Live Webcast 15th Annual Charm++ Workshop

As we design exascale applications and machines, it becomes important to be able to analyze and experiment with alternate designs of both machines and applications. These experiments have to be done before the machines are built since it will be too expensive to build a large number of alternate designs. One of the challenges in this process is how to represent application behavior in such machines. For analyzing network performance via simulations, for example, one can use pre-designed injection patterns, but they do not capture the feedback that occurs naturally in applications: if an incoming message is late, the ordering of events may change, and outgoing message injection will also change. To achieve a high fidelity simulation is therefore challenging. One method that has shown promise is that of emulation- followed-by-simulation: one carries out a full-scale emulation of the application with the correct number of nodes and control threads, facilitated by some overdecomposition based system such as Charm++ [1], FG-MPI[2], or AMPI [3]. The emulation captures dependencies between sequential computations and remote data in traces. The traces generated by emulation can then be fed to a multi-component simulator, where a variable resolution simulation can be carried out to predict performance and other attributes. We advocate this methodology and elaborate on research challenges involved in following it in exascale design. At exascale, we expect the components, which are pluggable entities similar to those used in existing frame- works such as BigSim [4, 5], SST [6], to simulate network, resilience support, power management, thermal constraints, operating system and file system. In addition, the adaptive runtime system, essential for scalable execution at exascale, needs to be (and can be) simulated in detail, with realistic code and strategies, in order to attain high fidelity.