cyclicexecutive

Cyclic executive, sometimes written as cyclic executive or cyclic scheduling, is a scheduling approach used in real-time and embedded systems where a fixed, repetitive timetable drives the execution of tasks. In a cyclic executive, time is divided into a sequence of frames, and a precomputed frame table specifies which tasks run in each frame and for how long. The resulting schedule is time-triggered and repeats with a hyperperiod, yielding deterministic behavior and bounded task latencies.

Architecture and operation: A central scheduler, or the executive, advances the system clock, enters frames, and activates the designated tasks for that frame. Tasks may finish within their frame or yield early; inter-frame communication typically uses buffers or handshakes. The schedule table is generated offline through schedulability analysis and validated against timing constraints. Variants include multirate cyclic schemes that assign different frame durations to accommodate tasks with different periods.

Characteristics and design considerations: Cyclic executaries offer predictability, low runtime overhead, and straightforward implementation, making them easy to verify for real-time deadlines. They, however, suffer from rigidity and limited adaptability to sporadic or highly dynamic workloads. Adding or removing tasks often requires regenerating and revalidating the entire schedule, and timing granularity is bounded by frame duration. The approach works best when workloads are well bounded and timing requirements are strict.

Applications and variants: Cyclic executives are widely used in aerospace, automotive control units, and other safety-critical embedded systems. They are commonly employed in time-triggered architectures and can be extended to multi-rate cyclic scheduling to handle heterogeneous task groups. They complement event-driven methods, providing a deterministic backbone for parts of the system.