Question

Lets assume we have a simple generational GC with only two generations, the “old” generation (objects who survived at least one collection) and the “young” generation (newly allocated). So how exactly would the GC determine a “young” object to be garbage without tracing the whole reference graph from the very roots? Or to put it a different way: What does the GC choose as roots for the trace when indending to collect the “young” generation only?

I’m interested in the general method but in specific examples from existing implementations as well.

Thanks!

Answer 1

There are a few techniques, which all boil down to maintaining knowledge of which old-gen objects (or ranges of old-gen memory) may contain references to young objects.

Pretty much all implementations I can think of maintain this knowledge by adding write barriers. Those write barriers trigger when a young-gen reference is stored in a old-gen object, and thereby cause execution of a small code snippet which remembers the new reference.

To store that knowledge, some GCs use card marking, where a compact bitmap is used to mark small-ish memory blocks as “contains references to younger generations”. Others maintain explicit “remembered sets”, which does something similar for individual objects. In both cases, young-gen collections then add the objects in the (remembered set/memory blocks marked by the card table) to the roots.

As for specific implementations:

• Mono uses remembered sets.
• PyPy has several GCs, the newest and shiniest (Minimark) uses remembered sets, with the addition of card marking for individual large arrays.
• .NET uses card marking.