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JVM Internals

JVM Internals. Douglas Q. Hawkins. JVM Internals. Bytecode Garbage Collection Optimizations Compile Time Run Time. Java Bytecode. Java Bytecode. Stack Based Local Variable Space. Local Variables. Operand Stack. 7. +. 10. 3. Operation Types. Load and Store Arithmetic and Logic

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JVM Internals

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  1. JVM Internals • Douglas Q. Hawkins

  2. JVM Internals • Bytecode • Garbage Collection • Optimizations • Compile Time • Run Time

  3. Java Bytecode

  4. Java Bytecode • Stack Based • Local Variable Space Local Variables Operand Stack 7 + 10 3

  5. Operation Types • Load and Store • Arithmetic and Logic • Type Conversion • Control Transfer • Object Creation and Manipulation • Operand Stack • Method Invocation

  6. Demo

  7. Garbage Collection

  8. Garbage Collection • Generational Garbage Collection • Segmented into Young, Old, and Permanent Generations • Types of Collectors • Parallel - across multiple threads • Concurrent - while program runs

  9. Demo

  10. Garbage Collection Pattern • Minor • Major • Major Again - for objects with finalize • Soft References • Major • Major Again - for objects with finalize • Throw OutOfMemoryError

  11. Optimizations

  12. Optimizations • Just In Time Compilation • Purely Interpreted • Ahead of Time Compilation • Almost No Compile Time Optimization • Most Optimizations are Runtime

  13. Compile Time Demo

  14. 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 100 0 Heap Allocations 100 0 Method Invocations 200 0 Lock Operations 100 0

  15. Common Sub-Expression Elimination int x = a + b; int y = a + b; int tmp = a + b; int x = tmp; int y = tmp;

  16. 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 < 0 || i >= nums.length ) { throw new ArrayIndexOutOfBoundsException(); } System.out.println( “nums[“ + i + “]=” + nums[ i ] ); }

  17. Loop Invariant Hoisting for ( int i = 0; i < nums.length; ++i ) { ... } int length = nums.length; for ( int i = 0; i < length; ++i ) { ... }

  18. Loop Unrolling int sum = 0; for ( int i = 0; i < 10; ++i ) { sum += i; } int sum = 0; sum += 1; ... sum += 9;

  19. Method Inlining Vector vector = ... double magnitude = vector.magnitude(); static always Vector vector = ... double magnitude = Math.sqrt( vector.u*vector.u + vector.v*vector.v ); final always private always virtual often Vector vector = ... double magnitude; if ( vector instance of Vector2D ) { magnitude = Math.sqrt( vector.u*vector.u + vector.v*vector.v ); } else { magnitude = vector.magnitude(); } reflective sometimes dynamic often

  20. 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 );

  21. Other Lock Optimizations • Biased Locking • Adaptive Locking - Thread sleep vs. Spin lock

  22. Escape Analysis Point p1 = new Point(x1, y1), p2 = new Point(x2, y2); synchronized ( p1 ) { synchronized ( p2 ) { 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 );

  23. Run Time Demo

  24. 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

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