Node-Level Performance Engineering

This online course organised in cooperation with NHR@FAU covers performance engineering approaches on the compute node level. Even application developers who are fluent in OpenMP and MPI often lack a good grasp of how much performance could at best be achieved by their code. This is because parallelism takes us only half the way to good performance. Even worse, slow serial code tends to scale very well, hiding the fact that resources are wasted. This course conveys the required knowledge to develop a thorough understanding of the interactions between software and hardware. This process must start at the core, socket, and node level, where the code gets executed that does the actual computational work. We introduce the basic architectural features and bottlenecks of modern processors and compute nodes. Pipelining, SIMD, superscalarity, caches, memory interfaces, ccNUMA, etc., are covered. A cornerstone of node-level performance analysis is the Roofline model, which is introduced in due detail and applied to various examples from computational science. We also show how simple software tools can be used to acquire knowledge about the system, run code in a reproducible way, and validate hypotheses about resource consumption. Finally, once the architectural requirements of a code are understood and correlated with performance measurements, the potential benefit of code changes can often be predicted, replacing hope-for-the-best optimizations by a scientific process. The course is a PRACE training event. Introduction Basic architecture of multicore systems: pipelines, SIMD, caches, sockets, memory The important role of system topology Tools: topology & affinity in multicore environments likwid-topology and likwid-pin, alternatives Roofline model basics Model assumptions and construction Simple examples Limitations of the Roofline model Tools: hardware performance counters Why hardware performance counters? likwid-perfctr Validating performance models Optimal use of parallel resources Single Instruction Multiple Data (SIMD) Cache-coherent Non-Uniform Memory Architecture (ccNUMA) Roofline case studies Tall & skinny dense matrix-matrix multiplication Sparse matrix-vector multiplication Jacobi (stencil) smoother Basics of performance engineering PE process Code profiling Proper benchmarking Reproducibility and documentation Beyond Roofline: The ECM performance model (optional) Prerequisites You have to be able to handle a Linux command line and file editing remotely. Basic knowledge of C, C++, or Fortran programming and of OpenMP is required. Hands-On Exercises will be done on a cluster at NHR@FAU. For the exercises you need an SSH client on your local computer.https://moodle.nhr.fau.de/course/view.php?id=