Conservative garbage collection on distributed shared memory systems

Abstract

In this paper we present the design and implementation of a conservative garbage collection algorithm for distributed shared memory (DSM) applications that use weakly-typed languages like C or C++, and evaluate its performance. In the absence of language support to identify references, our algorithm constructed a conservative approximation of the set of cross-node references based on local information only. It was also designed to tolerate memory inconsistency on DSM systems that use relaxed consistency protocols. These techniques enabled every node to perform garbage collections without communicating with others, effectively avoiding the high cost of cross-node communication in networks of workstations. We measured the performance of our garbage collector against explicit programmer management using three application programs. In two out of the three programs the performance of the GC version is within 15% of the explicit version. The results showed that the garbage collector has two effects on application programs. On one hand, it tends to reduce memory locality, increasing the communication cost; on the other hand, it may eliminate synchronization and memory accesses that would be incurred if memory were managed by the programmer reducing the communication cost.