To offset the significant power demands of hard disk drives in computer systems, drives are typically powered down during idle periods. This saves power, but accelerates duty cycle consumption, leading to earlier drive failure. Hybrid disks with a small amount of non-volatile flash memory (NVCache) are coming on the market. We present four I/O subsystem enhancements that exploit the characteristics of hybrid disks to improve system performance: 1) artificial idle periods, 2) a read-miss cache, 3) anticipatory spin-up, and 4) NVCache write-throttling. These enhancements reduce power consumption, duty cycling, NVCache block-erase impact, and the observed spinup latency of a hybrid disk, resulting in lower power consumption, greater reliability, and faster I/O.

One of the biggest bottlenecks in desktop-based computing is the hard disk with I/O write latency being a key contributor. I/O write latency stems from the mechanical nature of hard disks, with seek and rotational delays the major components. Hybrid disk drives place a small amount of flash memory (NVCache) on the drive itself which can be leveraged by the host and has the potential to increase I/O performance and reduce hard disk power consumption. We present an I/O scheduling algorithm, "Flash-Backed I/O Requests", which leverages the on-board flash to reduce write latency. Since flash memory and rotating media have different I/O characteristics, predominantly in random access context, an I/O scheduler can decide which media will most efficiently service I/O requests. Our results show that with Flash-Backed I/O requests, overall write latency can be reduced by up to 70%.

Being one of the few mechanical components in a typical computer system, hard drives consume a significant amount of the overall power used by a computer. Spinning down a hard drive reduces its power consumption, but only works when no disk accesses occur, limiting overall effectiveness. We have designed and implemented a technique to extend disk spin-down times using a small non-volatile storage cache called NVCache, which contains a combination of caching techniques to service reads and writes while the hard disk is in low-power mode. We show that combining NVCache with an adaptive disk spin-down algorithm, a hard disk’s power consumption can be reduced by up to 90%.

Demand for real-time capability in general-purpose systems is rising and as systems are retrofitted with scheduling features they become increasingly complex. To counter this trend we present the best-effort bandwidth server (BEBS), an aperiodic server for flexible and efficient support of best-effort applications in a real-time system. Recognizing that the responsiveness of a server depends on its period, and that not every best-effort task requires equal responsiveness, the algorithm adjusts its period based on run-time behavior of tasks. We created a prototype implementation of the system to demonstrate that it performs suitably as a general-purpose scheduler in comparison to Linux, and outperforms a common type of hierarchy used in existing general-purpose systems. The result is a system that integrates real-time scheduling with best-effort support, both simple and powerful enough to be used as the only scheduler in a general-purpose operating system.

Real-time systems are growing in complexity and real-time and soft real-time applications are becoming common in general-purpose computing environments. Thus, there is a growing need for scheduling solutions that simultaneously support processes with a variety of different timeliness constraints. Toward this goal we have developed the resource allocation/dispatching (RAD) integrated scheduling model and the rate-based earliest deadline (RBED) integrated multi-class real-time scheduler based on this model. We present RAD and the RBED scheduler and formally prove the correctness of the operations that RBED employs. We then describe our implementation of RBED and present results demonstrating how RBED simultaneously and seamlessly supports hard real-time, soft real-time, and best-effort processes.