Operating System Schedulers: How the Kernel Manages Tasks
Behind every program you run, the kernel juggles many tasks. The OS scheduler decides which task runs on the CPU and for how long. It must balance speed, fairness, and energy use, while staying responsive to user input. The scheduler moves tasks through ready, running, waiting, and completed states, keeping the system busy without wasting time.
Most modern kernels use a short-term scheduler that runs many times per second. Long- and medium-term schedulers decide what jobs enter the pool of ready tasks and how many stay in memory. The short-term scheduler chooses the next task to run and may preempt the current one if a higher-priority task arrives.
Key ideas include preemption, time slices, priorities, and run queues. Preemption lets a more important task interrupt a running one. Time slices cap how long a task can hold the CPU. Run queues organize tasks by state and priority, so the next candidate is easy to find.
Common approaches include Round Robin, Priority scheduling, and adaptive methods like multilevel feedback queues. Linux uses the Completely Fair Scheduler (CFS), which treats CPU time as a resource shared fairly by measuring virtual runtimes. Windows uses a different policy that blends priorities with aging to prevent starvation. Real-time systems may offer strict timing guarantees with dedicated queues.
Example: you type in a text editor while a background download runs. The editor remains responsive, and the download makes steady progress. If a video call starts with higher priority, the kernel can briefly give the call priority, keeping audio and video smooth.
Why it matters: good scheduling improves responsiveness, battery life, and overall throughput. For developers, understanding the basics helps write cooperative programs and predict behavior under load.
Key Takeaways
- Schedulers decide which task runs next and for how long.
- Preemption, time slices, and run queues balance responsiveness and fairness.
- Different OSs use different algorithms; Linux’s CFS is a well-known example.