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Chapter 9 of Inside the Java Virtual Machine
Garbage Collection
by Bill Venners

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Generational Collectors

One disadvantage of simple stop and copy collectors is that all live objects must be copied at every collection. This facet of copying algorithms can be improved upon by taking into account two facts that have been empirically observed in most programs in a variety of languages:

  1. Most objects created by most programs have very short lives.
  2. Most programs create some objects that have very long lifetimes. A major source of inefficiency in simple copying collectors is that they spend much of their time copying the same long-lived objects again and again.

Generational collectors address this inefficiency by grouping objects by age and garbage collecting younger objects more often than older objects. In this approach, the heap is divided into two or more sub-heaps, each of which serves one "generation" of objects. The youngest generation is garbage collected most often. As most objects are short-lived, only a small percentage of young objects are likely to survive their first collection. Once an object has survived a few garbage collections as a member of the youngest generation, the object is promoted to the next generation: it is moved to another sub-heap. Each progressively older generation is garbage collected less often than the next younger generation. As objects "mature" (survive multiple garbage collections) in their current generation, they are moved to the next older generation.

The generational collection technique can be applied to mark and sweep algorithms as well as copying algorithms. In either case, dividing the heap into generations of objects can help improve the efficiency of the basic underlying garbage collection algorithm.

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