Question

On the slides I am revising from it says the following:

Live objects can be identified either by maintaining a count of the number of references to each object, or by tracing chains of references from the roots.

Reference counting is expensive – it needs action every time a reference changes and it doesn’t spot cyclical structures, but it can reclaim space incrementally.

Tracing involves identifying live objects only when you need to reclaim space – moving the cost from general access to the time at which the GC runs, typically only when you are out of memory.

I understand the principles of why reference counting is expensive but do not understand what
"doesn’t spot cyclical structures, but it can reclaim space incrementally." means. Could anyone help me out a little bit please?

Thanks

Answer 1

Reference counting doesn’t spot cyclical structures…

Let’s say you have two objects O1 and O2. They reference each other: O1 -> O2 and O2 -> O1, and no other objects references them. They will both have reference count 1 (one referrer).

If neither O1 or O2 is reachable from a GC root, they can be safely garbage collected. This is not detected by counting references though, since they both have reference count > 0.

0 references is a sufficient but not necessary requirement for an object to be eligible for garbage collection.

…but it can reclaim space incrementally.

The incremental part refers to the fact that you can garbage collect some of the 0-referenced objects quickly, get interrupted and continue at another time without problems.

If a tracing-algorithm gets interrupted it will need to start over from scratch the next time it’s scheduled. (The tree of references may have changed since it started!)