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Maximize Power Savings Gains with Dynamic Voltage and Frequency Scaling

Key Takeaways

  • Dynamic voltage and frequency scaling (DVFS) is the power management technique of controlling voltage and frequency in computers, embedded systems, and peripheral devices to reduce power consumption.

  • The increase in voltage to the processors is called overvolting in DVFS, and a decrease of the same is called undervolting. Overvolting increases the runtime of the processors, whereas undervolting redefines the thermal management of the processor and related devices.

  • The advantages of DVFS are many: maximum power savings gains, extra battery life, extended lifespan of embedded systems and peripherals, and jump from active to passive cooling for processors.

Dynamic Voltage Scaling block text

Time and priority management have felt interdependent. I usually start working on challenging tasks first, and try to give myself more time for the most difficult tasks. I have seen my productivity increase greatly with this technique. Whether multitasking is being done by a human or a system, there is usually a management algorithm running in the background that promises energy-efficient functioning and best yield.

As our lifestyle gets busier and more technology-dependent, the proliferation of embedded systems, processors, and computers into daily life is increasing. The embedded systems in these intelligent appliances focus on improving performance or prolonging functions without too much battery drain or power consumption. Dynamic voltage and frequency scaling (DVFS) is one such approach in computer architecture—improving reliability, performance, and power-saving capabilities. In this power management strategy, the voltage supplied to internal circuits in a computer is controlled according to the requirements for accomplishing power savings.

Exploring Dynamic Voltage and Frequency Scaling

Data processing in a processor.

Figure.1 Data processing in a processor.

Computers or processors operate in a world of 1s and 0s. The stable operation of a processor is all about the right interpretation of logic states. In modern desktops, laptops, embedded systems, and processors, the logic implementation is through CMOS integrated circuits, which work within a voltage range of 0-1.5 V for low logic state and 3.5 to 5 V for high logic state. The speed at which these logic circuits handle the transitions from 1s to 0s or vice-versa defines the clock speed, or operating frequency, of the processor. The voltage applied to the circuitry is dependent on the frequency, and this opens the window to DVFS.

In the DVFS technique, the voltage supply to a processor is varied by varying the clock frequency. The switching power loss in a CMOS circuit is given by the equation:

where C is the capacitance of the logic gates, f is the operating frequency, V is the voltage supplied, and Pstatic is the static leakage power loss in the logic circuit. By changing the clock frequency of the processor, we can achieve an increase or decrease in the voltage supply to the processor. One of the merits of DVFS is its tremendous power-saving capabilities, as the power dissipation given in the equation above is directly proportional to the square of the supply voltage. This power management scheme can boost the performance, stability, and power utilization of a computer. 

DVFS is the advanced version of dynamic voltage scaling in computer architecture management. Dynamic voltage scaling leads to the following subsequent processes:

Slow logic state transitions → reduced maximum frequency of operation → reduced rate of program instruction execution →increase in runtime.  

When the processor is expected to complete the computation at a faster pace, the dynamic voltage scaling should be accompanied by dynamic frequency scaling. In such scenarios, DVFS is a savior, as it can raise or lower both voltage and frequency simultaneously and prevent any timing violations in the process. The DVFS technique is guided by the Critical Operating Point ❲COP❳, which supervises the voltage and frequency scaling to avoid fatal problems. Frequency scaling takes the basic, communication, or multimedia mode after overseeing the present functions of the processor.

CPU

DVFS can improve the function of a CPU.

Overvolting

In the DVFS technique, the power supplied to components is varied according to operational priorities. If you want a computation to finish faster, then overvolting in DVFS is your best choice. The process of increasing power supply in DVFS is called overvolting, and paves the path to higher speeds or ‘overclocking’ a process. 

In overvolting, the voltage supplied to the logic circuits increases the capacitance associated with it. It decreases the time taken to charge and discharge the capacitor—the short charging and discharging time aid the faster logic state transitions. The fast logic state transition accounts for a higher speed of operation in overvolting. 

However, overvolting comes with a common side effect—’overheating’—which triggers the occurrences of thermal runaways, hot carrier injection, electromigration, and IR drop in CMOS logic circuits. Frequent overvolting can result in system crashes, hangs, permanent hardware damage, and is detrimental to the lifespan to the processor.

Undervolting

Undervolting is the reverse of overvolting—the voltage supply to the processor is reduced to achieve undervolting. The transition time of logic gates is stretched for enabling undervolting in computer architecture. Undervolting is employed to improve power utilization and system reliability. The low voltage supply to the circuits revises the thermal management strategy of the computer internal circuits to passive cooling. Undervolting reduces the heat generated in the computer hardware, thereby allowing power saving by entirely or partially removing active cooling systems.

In aggressive undervolting, each node in the logic circuit is maintained below the standard nominal voltage value for higher power efficiency gains. The system stability is questioned in this case of undervolting, and the reliability issues and faults start to pop up in such DVFS systems.

Implementation of DVFS

How do you vary the fan speed with the help of a speed regulator? DVFS does the same with voltage given to the computer processor. However, the regulator control is not manual in DVFS compared to the fan regulator. The two approaches for voltage regulation in DVFS are:

  1. Software supported- In computer architecture, the voltage supply to the CPU, RAM, etc. are controlled using BIOS. Here no hardware reformation is made as a part of DVFS. 

  2. Hardware supported- For specific components such as motherboard northbridges and video cards, the software-based DVFS is not applicable. The hardware modifications required for implementing DVFS are called voltage mods, or Vmod, by the computer hardware engineering community. 

Shortfalls of DVFS

DVFS is efficient in reducing operating costs, improving system reliability, and saving energy in computer processors. However, DVFS faces adverse effects with trending technology advancements.

Multi-Core Processors and DVFS

In multi-core processing systems, the workloads on each core need to be analyzed before frequency scaling. With the increase in the number of cores, DVFS implementation gets more complex. In multi-core multi-threading systems, there requires a thread-to-core mapping, followed by a workload analysis on each core for determining the voltage and frequency of DVFS. 

Effect of IC Downscaling 

We know that the IC downscaling technology has grown to such heights that you can accommodate millions of transistors in a single chip. The downscaling trend has adversely impacted DVFS techniques in embedded systems and its peripheral devices. The DVFS application allows for excellent dynamic power saving, which is the first term on the right-hand side of the equation shown above. As the feature size of the transistor reduces, the share of static loss (second term in the right-hand side of the equation shown above) in the switching power goes above the dynamic power. The DVFS scheme is dedicated to lowering the dynamic power, not the static leakage power. The introduction of low-core voltages also diminishes the possibility of further voltage scaling. The objective of energy-saving is not fulfilled to a T by DVFS in the processors utilizing downscaled circuits. 

Overall, DVFS offers many benefits. If you wish to enhance the power savings gains, battery life, life span, and footprint of the embedded systems and its peripheral devices, you need to turn to the exact power-saving strategy offered by DVFS.