Barrier techniques for incremental tracing

This paper presents a classification of barrier techniques for interleaving tracing with mutator operation during an incremental garbage collection. The two useful tricolour invariants are derived from more elementary considerations of graph traversal. Barrier techniques for maintaining these invariants are classified according to the action taken at the barrier (such as scanning an object or changing its colour), and it is shown that the algorithms described in the literature cover all the possibilities except one. Unfortunately, the new technique is impractical. Ways of combining barrier techniques are also discussed.