Impact of Temporal Juxtaposition on the Isolated Phase Optimization Approach to Mapping an Algorithm to Mixed-Mode Architectures

~ Mixed-mode parallel processing systems are capa¬ ble of executing in either SIMD (synchronous) or MIMD (asyn¬ chronous) modes of parallelism. The ability to switch between the two modes at instruction level granularity with very little overhead allows the parallelism mode to vary for each portion of an algorithm. To fully exploit the capability of intermixing both SIMD and MIMD operations within a single program, one must determine the optimum mapping of an algorithm to the mixedmode architecture. The phase optimization technique, where the programmer makes an implicit assumption that by combining the best version of each phase the optimal implementation of the entire program will be achieved, generally works in a serial com¬ puter environment. The application of this approach to the selection of a mode of parallelism for each phase is investigated by presenting a detailed study of a practical image processing application, the Edge Guided Thresholding algorithm, and its mapping to a mixed-mode parallel architecture. The six func¬ tional phases of the algorithm, as well as their temporal juxtapo¬ sition, are analyzed along with experimental performance meas¬ urements obtained from the PASM parallel processing proto¬ type, a mixed-mode system. The results discussed here demon¬ strate a situation where the advantages of a mixed-mode approach are limited.