Supporting Ada 95 Passive Partitions in a Distributed Environment

Ada 95 passive partitions, containing passive library units, provide the means to distribute data within a network of workstations. This paper shows how passive partitions can be implemented via distributed shared virtual memory (DSM). DSM provides the logical view of a portion of memory shared between physically distributed workstations within a network. In this paper, we relate design issues and operational characteristics of DSM systems to the semantics of shared passive library units as speciied in the Ada 95 distributed system annex. We designed and implemented such a DSM system, operating at the granularity of pages, in a portable fashion over POSIX threads. The DSM system establishes its own communication services and is completely independent of any Ada runtime system support. Protected objects are supported via a novel prioritized protocol for distributed mutual exclusion. We integrated the DSM system with the Gnu Ada Translator (Gnat) and its environment to support active partitions. The DSM system integration required minimal changes to the Gnat environment and remains completely transparent to the user and the Ada runtime system. This is the rst implementation of shared passive library units within an Ada 95 compilation environment, to our knowledge. The DSM support can provide considerable performance advantages over alternative approaches with repeated remote accesses, since with DSM only the rst access to remote data results in communication overhead while consecutive accesses may take the same time as local memory accesses. Using the DSM paradigm, distributed applications can be designed to access distributed shared data eeciently. Furthermore, the shared memory model inherent to the Ada language can be used without modiications in a distributed environment.