Dissecting the Software-Based Measurement of CPU Energy Consumption: A Comparative Analysis

Information and Communications Technologies (ICT) are an increasingly important contributor to the environmental crisis. Computer scientists need tools for measuring the footprint of the code they produce and for optimizing it. Running Average Power Limit (RAPL) is a low-level interface designed by Intel that provides a measure of the energy consumption of a CPU (and more) without the need for additional hardware. Since 2017, it is available on most x86 processors, including AMD processors. More and more people are using RAPL for energy measurement, mostly like a black box without deep knowledge of its behavior. Unfortunately, this causes mistakes when implementing measurement tools. In this article, we propose to come back to the basic mechanisms that allow to use RAPL measurements and present a critical analysis of their operations. In addition to long-established mechanisms, we explore the suitability of the recent eBPF technology (formerly and abbreviation for extended Berkeley Packet Filter) for working with RAPL. We release an implementation in Rust that avoids the pitfalls we detected in existing tools, improving correctness, timing accuracy and performance, with desirable properties for monitoring and profiling parallel applications. We provide an experimental study with multiple benchmarks and processor models to evaluate the efficiency of the various mechanisms and their impact on parallel software. We show that no mechanism provides a significant performance advantage over the others. However, they differ significantly in terms of ease-of-use and resiliency. We believe that this work will help the community to develop correct, resilient and lightweight measurement tools.