Modeling Energy-Efficient Performance

The first step in modeling energy-efficient performance is to define workloads that represent everyday PC activities. The thermal design power envelope is a design spec. Measurement under load is fine for maximum power consumption, but PCs don’t run at full power all the time. Idle power is useful information too, but neither do they run at idle all the time. A test workload must mirror the kinds of activities routinely done during everyday interactions with the PC. It must also report results that have meaning to the buyer/user. Intel’s EEP approach reports both the level of performance that can be achieved on the PC as well the annual energy cost to run the PC and achieve that performance.

In an office environment, much activity is centered on “office suite” applications such as word processors, spreadsheets, slide presentation, database, virus scanning, and file compression/decompression. Web content creators use a different set of applications, including HTML editors, image editors, media encoders, as well as video editing packages. An effective methodology will use these types of real-world applications as its workload.

In addition to the right workload, an effective methodology needs to define a usage context for that workload that simulates how the PC will be used over the course of an average day. This should be a reasonable mixture of executing the defined workload, some amount of time the system sits idle, and the system being put into standby or shut off during non-working/use hours.

The final results of an effective methodology should report the delivered performance at a given level of energy cost, communicating both performance and the energy cost of delivering that performance. Metrics that attempt to combine these measures are best avoided as they can obscure both the delivered performance and energy cost. This is especially true of taking ratios, as there is not a guaranteed linear correlation of the value of performance or energy cost. Platform choices are more easily made when both performance and its energy cost are clearly articulated.

Finally, the challenge of choosing the right workload is that one size does not fit all. Different usage models – digital office, digital home, gaming – require different representative workloads. Since much of the concern about PC energy use is found in the enterprise, it follows that a digital office workload is the logical starting point.