15.2. Dynamic voltage and frequency scaling

Many systems operate under conditions where their workload is variable. Therefore it is useful to have the ability to reduce or increase the core performance to match the expected core workload. Clocking the core more slowly reduces dynamic power consumption.

Dynamic Voltage and Frequency Scaling (DVFS) is an energy saving technique that exploits:

This relationship is given as:

P = C × V2 × f

Where:

P

Is the dynamic power.

C

Is the switching capacitance of the logic circuit in question.

V

Is the operational voltage.

f

Is the operational frequency.

Power savings are achieved by adjusting the frequency of a core clock.

At lower frequencies the core can also operate at lower voltages. The advantage of reducing supply voltage is that it reduces both dynamic and static power.

There is an implementation specific relationship between the operational voltage for a given circuit and the range of frequencies that circuit can safely operate at. A given frequency of operation together with its corresponding operational voltage is expressed as a tuple and is known as an Operating Performance Point (OPP). For a given system, the range of attainable OPPs are collectively termed as the system DVFS curve.

Operating systems use DVFS to save energy and, where necessary, keep within thermal limits. The OS provides DVFS policies to manage the power consumed and the required performance. A policy aimed at high-performance selects higher frequencies and uses more energy. A policy aimed at saving energy selects lower frequencies and therefore results in lower performance.

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