On the fly garbage collection

• Author: Edsger W. Dijkstra, Leslie Lamport, A.J. Martin, C.S. Scholten, E.F.M. Steffens
• Name: On-the-Fly Garbage Collection: An Exercise in Cooperation
• Link: http://research.microsoft.com/en-us/um/people/lamport/pubs/garbage.pdf

WC 252 / RT 2min

In our abstract form of the problem, we consider a directed graph of varying structure but with a fixed number of nodes, in which each node has at most two outgoing edges. More precisely, each node may have a left-hand outgoing edge and may have a right-hand outgoing edge, but either of them or both may be missing. In this graph a fixed set of nodes exists, called “the roots.” A node is called “reachable” if it is reachable from at least one root via a directed path along the edges.

The subgraph consists of all reachable nodes and their interconnections is called ‘the data structure’; nonreachable nodes that do not belong to the data structure are called garbage.

Data structure can modified:

• Redirecting an outgoing edge of a reachable node towards an already reachable one.
• Redirecting an outgoing edge of a reachable node towards a not yet reachable one without outgoing edges.
• Adding–where an outgoing edge was missing an edge pointing from a reachable node towards an already reachable one.
• Adding–where an outgoing edge was missing an edge pointing from a reachable node towards a not yet reachable one without outgoing edges.
• Removing an outgoing edge of a reachable node

Mutator: redirect an outgoing edge of reachable node towards an already reachable one.

Collector:

• marking phase: mark all reachable nodes
• appending phase: append all unmarked nodes to the free list and remove the markings from all marked nodes

Notes

Free list - collection of nodes that have been identified as garbage.