Location consistency - a new memory model and cache consistency protocol

Abstract

Existing memory models and cache consistency protocols assume the memory coherence property which requires that all processor observe the same ordering of write operations to the same location. In this paper, we address the problem of defining a memory model that does not rely on the memory coherence assumption and also the problem of designing a cache consistency protocol based on such a memory model. We define a new memory consistency model, called Location Consistency (LC), in which the state of a memory location is modeled as a partially ordered multiset (pomset) of write and synchronization operations. We prove that LC is strictly weaker than existing memory models, but is still equivalent to stronger models for the common case of parallel programs that have no data races. We also describe a new multiprocessor cache consistency protocol based on the LC memory model. We prove that this LC protocol obeys the LC memory model. The LC protocol does not need to enforce single write ownership of memory blocks. As a result, the LC protocol is simpler and more scalable than existing and directory-based cache consistency protocols.