Intel® 64 and IA-32 Architectures Optimization Reference Manual

POWER OPTIMIZATION FOR MOBILE USAGES• Closing all open system resource handles such as files and I/O devices (thisshould include duplicated handles).• Disconnecting all communication links prior to the sleep transition and re-establishingall communication links upon waking up.• Synchronizing all remote activity, such as like writing back to remote files or toremote databases, upon waking up.• Stopping any ongoing user activity, such as streaming video, or a file download,prior to the sleep transition and resuming the user activity after the wake uptransition.Recommendation: Appropriately handling the suspend event enables more robust,better performing applications.10.4.5 Using Enhanced Intel SpeedStep ® TechnologyUse Enhanced Intel SpeedStep Technology to adjust the processor to operate at alower frequency and save energy. The basic idea is to divide computations intosmaller pieces and use OS power management policy to effect a transition to higherP-states.Typically, an OS uses a time constant on the order of 10s to 100s of milliseconds 6 todetect demand on processor workload. For example, consider an application thatrequires only 50% of processor resources to reach a required quality of service(QOS). The scheduling of tasks occurs in such a way that the processor needs to stayin P0 state (highest frequency to deliver highest performance) for 0.5 seconds andmay then goes to sleep for 0.5 seconds. The demand pattern then alternates.Thus the processor demand switches between 0 and 100% every 0.5 seconds,resulting in an average of 50% of processor resources. As a result, the frequencyswitches accordingly between the highest and lowest frequency. The powerconsumption also switches in the same manner, resulting in an average power usagerepresented by the equation Paverage = (Pmax+Pmin)/2.Figure 10-4 illustrates the chronological profiles of coarse-grain (> 300 ms) taskscheduling and its effect on operating frequency and power consumption.6. The actual number may vary by OS and by OS release.10-8