The performance realities of massively parallel processors: a case study

The authors present the results of an architectural comparison of SIMD (single-instruction multiple-data) massive parallelism, as implemented in the Thinking Machines Corp. CM-2, and vector or concurrent-vector processing, as implemented in the Cray Research Inc., Y-MP/8. The comparison is based primarily upon three application codes taken from the LANL (Los Alamos National Laboratory) CM-2 workload. Tests were run by porting CM Fortran codes to the Y-MP, so that nearly the same level of optimization was obtained on both machines. The results for fully configured systems, using measured data rather than scaled data from smaller configurations, show that the Y-MP/8 is faster than the 64 k CM-2 for all three codes. A simple model that accounts for the relative characteristic computational speeds of the two machines, and reduction in overall CM-2 performance due to communication or SIMD conditional execution, accurately predicts the performance of two of the three codes. The authors show the similarity of the CM-2 and Y-MP programming models and comment on selected future massively parallel processor designs