Everyone wants more computing power for their applications, and the industry has responded in two ways: first by increasing the speed of single CPU's, and second by deploying multiple processors in parallel. Much controversy exists over how best to balance processor speed against the number of processors employed. Is it better to have a single, very fast and very expensive CPU, thousands of very slow but very cheap CPU's, or is there some optimal mix in between? The value of single processors, measured in floating-point performance per dollar, is relatively easy to assess, but the corresponding value of parallel systems is obscured by the fact that applications are not generally perfectly parallel, with some loss in efficiency occurring due to sequential bottlenecks and communication overhead.