Least slack time scheduling

This algorithm is also known as least laxity first.

词语解释：Laxity 松懈的；马虎的；不严格的，Least-Laxity-First 松弛程度最小的优先，言下之意就是最紧张的任务优先执行。

LLF 已经被证明是单处理机的最佳选择，然而，该算法实施起来却不切实际。

The Least-Laxity-First (LLF) scheduling algorithm assigns higher priority to a task with the least laxity, and has been proved to be optimal for uniprocessor systems. The algorithm, however is impractical to implement because laxity tie results in the frequent context switches among the tasks. The Modified Least-Laxity-First (MLLF) scheduling algorithm proposed in this paper solves the problem of the LLF scheduling algorithm by reducing the number of context switches significantly. By reducing the system overhead due to unnecessary context switches, the MLLF scheduling algorithm avoids the degradation of system performance and conserves more system resources for unanticipated aperiodic tasks. We propose the MLLF scheduling algorithm and prove its optimality. We show the performance enhancement of the proposed MLLF scheduling algorithm by using simulation results.

Note

LLF 是面向周期性任务的实时调度算法。

松弛度 = 必须完成时间 - 其本身的运行时间 - 当前时间

下面几张图可以帮助理解 LLF调度算法：

图一是在无竞争状态下任务A 和任务B 的运行状况。

图二是在任务A 具有更高优先级的运行状况（抢占式调度）

图三是在任务B 具有更高优先级的运行状况（抢占式调度）

图四是采用 LLF调度算法各任务的运行状况（抢占式调度）

时间线都排不满的调度方式肯定是要被淘汰的！ :p

图片来源 http://www.chegg.com/homework-help/least-laxity-first-llf-real-time-scheduling-algorithm-period-chapter-10-problem-3p-solution-9780133805918-exc

我发现汤子丹书上的图就是COPY 自这里的，然而注解的文字又完全让人看不懂。