Scope consistency: A bridge between release consistency and entry consistency

The large granularity of communication and coherence in shared virtual memory systems causes problems with false sharing and extra communication. Relaxed memory consistency models have been used to alleviate these problems, but at a cost in programming complexity. Release Consistency (RC) and Lazy Release Consistency (LRC) are accepted to offer a reasonable tradeoff between performance and programming complexity. Entry Consistency (EC) offers a more relaxed consistency model, but it requires explicit association of shared data objects with synchronization variables. The programming burden of providing such associations can be substantial. This paper proposes a new consistency model for shared virtual memory, called Scope Consistency (ScC), which offers most of the potential performance advantages of the EC model without requiring explicit bindings between data and synchronization variables. Instead, ScC dynamically detects the bindings implied by the programmer allowing a programming interface as simple as that of RC or LRC in most cases. We compare ScC with Automatic Update Release Consistency (AURC), a modified LRC protocol which takes advantage of new network interfaces that provide automatic update support. AURC already improves performance substantially over the all-software LRC. For three of the five applications we used, ScC further improves the speedups achieved by AURC by about 10%. We also show how ScC may be used without any hardware support.