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Published byNatalie Jackson Modified over 10 years ago
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JVM Internals Douglas Q. Hawkins
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JVM Internals Bytecode Garbage Collection Optimizations Compile Time Run Time
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Java Bytecode
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Stack Based Local Variable Space Local Variables Operand Stack 3 7+ 10
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Operation Types Load and Store Arithmetic and Logic Type Conversion Control Transfer Object Creation and Manipulation Operand Stack Method Invocation
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Demo
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Garbage Collection
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Generational Garbage Collection Segmented into Young, Old, and Permanent Generations Types of Collectors Parallel - across multiple threads Concurrent - while program runs
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Demo
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Garbage Collection Pattern MinorMajor Major Again - for objects with finalize Soft References Major Major Again - for objects with finalize Throw OutOfMemoryError
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Optimizations
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Optimizations Just In Time Compilation Purely Interpreted Ahead of Time Compilation Almost No Compile Time Optimization Most Optimizations are Runtime
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Compile Time Demo
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Is This Optimized? double sumU = 0, sumV = 0; 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...? Loop Iterations Heap Allocations Method Invocations Lock Operations 100 100 200 100 0 0 0 0
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Common Sub-Expression Elimination int x = a + b; int y = a + b; int tmp = a + b; int x = tmp; int y = tmp;
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Array Bounds Check Elimination int[] nums =... for ( int i = 0; i < nums.length; ++i ) { System.out.println( nums[ + i + ]= + nums[ i ] ); } int[] nums =... for ( int i = 0; i < nums.length; ++i ) { if ( i = nums.length ) { if ( i = nums.length ) { throw new ArrayIndexOutOfBoundsException(); throw new ArrayIndexOutOfBoundsException(); } System.out.println( nums[ + i + ]= + nums[ i ] ); }
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Loop Invariant Hoisting for ( int i = 0; i < nums.length; ++i ) {...} int length = nums.length; for ( int i = 0; i < length; ++i ) {...}
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Loop Unrolling int sum = 0; for ( int i = 0; i < 10; ++i ) { sum += i; sum += i;} int sum = 0; sum += 1;... sum += 9;
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Method Inlining Vector vector =... double magnitude = vector.magnitude(); Vector vector =... double magnitude = Math.sqrt( vector.u*vector.u + vector.v*vector.v ); vector.u*vector.u + vector.v*vector.v ); Vector vector =... double magnitude; if ( vector instance of Vector2D ) { magnitude = Math.sqrt( magnitude = Math.sqrt( vector.u*vector.u + vector.v*vector.v ); } else { magnitude = vector.magnitude(); magnitude = vector.magnitude();} static final private virtual reflective dynamic always always always often sometimes often
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Lock Coarsening StringBuffer 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 );
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Other Lock Optimizations Biased Locking Adaptive Locking - Thread sleep vs. Spin lock
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Escape Analysis Point p1 = new Point(x1, y1), p2 = new Point(x2, y2); synchronized ( p2 ) { synchronized ( p2 ) { double dx = p1.getX() - p2.getX(); double dx = p1.getX() - p2.getX(); 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 = y1 - y2; double distance = Math.sqrt( dx*dx + dy*dy );
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Run Time Demo
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Resources Brian Goetz Developer Works Articles Tony Printezis Garbage Collection in the Java HotSpot Virtual Machine - Garbage Collection in the Java HotSpot Virtual Machine - http://www.devx.com/Java/Article/21977 http://www.devx.com/Java/Article/21977 Java Specialist Newsletter - http://www.javaspecialists.eu/ 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
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