Performance implications of communication mechanisms in all-software global address space systems

Abstract

Global addressing of shared data simplifies parallel programming and complements message passing models commonly found in distributed memory machines. A number of programming systems have been designed that synthesize global addressing purely in software on such machines. These systems provide a number of communication mechanisms to mitigate the effect of high communication latencies and overheads. This study compares the mechanisms in two representative all-software systems: CRL and Split-C. CRL uses region-based caching while Split-C uses split-phase and push-based data transfers for optimizing communication performance. Both systems take advantage of bulk data transfers.By implementing a set of parallel applications in both CRL and Split-C, and running them on the IBM SP2, Meiko CS-2 and two simulated architectures, we find that split-phase and push-based bulk data transfers are essential for good performance. Region-based caching benefits applications with irregular structure and with sufficient temporal locality, especially under high communication latencies. However, caching also hurts performance when there is insufficient data reuse or when the size of caching granularity is mismatched with the communication granularity. We find the programming complexity of the communication mechanisms in both languages to be comparable. Based on our results, we recommend that an ideal system intended to support diverse applications on parallel platforms should incorporate the communication mechanisms in CRL and Split-C.