DVFS is Dead, Long Live Holistic DVFS

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03 October 2017

An industry survey some three years ago indicated only about one-quarter of all chip designers were using dynamic voltage and frequency scaling (DVFS). It’s likely most of those people who said they were implementing DVFS are deeply dependent upon a mobile operating system. For instance, Android offers a range of CPU governor software modules to optimize frequency and voltage levels for application throughput. While DVFS is already enabled in most hardened CPU and GPU cores, clusters and/or subsystems, the majority of SoC designers not tied to Android opt for simpler techniques like clock and power gating. Is there an opportunity for a more holistic DVFS approach that more SoC designers would embrace?

Software sort of in control

The problem many teams cite with the current state of DVFS technology is that improving battery life requires optimizing for energy use, while most software-based control approaches instead minimize power dissipation based on recent activity. How could one expect software to minimize energy, when it can predict neither the throughput requirements nor the energy trade-offs between the different power savings techniques? Add on the implications of process corners and temperature variation, and the energy cost just to run a more sophisticated prediction model in software, and designers start to look for easier solutions.

Case in point: a team at Georgia Tech studying the current Android DVFS solution published their findings earlier this year. In short, they highlight the existing stock Android governor framework is too general purpose, focused on power reduction rather than delivering overall energy savings. Worse still, multiple software governors “work independently of each other,” leading to performance and energy losses as they undo each other’s optimizations. *

The Georgia Tech team proposes a solution implementing coordinated control of CPU frequencies and memory bandwidth against a set of six applications executing on a Google Nexus 6 smartphone running Android Marshmallow 6.0. Their software-coordinated controller delivers a 4% to 31% energy savings across these use cases with less than a 1% performance loss in all cases. It’s on a highly simplified set of mobile energy use cases, but their study points to the potential for a better DVFS solution – one implemented by the SoC designer who has the best knowledge of the power versus throughput characteristics of each power savings technique.

Read the full blog here

Last modified on Tuesday, 03 October 2017 05:43