Intel® 64 and IA-32 Architectures Optimization Reference Manual

POWER OPTIMIZATION FOR MOBILE USAGES10.2 MOBILE USAGE SCENARIOSIn mobile usage models, heavy loads occur in bursts while working on battery power.Most productivity, web, and streaming workloads require modest performanceinvestments. Enhanced Intel SpeedStep Technology provides an opportunity for anOS to implement policies that track the level of performance history and adapt theprocessor’s frequency and voltage. If demand changes in the last 300 ms 3 , the technologyallows the OS to optimize the target P-state by selecting the lowest possiblefrequency to meet demand.Consider, for example, an application that changes processor utilization from 100%to a lower utilization and then jumps back to 100%. The diagram in Figure 10-1shows how the OS changes processor frequency to accommodate demand and adaptpower consumption. The interaction between the OS power management policy andperformance history is described below:1CPU demand3Frequency& Power452Figure 10-1. Performance History and State Transitions1. Demand is high and the processor works at its highest possible frequency (P0).2. Demand decreases, which the OS recognizes after some delay; the OS sets theprocessor to a lower frequency (P1).3. The processor decreases frequency and processor utilization increases to themost effective level, 80-90% of the highest possible frequency. The sameamount of work is performed at a lower frequency.4. Demand decreases and the OS sets the processor to the lowest frequency,sometimes called Low Frequency Mode (LFM).5. Demand increases and the OS restores the processor to the highest frequency.3. This chapter uses numerical values representing time constants (300 ms, 100 ms, etc.) on powermanagement decisions as examples to illustrate the order of magnitude or relative magnitude.Actual values vary by implementation and may vary between product releases from the samevendor.10-2