Intel ARCHITECTURE IA-32 User Manual download pdf (Page 427)

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101

Power Optimization for Mobile Usages 9

9-13

The same application can be written in such a way that work units are

divided into smaller granularity, but scheduling of each work unit and

Sleep() occurring at more frequent intervals (e.g. 100 ms) to deliver the

same QOS (operating at full performance 50% of the time). In this

scenario, the OS observes that the workload does not require full

performance for each 300 ms sampling. Its power management policy

may then commence to lower the processor’s frequency and voltage

while maintaining the level of QOS.

The relationship between active power consumption, frequency and

voltage is expressed by the equation:

In the equation: ‘V’ is core voltage, ‘F’ is operating frequency, and ‘α’

is the activity factor. Typically, the quality of service for 100%

performance at 50% duty cycle can be met by 50% performance at

100% duty cycle. Because the slope of frequency scaling efficiency of

most workloads will be less than one, reducing the core frequency to

50% can achieve more than 50% of the original performance level. At

the same time, reducing the core frequency to 50% allows for a

significant reduction of the core voltage.

Because executing instructions at higher P-state (lower power state)

takes less energy per instruction than at P0 state, Energy savings relative

to the half of the duty cycle in P0 state (Pmax /2) more than compensate

for the increase of the half of the duty cycle relative to inactive power

consumption (Pmin /2). The non-linear relationship between power

consumption to frequency and voltage means that changing the task unit

to finer granularity will deliver substantial energy savings. This

optimization is possible when processor demand is low (such as with

media streaming, playing a DVD, or running less resource intensive

applications like a word processor, email or web browsing).

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