SGen – Finalization And Weak References

Introduction

In this installment of my series on SGen, Mono’s new garbage collector, we shall be looking at how finalizers and weak references are implemented, and why you (almost certainly) should not use finalizers.

Tracking Object Lifetime

Both finalizers and weak references need to track the lifetime of certain objects in order to take an action when those objects become unreachable. To that end SGen keeps lists of finalizable objects and weak references which it checks against at the end of every collection.

If the object referred to by a weak reference has become unreachable, the weak reference is nulled.

If a finalizable object is deemed unreachable by the collector, it is put onto the finalization queue and it is marked, since it must be kept alive until the finalizer has run. Of course, all objects that it references have to be marked as well, so the main collection loop is activated again.

Generations

A nursery collection cannot collect an object in the major heap, so it is not necessary to check the status of old objects after a nursery collection. That is why SGen keeps separate finalization and weak reference lists for the nursery and major heaps.

Invoking Finalizers

SGen uses a dedicated thread for invoking finalizers. The finalization queue is processed one object at a time. As long as an object is in the finalization queue it is also considered live, i.e. the finalization queue is a GC root.

Resurrection

Resurrecting an object means making it reachable again from within its finalizer or a finalizer that can still reach the object (or via a tracking weak reference). The garbage collector does not have to treat this case specially—until the finalizer has run the object is considered live by virtue of its being in the finalization queue, and afterwards it is live because it is reachable through some other root(s).

Tracking Weak References

If an object is weakly referenced and has a finalizer, the weak reference will be nulled during the same collection as the finalizer is put in the finalization queue. That is not always desirable, especially for objects that might be resurrected.

Tracking references solve the problem by keeping the reference intact at least until the finalizer has run. Once the finalizer has finished, a tracking reference acts like a standard weak reference, i.e. it will be nulled once the object becomes unreachable, typically during the next collection (unless the object was resurrected).

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