fixedpriority

Fixed priority scheduling is a CPU scheduling approach in which each task is assigned a static priority. These priorities do not change during system execution, and the scheduler selects the highest-priority ready task to run. In preemptive fixed-priority systems, a newly ready higher-priority task can interrupt a currently running lower-priority task. In non-preemptive variants, once a task begins execution it runs until it blocks or completes, regardless of other tasks becoming ready.

This approach is common in real-time and embedded systems where predictability and repeatable timing are essential. Among fixed-priority schemes, rate-monotonic scheduling assigns higher priorities to tasks with shorter periods, making it a widely cited algorithm for periodic tasks. Deadline-monotonic scheduling is another fixed-priority variant that assigns priorities based on earlier deadlines. In many simple fixed-priority setups, task deadlines are assumed to coincide with task periods.

Schedulability analysis for fixed-priority systems relies on worst-case execution time estimates and task periods. For rate-monotonic scheduling, a classical sufficient condition states that the total utilization must not exceed a bound that depends on the number of tasks (the Liu-Layland bound). More precise analysis uses response-time calculations to determine whether each task meets its deadline under the fixed priority order.

Advantages of fixed priority include simplicity, low runtime overhead, and strong predictability, which is valuable for hard real-time requirements. Disadvantages include potential starvation of low-priority tasks under heavy higher-priority load and reduced flexibility in adapting to changing workloads. Blocking and priority inversion can occur in preemptive systems without mitigation, such as priority inheritance protocols.

Examples of systems using fixed-priority scheduling include various real-time operating systems and embedded platforms, such as FreeRTOS, VxWorks, and QNX.