

Operand Stack

Java Bytecode

Stack Based

Local Variables

Operand Stack

Java Bytecode

Stack Based

Local Variables

Operand Stack

Java Bytecode

Stack Based

Local Variables

Operand Stack

Operation Types

Load and Store

Arithmetic and Logic

Type Conversion

Control Transfer

Object Creation and Manipulation

Operand Stack

Method Invocation

Garbage Collection

Generational Garbage Collection

Segmented into Young, Old, and Permanent Generations

Types of Collectors

Parallel - across multiple threads

Concurrent - while program runs

Young Generation

SurvivorSpaces

Old Generation

Tenured

Young Generation

SurvivorSpaces

Old Generation

Tenured

Young Generation

SurvivorSpaces

Old Generation

Tenured

A B C D

Young Generation

SurvivorSpaces

Old Generation

Tenured

A B C D E F G

Young Generation

SurvivorSpaces

Old Generation

Tenured

A B C D E F G

Young Generation

SurvivorSpaces

Old Generation

Tenured

Young Generation

SurvivorSpaces

Old Generation

Tenured

Young Generation

SurvivorSpaces

Old Generation

Tenured

H I J K L M N

Young Generation

SurvivorSpaces

Old Generation

Tenured

H I J K L M N

Young Generation

SurvivorSpaces

Old Generation

Tenured

K L M N

Young Generation

SurvivorSpaces

Old Generation

Tenured

Young Generation

SurvivorSpaces

Old Generation

Tenured B E J

Garbage Collection Pattern

Major Again - for objects with finalize

Soft References

Major Again - for objects with finalize

Throw OutOfMemoryError

Optimizations

Just In Time Compilation

Purely Interpreted

Ahead of Time Compilation

Almost No Compile Time Optimization

Most Optimizations are Runtime

Compile Time Demo

Is This Optimized?double sumU = 0, sumV = 0;for ( int i = 0; i < 100; ++i ) { Vector2D vector = new Vector2D( i, i ); synchronized ( vector ) { sumU += vector.getU(); sumV += vector.getV(); }}

How many...?

Loop Iterations

Heap Allocations

Method Invocations

Lock Operations

How many...?

Loop Iterations

Heap Allocations

Method Invocations

Lock Operations

How many...?

Loop Iterations

Heap Allocations

Method Invocations

Lock Operations

Common Sub-Expression Eliminationint x = a + b;int y = a + b;

int tmp = a + b;int x = tmp;int y = tmp;

Array Bounds Check Eliminationint[] nums = ...for ( int i = 0; i < nums.length; ++i ) {

System.out.println( “nums[“ + i + “]=” + nums[ i ] );}

Array Bounds Check Eliminationint[] nums = ...for ( int i = 0; i < nums.length; ++i ) {

int[] nums = ...for ( int i = 0; i < nums.length; ++i ) { if ( i < 0 || i >= nums.length ) { throw new ArrayIndexOutOfBoundsException(); }

Loop Invariant Hoisting

for ( int i = 0; i < nums.length; ++i ) {...}

Loop Invariant Hoisting

for ( int i = 0; i < nums.length; ++i ) {...}

int length = nums.length;for ( int i = 0; i < length; ++i ) {...}

Loop Unrolling

int sum = 0;for ( int i = 0; i < 10; ++i ) { sum += i;}

Loop Unrolling

int sum = 0;for ( int i = 0; i < 10; ++i ) { sum += i;}

int sum = 0;sum += 1;...sum += 9;

Method InliningVector vector = ...double magnitude = vector.magnitude();

Vector vector = ...double magnitude = Math.sqrt( vector.u*vector.u + vector.v*vector.v );

Vector vector = ...double magnitude;if ( vector instance of Vector2D ) { magnitude = Math.sqrt(

vector.u*vector.u + vector.v*vector.v );} else { magnitude = vector.magnitude();}

Vector vector = ...double magnitude;if ( vector instance of Vector2D ) { magnitude = Math.sqrt(

vector.u*vector.u + vector.v*vector.v );} else { magnitude = vector.magnitude();}

static

private

virtual

reflective

dynamic

always

sometimes

Lock CoarseningStringBuffer buffer = ...buffer.append( “Hello” );buffer.append( name );buffer.append( “\n” );

StringBuffer buffer = ...lock( buffer ); buffer.append( “Hello” ); unlock( buffer );lock( buffer ); buffer.append( name ); unlock( buffer );lock( buffer ); buffer.append( “\n” ); unlock( buffer );

StringBuffer buffer = ...lock( buffer ); buffer.append( “Hello” );buffer.append( name );buffer.append( “\n” );unlock( buffer );

Other Lock Optimizations

Biased Locking

Adaptive Locking - Thread sleep vs. Spin lock

Escape AnalysisPoint p1 = new Point( x1, y1 ), p2 = new Point( x2, y2 );

synchronized ( p2 ) { double dx = p1.getX() - p2.getX();

synchronized ( p1 ) {

double dy = p1.getY() - p2.getY();

double distance = Math.sqrt( dx*dx + dy*dy );

double dx = p1.getX() - p2.getX(); double dy = p1.getY() - p2.getY(); double distance = Math.sqrt( dx*dx + dy*dy );

double dx = x1 - x2;double dx = y1 - y2;double distance = Math.sqrt( dx*dx + dy*dy );

Run Time Demo

Resources

Brian Goetz

Developer Works Articles

Tony Printezis

Garbage Collection in the Java HotSpot Virtual Machine - http://www.devx.com/Java/Article/21977

Java Specialist Newsletter - http://www.javaspecialists.eu/

http://java.sun.com/javase/6/docs/technotes/guides/vm/cms-6.html

http://java.sun.com/docs/hotspot/gc1.4.2/faq.html

http://www.fasterj.com/articles/G1.html

http://www.informit.com/guides/content.aspx?g=java&seqNum=27

JVM Internals - Garbage Collection & Runtime Optimizations

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