Cross-Layer Customization for Rapid and Low-Cost Task Preemption in Multitasked Embedded Systems
Preemptive multitasking is widely used in many low-cost and real-time embedded applications for its superior hardware utilization. The frequent and asynchronous context switches, however, require the preservation and restoration of the task state, thus resulting in a large number of memory transfer instructions. As a consequence, task responsiveness and application throughput can be significantly deteriorated. To address this problem we propose a cross-layer customization framework which through the close cooperation of compiler, OS, and hardware architecture achieves rapid and low-cost task switch. Application information extracted during compile-time regarding state liveness is exploited in order to preserve a minimal amount of task state on task preemption. We introduce two complementary techniques to implement the application-aware state preservation. The first technique utilizes compiler-generated custom routines which preserve/restore an extremely small live context at judiciously selected points in the application code. The second technique requires more sophisticated hardware support. It employs an OS-controlled register file mapping to achieve a rapid context switch. By mapping a small fraction of the register file in a single clock cycle, a context switch is achieved requiring no memory transfers for the majority of cases to preserve/restore the live state. The effect of aggressively replicated register files, where each task is given its own replica, is achieved with the hardware cost of only adding from 25% to 50% extra physical registers. Through the utilization of these novel mechanisms, a significant improvement on task response time is achieved as the context-switch cost is minimized.
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