2.3 Generation-Based Collectors

Old versus New Garbage   

In many programs, people have observed that object lifetime tends toward the extreme possibilities.

• temporary objects that are created, used, and become garbage almost immediately
• long-lived objects that do not become garbage until program termination

Generational GC   

Generational collectors take advantage of this behavior by dividing the heap into "generations".

• The area holding the older generation is scanned only occasionally.
• The area holding the youngest generation is scanned frequently for possible garbage.
  • an object in the young generation area that survives a few garbage collection passes is moved to the older generation area

The actual scanning process is a modified mark and sweep. But because relatively few objects are scanned on each pass, the passes are short and the overall cost of GC is low.

To keep the cost of a pass low, we need to avoid scanning the old objects on the heap. The problem is that some of those objects may have pointers to the newer ones. Most generational schemes use traps in the virtual memory system to detect pointers from "old" pages to "new" ones to avoid having to explicitly scan the old area on each pass.