Vector vs. Scalar Processors: A Performance Comparison Using a Set of Computational Science Benchmarks

Despite a significant decline in their popularity in the last decade vector processors are still with us, and manufacturers such as Cray and NEC are bringing new products to market. We have carried out a performance comparison of three full-scale applications, the first, SBLI, a Direct Numerical Simulation code from Computational Fluid Dynamics, the second, DL_POLY, a molecular dynamics code and the third, POLCOMS, a coastal-ocean model. Comparing the performance of the Cray X1 vector system with two massively parallel (MPP) micro-processor-based systems we find three rather different results. The SBLI PCHAN benchmark performs excellently on the Cray X1 with no code modification, showing 100% vectorisation and significantly outperforming the MPP systems. The performance of DL_POLY was initially poor, but we were able to make significant improvements through a few simple optimisations. The POLCOMS code has been substantially restructured for cache-based MPP systems and now does not vectorise at all well on the Cray X1 leading to poor performance. We conclude that both vector and MPP systems can deliver high performance levels but that, depending on the algorithm, careful software design may be necessary if the same code is to achieve high performance on different architectures.