Tuesday, October 11, 2005

Better GC in JDK6 (Mustang)

As noted in OSnews, IBM has a great article on GC in the Java VM entitled Java Urban Legends. One of the developments in the ever-improving JVM really caught my eye:

Escape analysis is an optimization that has been talked about for a long time, and it is finally here -- the current builds of Mustang (Java SE 6) can do escape analysis and convert heap allocation to stack allocation (or no allocation) where appropriate. The use of escape analysis to eliminate some allocations results in even faster average allocation times, reduced memory footprint, and fewer cache misses. Further, optimizing away some allocations reduces pressure on the garbage collector and allows collection to run less often.

Escape analysis can find opportunities for stack allocation even where it might not be practical to do so in the source code, even if the language provided the option, because whether a particular allocation gets optimized away is determined based on how the result of an object-returning method is actually used in a particular code path. The Point returned from getLocation() may not be suitable for stack allocation in all cases, but once the JVM inlines getLocation(), it is free to optimize each invocation separately, offering us the best of both worlds: optimal performance with less time spent making low-level, performance-tuning decisions.

The referenced code before optimization:

public double getDistanceFrom(Component other) {
    Point otherLocation = new Point(other.x, other.y);
    int deltaX = otherLocation.x - location.x;
    int deltaY = otherLocation.y - location.y;
    return Math.sqrt(deltaX*deltaX + deltaY*deltaY);
}

And after:

public double getDistanceFrom(Component other) {
    int tempX = other.x, tempY = other.y;
    int deltaX = tempX - location.x;
    int deltaY = tempY - location.y;
    return Math.sqrt(deltaX*deltaX + deltaY*deltaY);
}

A nice win for Sun's JVM, to be sure.

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