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Keywords: CCA, Performance modeling, CFRFS Combustion, TAU
Simulations on structured adaptively refined meshes (SAMR) pose unique problems in the context of performance evaluation and modeling. Adaptively refined meshes aim to concentrate grid points in regions of interest while leaving the bulk of the domain sparsely tessellated. Structured adaptively refined meshes achieve this by having overlaid grids of different refinement. Numerical algorithms employing explicit multi-rated time- stepping methods apply a computational "kernel" to the finer meshes at a higher frequency than at the coarser meshes. Each application of the kernel at a given level of refinement is followed up by a communication step where data is exchanged with neighboring subdomains. The SAMR approach is adaptive, i.e. its characteristics change as the simulation evolves in time. Thus, scalability depends on the number of processors and the time-integrated effect of the physics of the problem. The time-integrated effect renders the estimation of a general metric of scalability difficult and often impossible. Generally, as reported in the literature, for realistic problems and configurations, SAMR simulations do not scale well. For this work we analyzed two different hydrodynamic problems and present how communication costs scale with various aspects of the domain decomposition. Approach: The codes that we analyzed solve PDEs to simulate reactive flows and flows with shock waves. The codes were run until the incremental decrease in run times (with increasing processors) approached zero. It was found that the nature of the problem changed vastly during the run - even runs which showed poor scaling had periods of evolution where the domain decomposition showed "good" scaling characteristics, i.e compute loads were higher than communication loads. The computational load was found to be evenly balanced across the processors - the lack of scalability was due to the dominance of communication and synchronization costs over computational costs. We identified and analyzed phases in the evolution of the problem where the simulation exhibited good and bad scaling. Communication costs were analyzed with respect to the levels of refinement of the grid as well as the data-exchange radius for each of the runs. This is a thorough performance analysis of SAMR hydrodynamics codes, performed for the first time in CCA-compliant codes, tackling the time-dependent nature of the communication overheads. Both the codes that we analyzed employ the Common Component Architecture (CCA) paradigm and were run within the CCAFFEINE framework. The adaptive mesh package used (that performs the bulk of the communications) was GrACE (Rutgers, The State University of New Jersey). The measurements were performed using the CCA version of TAU (Tuning and Analysis Utilities). The tests were performed on "platinum" at NCSA (University of Illinois, Urbana Champaign), a Linux cluster of dual-node Pentium III 1 GHz processors, connected via a Myrinet interconnect. Visual: As a part of the visual presentation, we will present a color poster with our performance analysis results and hold a demonstration of the composition and execution of CCA codes. Animations of the adaptively refined grid will also be shown.
Created: Wed Dec 10 9:50:31 US/Pacific 2003
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