JVM InternalsJVM InternalsDouglas Q. HawkinsDouglas Q. Hawkins

JVM InternalsJVM Internals

BytecodeBytecode

Garbage CollectionGarbage Collection

OptimizationsOptimizations

Compile TimeCompile Time

Run TimeRun Time

Java BytecodeJava Bytecode

Java BytecodeJava Bytecode

Stack BasedStack Based

Local Variable SpaceLocal Variable Space

Local VariablesLocal Variables

Operand StackOperand Stack

3377 ++1010

Operation TypesOperation Types

Load and StoreLoad and Store

Arithmetic and LogicArithmetic and Logic

Type ConversionType Conversion

Control TransferControl Transfer

Object Creation and ManipulationObject Creation and Manipulation

Operand StackOperand Stack

Method InvocationMethod Invocation

DemoDemo

Garbage CollectionGarbage Collection

Garbage CollectionGarbage Collection

Generational Garbage CollectionGenerational Garbage Collection

Segmented into Young, Old, and Permanent Segmented into Young, Old, and Permanent GenerationsGenerations

Types of CollectorsTypes of Collectors

Parallel - across multiple threadsParallel - across multiple threads

Concurrent - while program runsConcurrent - while program runs

DemoDemo

Garbage Collection Garbage Collection PatternPattern

MinorMinor

MajorMajor

Major Again - for objects with finalizeMajor Again - for objects with finalize

Soft ReferencesSoft References

MajorMajor

Major Again - for objects with finalizeMajor Again - for objects with finalize

Throw OutOfMemoryErrorThrow OutOfMemoryError

OptimizationsOptimizations

OptimizationsOptimizations

Just In Time CompilationJust In Time Compilation

Purely InterpretedPurely Interpreted

Ahead of Time CompilationAhead of Time Compilation

Almost No Compile Time Almost No Compile Time OptimizationOptimization

Most Optimizations are RuntimeMost Optimizations are Runtime

Compile Time DemoCompile Time Demo

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

How many...?How many...?

Loop IterationsLoop Iterations

Heap AllocationsHeap Allocations

Method InvocationsMethod Invocations

Lock OperationsLock Operations

100100

100100

200200

100100

00

00

00

00

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

int int tmptmp = = a + ba + b;;int x = int x = tmptmp;;int y = int y = tmptmp;;

Array Bounds Check Array Bounds Check EliminationEliminationint[] nums = ...int[] nums = ...for ( int i = 0; i < nums.length; ++i ) {for ( int i = 0; i < nums.length; ++i ) {System.out.println( “nums[“ + i + “]=” + nums[ System.out.println( “nums[“ + i + “]=” + nums[ i ] );i ] );}}

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

Loop Invariant HoistingLoop Invariant Hoisting

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

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

Loop UnrollingLoop Unrolling

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

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

Method InliningMethod InliningVector vector = ...Vector vector = ...double magnitude = double magnitude = vector.magnitudevector.magnitude();();

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

Vector vector = ...Vector vector = ...double magnitude;double magnitude;if ( vector instance of Vector2D ) {if ( vector instance of Vector2D ) { magnitude = Math.sqrt(magnitude = Math.sqrt(vector.u*vector.u + vector.v*vector.v );vector.u*vector.u + vector.v*vector.v );} else {} else { magnitude = vector.magnitude();magnitude = vector.magnitude();}}

staticstatic

finalfinal

privateprivate

virtualvirtual

reflectivereflective

dynamicdynamic

alwaysalways

alwaysalways

alwaysalways

oftenoften

sometimessometimes

oftenoften

Lock CoarseningLock CoarseningStringBuffer buffer = ...StringBuffer buffer = ...buffer.buffer.appendappend( “Hello” );( “Hello” );buffer.buffer.appendappend( name );( name );buffer.buffer.appendappend( “\n” );( “\n” );StringBuffer buffer = ...StringBuffer buffer = ...locklock( buffer ); buffer.append( “Hello” ); ( buffer ); buffer.append( “Hello” ); unlockunlock( buffer );( buffer );locklock( buffer ); buffer.append( name ); ( buffer ); buffer.append( name ); unlockunlock( buffer );( buffer );locklock( buffer ); buffer.append( “\n” ); ( buffer ); buffer.append( “\n” ); unlockunlock( buffer );( buffer );StringBuffer buffer = ...StringBuffer buffer = ...locklock( buffer ); ( buffer ); buffer.append( “Hello” );buffer.append( “Hello” );buffer.append( name );buffer.append( name );buffer.append( “\n” );buffer.append( “\n” );unlockunlock( buffer );( buffer );

Other Lock Other Lock OptimizationsOptimizations

Biased LockingBiased Locking

Adaptive Locking - Thread sleep vs. Spin lockAdaptive Locking - Thread sleep vs. Spin lock

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

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

synchronized ( p1 ) {synchronized ( p1 ) {

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

}}}}

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

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

Run Time DemoRun Time Demo

ResourcesResources

Brian GoetzBrian Goetz

Developer Works ArticlesDeveloper Works Articles

Tony PrintezisTony Printezis

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

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

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

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

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

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