earliestdeadlinefirst

Earliest Deadline First (EDF) is a dynamic-priority preemptive scheduling algorithm used in real-time systems. At any moment, the scheduler selects the ready task with the smallest deadline, assigning CPU time to that task. Deadlines can be absolute (calendar time) or relative to a task’s release, depending on convention.

In operation, the system maintains a ready queue ordered by deadlines. When a new task arrives or the current task completes, the scheduler may preempt the running task if a more urgent task becomes ready. Under EDF, a feasible schedule exists for a uniprocessor with independent tasks and known deadlines if the total workload can be accommodated; in many formulations, the processor utilization U = sum(Ci/Di) must not exceed 1 for relative deadlines.

Optimality on a single processor: EDF is considered optimal for preemptive, uniprocessor real-time scheduling, in the sense that if any schedule can meet all deadlines, EDF can as well. This makes EDF attractive for feasibility analysis and utilization-based tests. On multiprocessor systems, however, EDF is not generally optimal, and feasibility becomes more complex; variants such as Global EDF and Partitioned EDF are used, with performance dependent on task characteristics and scheduling policies.

Advantages and limitations: EDF offers high CPU utilization and straightforward feasibility checks, adapting to varying workloads and sporadic tasks. The approach requires precise deadline information and incurs overhead from preemption and deadline tracking. In practice, implementation choices, including preemption cost and timer precision, influence performance.

Applications and history: EDF is a foundational algorithm in real-time systems theory and practice, widely discussed in literature from the 1970s by researchers such as Liu and Layland. It remains a core option in embedded systems, control applications, and real-time operating systems where deadlines are critical.