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JVM performance optimization, Part 3: Garbage collection

Choose the right garbage collector for your application needs

By Eva Andreasson, JavaWorld.com, 10/10/12

Page 3 of 8

The main difficulty with reference counting collectors is keeping the reference counts accurate. Another well-known challenge is the complexity associated with handling circular structures. If two objects reference each other and no live object refers to them, their memory will never be released. Both objects will forever remain with a non-zero count. Reclaiming memory associated with circular structures requires major analysis, which brings costly overhead to the algorithm, and hence to the application.

Tracing collectors

Tracing collectors are based on the assumption that all live objects can be found by iteratively tracing all references and subsequent references from an initial set of known to be live objects. The initial set of live objects (called root objects or just roots for short) are located by analyzing the registers, global fields, and stack frames at the moment when a garbage collection is triggered. After an initial live set has been identified, the tracing collector follows references from these objects and queues them up to be marked as live and subsequently have their references traced. Marking all found referenced objects live means that the known live set increases over time. This process continues until all referenced (and hence all live) objects are found and marked. Once the tracing collector has found all live objects, it will reclaim the remaining memory.

Tracing collectors differ from reference-counting collectors in that they can handle circular structures. The catch with most tracing collectors is the marking phase, which entails a wait before being able to reclaim non-referenced memory.

Tracing collectors are most commonly used for memory management in dynamic languages; they are by far the most common for the Java language and have been commercially proven in production environments for many years. I'll focus on tracing collectors for the remainder of this article, starting with some of the algorithms that implement this approach to garbage collection.