[gclist] Instanteneous GC

Tim Sweeney tim@epicgames.com
Wed, 28 Jul 1999 23:49:02 -0400


Thanks for all the feedback everyone has given regarding optimizing a game
programming language for garbage-collection efficiency.  It has been a great

I've started prototyping my collector and its data structures, and will
probably be seeking more advice once I get into all the hairy details.  I'm
having a hard time finding the right set of tradeoffs to make, regarding
immediate vs incremental collection, and robustness of support for

Once idea I've always liked, despite its huge implementation hurdles, is
"immediate garbage collection", where all references to objects are subject
to a write-barrier and some logic that causes all objects to be collected
the very instant they become garbage.  I'm not talking about a simple
reference counting scheme (which has pitfalls with cyclic references), I
mean a fully general GC scheme that notices immediately when groups of
objects in the rooted graph become disjoint.

I like the concept because I'm a big fan of deterministic behaviour, and
I've always been a little uneasy about the idea of garbage objects
"eventually" being cleaned up.

Ok, so here's what I've come up with:

1. Absolutely naieve implementation: Each time a reference is updated or
goes out of scope, start at the root object and do a full mark-and-sweep
through all objects and determine which ones are garbage. Advantage: always
instantaneous. Disadvantage: Incredibly inefficient.

2. Faster implementation: Assign each object a positive number, starting
with 0 at the root.  For each object, maintain for lists of references:
(incoming, outgoing) x (to lower numbered objects, to higher numbered
objects).  Enforce a rule that every object always has at least one incoming
reference from a lowered number object.  Whenever a reference changes that
breaks this rule, move the offending object to the end of the list (make it
the highest numbered object), and readjust the other objects--which may
cause them to recursively fall to the bottom of the list.  In this process,
some objects that land at the bottom of the list will have no upward
references, and those objects have immediately become garbage.  The overhead
of this is (on a 32 bit machine) 8 bytes per object + 24 bytes per reference
variable.  This has a lot of O(n^2) performance cases, for example many
double-linked list implementations, but I think the algorithm can be
restructured to never be worse than O(n).  Still this is impractically slow.

So, my question is: is there any reasonably efficient way to do
instantaneous garbage collection, using a write-barrier function that can
immediately determine that objects have become garbage?