Energy optimization for a two-device data flow chain

Many applications running on today's portable devices use multiple power-consuming devices simultaneously, often in the form of a data flow chain which involves transfer of data between devices through buffers. Some of these devices have the ability to scale their performance and power simultaneously by tuning one of their parameters (generically called the device speed). We address the problem of minimizing the energy consumed by a two-device data flow chain by choosing the speed profiles of the two devices and the "cycle time" of the intermediate buffer. Determining the speed profiles (functions of time) to minimize the energy functional, in general, requires variational techniques. However, based on certain observations about device power-speed relations and application performance constraints, we were able to solve the problem analytically in two steps - device characterization and cycle time optimization. The effectiveness of the technique was demonstrated for two practical applications of dataflow chains - CD recording and VCD playback with up to 45% and 64% energy improvements, respectively.