Operating System Support for Flexible Coherence in Distributed Shared Memory

COMMOS1 is an operating system architecture developed to support shared persistent data objects in distributed systems. This paper describes its support for flexible coherence. The approach is based on a microkernel, typed memory objects and integrated coherence control. The coherence server is clearly separated from the external pager. This separation makes it easier to provide multiple coherence protocols and also supports addition of new coherence protocols by the addition of new coherence managers and servers. The support for typed memory objects makes it easier for applications to choose the most suitable protocols for different types of object to avoid protocol mismatch. Low-level coherence control is integrated with high-level concurrency control so that the number of messages required to maintain memory coherence is reduced and systemwide synchronisation is realised without severely impacting the system performance. In this paper, various design issues are discussed and the design of a flexible coherence mechanism, which accommodates multiple coherence protocols, is detailed. The prototype implementation and performance measurements are presented.