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Node-Level Performance Engineering

DescriptionThe gap between peak performance and application performance is continuing to open. Paradoxically, bad node-level performance leads to highly scalable code, but at the price of increased overall time to solution. Consequently, valuable resources are wasted, often on a massive scale. If the user cares about time to solution on any scale, optimal performance on the node level is often the key factor. We convey the architectural features of current processor chips, multiprocessor nodes, and accelerators, as far as they are relevant for the practitioner. Peculiarities like SIMD vectorization, shared vs. separate caches, data transfer bottlenecks, and ccNUMA characteristics are introduced, and the influence of system topology and affinity on the performance of typical parallel programming constructs is demonstrated. Performance engineering and performance patterns are suggested as powerful tools that help the user understand the bottlenecks at hand and to assess the impact of possible code optimizations. A cornerstone of these concepts is the roofline model, which is described in detail, including useful case studies, limits of its applicability, and possible refinements. We also show how simple performance tools can support node-level performance analysis by providing the developer with useful information about the bottlenecks of their code.

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