Comprehensive, Concurrent, and Robust Garbage Collection in the Distributed, Object-Based System Emerald

Garbage collection schemes are investigated to provide a solution well-suited for the distributed object-based system, Emerald. A distributed garbage collection scheme is implemented. It collects all garbage (comprehensive), works while the system is running user applications (concurrent), and copes with partial failures due to the distributed nature of the system (robustness). The collection scheme reaches its goals by using distributed control, a dual-collector scheme, and an object protection mechanism. Emerald is a distributed, object-based system running on a set of workstations connected by a local-area network. The Emerald garbage collection scheme employs two mark-and-sweep collectors on each node in the distributed system. To reduce the latency introduced, both collectors work concurrently with other processes by protecting unmarked objects during the mark-phase, and interleaving the sweep-phase with the allocator. A comprehensive collection of all garbage in the entire distributed system is achieved by the cooperation of one collector from each node. These global collectors exchange information about reachable objects during their mark-phase and synchronize their initialization and termination of the mark-phase, without using a coordinating node. The scheme is robust to temporary node failures and depends only on nodes being pair-wise able to exchange messages. As the comprehensive collection depends on information about the entire distributed graph of objects and references between them, any node failure may postpone the termination of the mark-phase. To achieve more expedient collection the other set of collectors does node-local collection on each node using a root set extended with the objects potentially known from other nodes. The scheme has been tested in the Emerald system running on four VAXstation 2000 workstations in a local-area network. Measurements show a garbage collection overhead during normal operation on less than 10%. The average pause introduced by garbage collection may be limit to 0:01 seconds.