Ghosts in the machine: interfaces for better power management

We observe that the modularity of current power management algorithms often leads to poor results. We propose two new interfaces that pierce the abstraction barrier that inhibits device power management. First, an OS power manager allows applications to query the current power mode of I/O devices to evaluate the performance and energy cost of alternative strategies for reading and writing data. Second, we allow applications to disclose ghost hints that enable better power management in the presence of multiple devices. Adaptive applications issue ghost hints to device power managers when they are forced to use a poor I/O path because a device is not in an ideal power mode; such hints allow devices to implement proactive power management strategies that do not depend upon passive load observation. Using these new interfaces, we implement a middleware layer that supports adaptive disk cache management. On an iPAQ handheld running Linux, our cache manager reduces interactive response time for a Web browser by 27% and decreases total energy usage by 9%. For a mail reader, the cache manager decreases response time by 42% and energy use by 5%.