We present a parallel simulator -- BigSim -- for predicting performance
of machines with a very large number of processors.
The simulator provides the ability to make performance
predictions for machines such as BlueGene/L, based on actual
execution of real applications. We present this capability using
case-studies of some application benchmarks. Such a simulator is
useful to evaluate the performance of specific applications on such
machines even before they are built. A sequential simulator may be
too slow or infeasible. However, a parallel simulator faces problems
of causality violations. We describe our scheme based on ideas
from parallel discrete event simulation and utilize inherent determinacy
of many parallel applications. We also explore techniques for
optimizing such parallel simulations of machines with large number of
processors on existing machines with fewer number of processors.
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