Multiprocessors based on simultaneous multithreaded (SMT) or multicore
(CMP) processors are continuing to gain a significant share in both
high-performance and mainstream computing markets. In this paper we
evaluate the performance of OpenMP applications on these two parallel
architectures. We use detailed hardware metrics to identify
architectural bottlenecks. We find that the high level of resource
sharing in SMTs results in performance complications, should more than
1 thread be assigned on a single physical processor. CMPs, on the
other hand, are an attractive alternative. Our results show that the
exploitation of the multiple processor cores on each chip results in
significant performance benefits. We evaluate an adaptive, run-time
mechanism which provides limited performance improvements on SMTs,
however the inherent bottlenecks remain difficult to overcome. We
conclude that out-of-the-box OpenMP code scales better on CMPs than
SMTs. To maximize the efficiency of OpenMP on SMTs, new capabilities
are required by the runtime environment and/or the programming
interface.