'Prefetching' scheme saves time, energy

Most desktop computers today power down their hard drives when not in use to save energy. While this may be energy efficient, it also incurs minor, though sometimes frustrating, delays. Waiting as long as eight seconds for a hard drive to power back up to retrieve a document is not the best use of any office worker's time. So two University of Arizona researchers have figured out a way to save energy and maintain access to data through a concept they call 'context-aware prefetching.'

"The idea is to reduce or eliminate delays," said Igor Crk, who along with Chris Gniady wrote up their findings in the paper titled "Context-Aware Mechanisms for Reducing Interactive Delays of Energy Management in Disks." Crk presented the team's work at the Usenix conference held last month in Boston.

The idea is to run a small program that logs user actions and notes which chains of actions typically lead to interactions with the hard disk. For example, if a user opens a word processing program and then executes a series of keystrokes to open a document, that whole series of actions is a good predictor that the hard drive will need to deliver a file in a few seconds.

"Monitoring user interactions with applications provides an opportunity for predicting upcoming power mode transitions and, as a result, eliminating the delays associated with these transitions," the paper states.

The team wrote a program that, when sensing a sequence of actions that typically leads to hard-drive use, will issue a command to the hard drive to spin-up. So, by the time the user actually requests the file, the hard drive, already up and running, can return it immediately.

Prefetching is widely used in many systems, but in most of these approaches, the hard drive is activated whenever a user does anything at all on a computer. Only when the user does not interact with the computer for a set period of time does the disk power down. Crk explained that this approach is not as efficient insofar the hard drive stays powered on for long periods of time while nothing is actually written or read from the disk.

The team's approach accurately predicted hard-drive usage 79 percent of the time. As a result, this approach "is able to reduce spin-up delays on average by 35 percent (more than 3 seconds), while maintaining low energy consumption," according to the paper. By contrast, traditional prefetching predicted hard drive use 81 percent of the time, although it incorrectly predicted usage 52 percent of the time, making that approach much more energy-intensive.

Crk, speaking with GCN after the presentation, noted that context-aware prefetching could be easily implemented in most operating systems. Most all commercial hard drives have commands that be evoked to spin up disks. Crk explained that the program they created consumed very little in the way of CPU resources, so it would go unnoticed by users. (The program is available for those willing to sign a nondisclosure agreement and only use for the software for non-commercial use.)

Crk also said that this technique could also be used with wireless cards, which could help save battery life on mobile and laptop computers. The idea would be the same, though the program would wake up the wireless communications card rather than the hard drive.

Usenix, the Advanced Computing Systems Association, is a nonprofit association for technicians, scientists, systems administrators and engineers to share information on developments in the field of computer science.

About the Author

Joab Jackson is the senior technology editor for Government Computer News.

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