Evaluating the effectiveness of system scheduling and energy savings in embedded real-time systems with low-computing resources is the problem addressed in this paper. In such systems, the characteristics of the implemented scheduling policy play a relevant role in both schedulability and energy consumption. Ideally, the scheduling policy should provide higher schedulability bounds and low runtime overheads, allowing for better usage of available slack in the schedule for energy saving purposes. Due its low overhead and simple implementation, the usual scheduling policy employed in real-time embedded systems is based on fixed priority scheduling (FPS). Under this scheme, as the priority of all system tasks are assigned at design time, a simple priority vector suffices to indicate the current ready task to run. System schedulability, however, is usually lower than that provided by dynamic priority scheduling (DPS) according to which task priorities are assigned at runtime. Managing dynamic priority queues incurs higher overheads, though. Deciding whether DPS is a viable choice for such embedded systems requires careful evaluation. We evaluate two implementations of Earliest Deadline First (EDF), a classical DPS policy, implemented in FreeRTOS running on an ARM-M4 architecture. EDF is compared against an optimal FPS, namely Rate-Monotonic (RM). Further, two mechanisms for energy savings are described. They differ by the manner they compute the slack available in an EDF schedule, statically (SS-EDF) or dynamically (DS-EDF). These two approaches are experimentally evaluated. Results indicate that EDF can be effectively used for energy savings.

在具有低计算资源的嵌入式实时系统中评估系统调度和节能的有效性是本文要解决的问题。在这样的系统中，实施的调度策略的特性在可调度性和能量消耗方面都起着相关的作用。理想情况下，调度策略应提供更高的可调度性界限和较低的运行时开销，从而允许更好地利用调度中的可用松弛部分以达到节能目的。由于开销低且实现简单，实时嵌入式系统中采用的常用调度策略基于固定优先级调度 (FPS)。在这种方案下，由于所有系统任务的优先级都是在设计时分配的，一个简单的优先级向量就足以指示当前准备运行的任务。然而，系统可调度性 通常低于动态优先级调度 (DPS) 提供的，根据动态优先级调度在运行时分配任务优先级。但是，管理动态优先级队列会产生更高的开销。确定 DPS 是否是此类嵌入式系统的可行选择需要仔细评估。我们评估了最早截止日期优先 (EDF) 的两种实现，这是一种经典的 DPS 策略，在运行于 ARM-M4 架构的 FreeRTOS 中实现。将 EDF 与最佳 FPS，即单调速率 (RM) 进行比较。此外，还描述了两种节能机制。它们的不同之处在于计算 EDF 计划中可用松弛的方式，静态 (SS-EDF) 或动态 (DS-EDF)。这两种方法是通过实验评估的。结果表明，EDF 可以有效地用于节能。