LIMITATIONS AND CAPABILITIES OF WEAK MEMORY CONSISTENCY SYSTEMS

Abstract

This dissertation develops and exploits a formalism for specifying memory consistency models. This formalism lays down the foundations for describing memory consistency models at various levels, and develops techniques to prove the equivalence between models defined at different levels. Two levels, called non-operational and operational, are addressed in this dissertation. The non-operational level describes these models in terms of program instructions or procedures, while the operational level describes them in terms of implementation events. Formal techniques are developed to prove the equivalence of rigorous specifications at both levels. This formalism is then exploited to define the memory consistency models of two state-of-the-art multiprocess systems: the SPARC version 8 architecture and the Java Virtual Machine. These models are defined at both operational and non-operational levels. These operational and non-operational descriptions are proved equivalent. The SPARC models provide "reasonably" weak memory consistency models that are capable of avoiding the use of explicit synchronization primitives for certain problems. However, Java provides a consistency model that is completely dependent on synchronization primitives, without which no form of coordination between different threads is possible. Fundamental process coordination problems have been extensively studied for traditional systems with strong memory consistency. This dissertation revisits the critical section and the producer/consumer problems in the context of weak memory consistency models. It establishes that the majority of known weak memory consistency models are incapable of supporting a solution to the critical section problem without the use of explicit synchronization primitives. Surprisingly, most of these models are capable of supporting solutions to versions of the producer/consumer problem without the use of these primitives.