Schedulers

Schedulers are software components that determine the order in which tasks are executed in a computing environment. They allocate limited resources, such as CPU time, I/O bandwidth, or network capacity, with the aim of improving overall system performance and fairness.

In operating systems, the CPU scheduler selects which ready process or thread runs next. Disk and network subsystems may employ their own schedulers to order I/O requests or packets. Scheduling policies are chosen to meet domain requirements and can be preemptive or non-preemptive.

Common scheduling approaches include first-come, first-served; round-robin, which assigns fixed time slices; priority-based scheduling, sometimes with aging to prevent starvation; and multilevel feedback queues, which adjust priority based on observed behavior. In real-time systems, schedulers enforce timing constraints using methods such as rate monotonic or earliest-deadline-first policies.

Implementation relies on data structures such as ready queues, priority queues, and I/O queues, along with time slicing, context switching, and queue management. Some systems use cooperative scheduling, where tasks yield control voluntarily, while others rely on preemption by a timer interrupt.

Applications extend beyond the operating system to programming frameworks, where schedulers manage asynchronous tasks, thread pools, or distributed job orchestration. Evaluation of schedulers considers metrics like turnaround time, waiting time, response time, throughput, CPU utilization, and fairness.

See also: scheduling theory; queueing theory; time-sharing; real-time scheduling.