Intel® 64 and IA-32 Architectures Optimization Reference Manual

POWER OPTIMIZATION FOR MOBILE USAGESCPU demandAverage powerFrequency& PowerFigure 10-4. Profiles of Coarse Task Scheduling and Power ConsumptionThe same application can be written in such a way that work units are divided intosmaller granularity, but scheduling of each work unit and Sleep() occurring at morefrequent intervals (e.g. 100 ms) to deliver the same QOS (operating at full performance50% of the time). In this scenario, the OS observes that the workload doesnot require full performance for each 300 ms sampling. Its power managementpolicy may then commence to lower the processor’s frequency and voltage whilemaintaining the level of QOS.The relationship between active power consumption, frequency and voltage isexpressed by the equation:In the equation: ‘V’ is core voltage, ‘F’ is operating frequency, and ‘α’ is the activityfactor. Typically, the quality of service for 100% performance at 50% duty cycle canbe met by 50% performance at 100% duty cycle. Because the slope of frequencyscaling efficiency of most workloads will be less than one, reducing the corefrequency to 50% can achieve more than 50% of the original performance level. Atthe same time, reducing the core frequency to 50% allows for a significant reductionof the core voltage.Because executing instructions at higher P-state (lower power state) takes lessenergy per instruction than at P0 state, Energy savings relative to the half of the dutycycle in P0 state (Pmax /2) more than compensate for the increase of the half of theduty cycle relative to inactive power consumption (Pmin /2). The non-linear relationshipbetween power consumption to frequency and voltage means that changing thetask unit to finer granularity will deliver substantial energy savings. This optimizationis possible when processor demand is low (such as with media streaming, playing aDVD, or running less resource intensive applications like a word processor, email orweb browsing).An additional positive effect of continuously operating at a lower frequency is thatfrequent changes in power draw (from low to high in our case) and battery currenteventually harm the battery. They accelerate its deterioration.10-9