Voltage and frequency scaling

Dynamic Voltage and Frequency Scaling (DVFS) is a technique that consists of varying the frequency and voltage of a microprocessor in realtime according to processing needs. This technique is used both for power-saving during off peak times, and as a protective measure to avoid overtemperature conditions.

Description

The power consumption inside of the CPU greatly depends on the current flow through it. The greatest current in the CMOS CPU occurs during the times of gate switching, because both the pull up and pull down transistors have reduced impedance and as a result allow a higher current to flow through. Furthermore, the current is also proportional to voltage, with the power being proportional to voltage squared. As a result $P=V^{2}*F$. Therefore to reduce the power consumption of the cpu both voltage and frequency can be scaled down. However, frequency scaling comes at a cost of reduced performance, since the cpu performance is directly proportional to the running clock frequency. On the other hand, the voltage is typically set at a level to prevent noise from affecting the results, and the only way to reduce the voltage lower is to also reduce the frequency. As a result voltage and frequency should be scaled together.

This reduction in running frequency and voltage of the cpu brings about a lot of transients that have to go through the cpu, this in some cases results in additional logic, and a requirement for a delay right after the scaling occurs.

The penalties in the delay after scaling and the performance penalty can have a significant affect on the cpu performance. Therefore to avoid the actual scaling a number of practices are taken, including applying DVFS to only parts of the cpu with drivers between the functional units, additional cooling and dynamic scheduling. Many of this practices are the hot subject of research in the computer engineering community.

Sources

• ACM Queue - Energy Management on Handheld Devices