(PPoPP'97) Relaxed Consistency and Coherence Granularity in DSM Systems: A Performance Evaluation

During the past few years, two main approaches have been taken to improve the performance of software shared memory implementations: relaxing consistency models and providing ne-grained access control. Their performance tradeos, however, are not well understood. This paper studies these tradeos on a platform that provides access control in hardware but runs coherence protocols in software. We compare the performance of three protocols across four coherence granularities, using 12 applications on a 16-node cluster of workstations. Our results show that no single com-bination of protocol and granularity performs best for all the applications. The combination of a sequentially consistent (SC) protocol and ne granularity works well with 7 of the 12 applications. The combination of a multiple-writer, home-based lazy release consistency (HLRC) protocol and page granularity works well with 8 out of the 12 applications. For applications that suer performance losses in moving to coarser granularity under sequential consistency, the performance can usually be regained quite eectively using relaxed protocols, particularly HLRC. We also nd that the HLRC protocol performs substantially better than a single-writer lazy release consistent (SW-LRC) protocol at coarse granularity for many irregular applications. For our applications and platform, when we use the original versions of the applications ported directly from hardware-coherent shared memory, we nd that the SC protocol with 256-byte granularity performs best on average. However, when the best versions of the applications are compared, the balance shifts in favor of HLRC at page granularity.