Handling Conflicts with Compiler's Help in Software Transactional Memory Systems

Abstract

Atomic sections are supported in software through the use of optimistic concurrency by using Software Transactional Memory (STM). However STM implementations incur high overheads which reduce the wide-spread use of this approach by programmers. Conflicts are a major source of overheads in STMs. The basic performance premise of a transactional memory system is the optimistic concurrency principle wherein data updates executed by the transactions are to disjoint objects/memory locations, referred to as Disjoint Access Parallel (DAP). Otherwise, the updates conflict, and all but one of the transactions are aborted. Such aborts result in wasted work and performance degradation. While contention management systems in STM implementations try to reduce conflicts by various runtime feedback control mechanisms, they are not aware of the application's structure and data access patterns and hence typically act after the conflicts have occurred. In this paper we propose a scheme based on compiler analysis, which can identify static atomic sections whose instances, when executed concurrently by more than one thread always conflict. Such an atomic section is referred to as Always Conflicting Atomic Section (ACAS). We propose and evaluate two techniques Selective Pessimistic Concurrency Control (SPCC) and compiler inserted Early Conflict Checks (ECC) which can help reduce the STM overheads caused by ACAS. We show that these techniques help reduce the aborts in 4 of the STAMP benchmarks by up to 27.52% while improving performance by 1.24% to 19.31%.