In this section, we examine the characteristics of Transmeta and AMD processors to which we ported RightSpeed. As we do so, we will discuss how these characteristics influence how we should use PACE on these processors.
First, we introduce some definitions. A setting is a speed and voltage combination at which a processor can properly operate. The efficiency of a setting is the amount by which power consumption is reduced by using this setting instead of emulating its speed using the best possible combination of all other settings. For example, suppose there are three settings: 300 MHz consuming 2 W, 500 MHz consuming 3.6 W, and 700 MHz consuming 6 W. We can emulate 500 MHz by running half the time at 300 MHz and half the time at 700 MHz. This consumes 4 W, while the 500 MHz setting consumes only 3.6 W, so the 500 MHz setting has efficiency 10%. We can emulate 300 MHz by running 60% of the time at 500 MHz and turning the CPU off 40% of the time; this emulation has average power consumption 2.16 W, so the 300 MHz setting has efficiency 7.4%. If a setting has efficiency of 0% or less, it is not worthwhile, i.e., one should never use it since one can get lower power consumption at the same speed using other settings.
For PACE to be effective, a processor must have at least three worthwhile speed/voltage settings. Furthermore, the more settings, and the higher their efficiency, the more effective PACE will be. This is because PACE works by choosing among speed schedules with identical performance to find the one with least expected energy consumption. If there is little choice in such speed schedules, and/or if there is little difference between choosing one setting versus emulating that setting's speed with other settings, there will likely be little benefit to choosing among them.