The use of Dynamic Voltage and Frequency Scaling (DVFS) by Energy-Efficient (EE) computer systems considerably increases the requirements regarding the design of efficient system clocks. On the one hand, the operation of a system clock must support the independent operating frequencies of the processor core units, the dynamic migration of the running processes between the processors core units, and the use of synchronization and time interpolation techniques to maintain the accuracy of the system clock. On the other hand, an efficient system clock has to minimize the overhead of its own operation, aiming at energy efficiency of EE computer systems.In this paper, we present the design and evaluation of the RVEC virtual system clock for the EE Wireless Raspberry Pi (RasPi) platform. In the RasPi platform, the use of DVFS for reducing the energy consumption hinders the direct use of the cycle count of the ARM11 processor core for building an efficient system clock. Therefore, a distinct feature of RVEC is to obviate this obstacle, such that it can make use of the cycle count circuit for precise and accurate time measurements, concurrently with the use of DVFS by the operating system of the ARM11 processor core. Specifically, this paper presents the design and experimental evaluation of an implementation of the RVEC virtual system clock in the Linux kernel of the RasPi platform with DVFS. Our experimental results validate the RVEC virtual system clock as an efficient system clock for the EE RasPi platform that runs the Linux operating system.

Projetos de redes de sensores sem fio requerem plataformas computacionais energeticamente eficientes como a plataforma Raspberry Pi (RPI). Para esse fim, um mecanismo típico que tais plataformas oferecem é o DVFS (Dynamic Voltage and Frequency Scale). Porém, o uso desse mecanismo pode afetar negativamente o desempenho dos circuitos contadores de tempo da plataforma RPI, em oposição a sua eficiência energética. Este trabalho propõe o relógio RVEC como uma nova solução que viabiliza o uso do contador de ciclos do processador ARM da plataforma RPI enquanto que garante a temporização ser estritamente crescente e precisa. A solução RVEC também provê resolução de nanosegundos com um custo de acesso equivalente aos dos relógios de sistemas.