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Athanasios E. Papathanasiou, and Michael L. Scott .
Computer Science Department
Work-in-Progress Talk,
OSDI '02 |
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Several important devices, such as hard disks and wireless networks, support low-power modes that can save significant energy if the device is inactive for relatively long periods of time (typically tens of seconds). Failure to remain in the low power state for a sufficient period of time, called breakeven time period, leads to increased energy consumption. Unfortunately, modern file systems are typically designed to maximize throughput and to minimize I/O latency, not to minimize energy. The result is that even under a light workload, idle times tend to be too short to exploit for energy savings. |
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As an example of file system behavior, consider how the Linux kernel reacts during 5 minutes of mp3 playback. The kernel's conservative prefetching algorithm requests 128KB of data approximately every 8 seconds. The result is that 66% of the total idle time appears in intervals of 8 seconds or less, while only 12% appears in intervals larger than 12 seconds, which is the spin-down breakeven point for an IBM TravelStar disk. This is unfortunate given that the disk's sustainable bandwidth significantly exceeds that required by mp3 playback. During periods of activity the disk is used at less than 1% of its sustainable bandwidth. This type of file system behavior also appears during light write workloads. |
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We believe that a central goal of file systems used in mobile platforms should be to create a bursty access pattern that increases the average length of idle intervals for devices with low power modes and maximizes the device's utilization when it is active. As shown in the graph a bursty file system gathers multiple requests depending on memory availability and services them periodically in a single disk burst. This significantly increases the total amount of time that the disk can spend in a low-power state, and reduces the number of expensive power-up transitions. |
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We are currently modifying the Linux ext2 file system in order to increase the burstiness of disk activity. Toward this end we are currently exploring aggressive prefetching, based on hints to increase accuracy, bursty periodic update policies with increased variance in time-to-stable-storage and scheduling algorithms that embody knowledge of device power characteristics, and coordinate I/O activities across all running applications. In order to mitigate the effects of possible disk congestion, we introduce a notion of urgency, and use it to order requests. The general idea is that requests for data that are going to be used earlier or requests that are important for system reliability should be serviced first. |
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Using trace-driven simulation, we have found that by re-shaping the low-level disk access pattern we can save up to 55% of total disk energy in such common applications as mp3 playback and encoding, data copying, and games. This is in comparison to a baseline IBM TravelStar disk with the best reactive spin-down policy. |
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For more information and an interesting technical report visit the URL on the slide. |
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