ThreadScheduling

ThreadScheduling is the component of an operating system that decides which runnable thread runs on a CPU at any moment. It aims to maximize CPU utilization, provide responsive performance, and ensure fairness among threads, while meeting any real-time constraints when present. Threads are the basic units of execution within processes, and the scheduler coordinates multiple threads across cores, handling context switches and synchronization with the kernel.

Most systems use preemptive scheduling, allowing the running thread to be interrupted by a timer tick to run another thread. Non-preemptive (cooperative) scheduling keeps a thread running until it blocks or yields. Preemption improves responsiveness but adds overhead and complexity.

Key concepts include ready and blocked states, per-CPU run queues, and context switching. The short-term scheduler selects the next thread from the ready queue; a context switch saves and restores thread state. Long- or mid-term schedulers determine the degree of concurrency by admitting or removing threads from execution.

Common strategies include round robin, which time-slices threads; static or dynamic priority scheduling; and multilevel feedback queues, which adjust priority based on behavior. Real-time systems use guarantees such as rate-monotonic or earliest-deadline-first scheduling to meet deadlines. Fair scheduling schemes aim to allocate CPU time proportionally to thread weights.

Implementations vary. Linux uses the Completely Fair Scheduler to balance fairness and latency; Windows employs a priority-based preemptive scheduler with aging; many runtimes defer to the host OS.

Performance considerations include the cost of context switches and cache locality. Avoiding starvation, using aging, and aligning scheduling with workload characteristics are important design goals.