Online "What-if" Metrics

In this talk I will describe a new technique to assist programmers with making informed decisions about how to tune their parallel programs. The technique permits programmers, while their programs are running, to ask "what-if questions about potential tuning alter-natives. Two examples of "what-if" questions will be presented. First, I will describe a non-trace based algorithm to compute the critical path profile of the execution of a mes-sage passing parallel program. The algorithm permits starting or stopping the critical path computation during program execution and reporting intermediate values. Second, I will describe a metric called Load Balancing Factor (LBF) that accesses the impact of moving where a computation is performed rather than reducing its execution time. LBF can be computed at either process or procedure granularity. Finally, I will present a brief case study that quantifies the runtime overhead of our algorithms and demonstrates their ability to accurately predict the performance of the tuning options.