Abstract
This paper presents refinements to the execution-cache-memory performance model and a previously published power model for multicore processors. The combination of both enables a very accurate prediction of performance and energy consumption of contemporary multicore processors as a function of relevant parameters such as number of active cores as well as core and Uncore frequencies. Model validation is performed on Intel Sandy Bridge-EP, Broadwell-EP, and AMD Epyc processors. Production-related variations in chip quality are demonstrated through a statistical analysis of the fit parameters obtained on one hundred Broadwell-EP CPUs of the same model. Insights from the models are used to explain the performance- and energy-related behavior of the processors for scalable as well as saturating (i.e., memory-bound) codes. In the process we demonstrate the models’ capability to identify optimal operating points with respect to highest performance, lowest energy-to-solution, and lowest energy-delay product and identify a set of best practices for energy-efficient execution.
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Notes
- 1.
Consider, e.g., the 18-core Broadwell-EP chip, which offers 17 different Uncore and 12 different CPU core frequencies, for which a total of 3672 measurements (each with a non-negligible runtime to reach operating temperature equilibrium) are required. In contrast, setting up the model requires only four, six, and nine measurements on the AMD Epyc, Intel Sandy Bridge-EP, and Broadwell-EP processors, respectively.
- 2.
The term Uncore refers to all parts of the chip that are not part of the core design, such as, e.g., shared last-level cache, ring interconnect, and memory controllers.
- 3.
- 4.
The coefficient of variation is used to measure the relative variance of a sample. It is defined as the ratio of the standard deviation \(\sigma \) to the mean \(\mu \) of a sample.
- 5.
For n active cores, the probability of a core’s memory access encountering a busy bus is \(u(n-1)\); when the bus is busy, the penalty \(p_\mathrm {0}\), which increases with the number of cores, is applied.
- 6.
On Sandy and Ivy Bridge processors the Uncore is clocked at the same frequency as the CPU cores and can thus only be set indirectly.
- 7.
Wall clock time can also be used, which essentially mirrors the plot about the y axis.
- 8.
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Hofmann, J., Hager, G., Fey, D. (2018). On the Accuracy and Usefulness of Analytic Energy Models for Contemporary Multicore Processors. In: Yokota, R., Weiland, M., Keyes, D., Trinitis, C. (eds) High Performance Computing. ISC High Performance 2018. Lecture Notes in Computer Science(), vol 10876. Springer, Cham. https://doi.org/10.1007/978-3-319-92040-5_2
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