Garbage collecting the world

Distributed symbolic computations involve the existence of remote references allowing an object, local to a processor, to designate another object located on another processor. To reclaim inaccessible objects is the non trivial task of a distributed Garbage Collector (GC). We present in this paper a new distributed GC algorithm which (i) is fault-tolerant, (ii) is largely independent of how a processor garbage collects its own data space, (iii) does not need centralized control nor global stop-the-world synchronization, (iv) allows for multiple concurrent active GCs, (v) does not require to migrate objects from processor to processor and (vi) eventually reclaims all inaccessible objects including distributed cycles. These results are mainly obtained through the concept of a group of processors (or processes). Processors of a same group cooperate together to a GC inside this group; this GC is conservative with respect to the outside of the group. A processor contributes to the global GC of all groups to which it belongs. Garbage collection on small groups reclaims quickly locally distributed garbage clusters, while garbage collection on large groups ultimately reclaims widely distributed garbage clusters, albeit more slowly. Groups can be reorganized dynamically, in particular to tolerate failures of some member processors. These properties make the algorithm usable on very large and evolving networks of processors. Other than distributed symbolic computations, possible applications include for example distributed file or database systems.