The Fundamentals of Operating Systems Scheduling

Scheduling decides which process runs next on the CPU and for how long. A good scheduler keeps the system responsive, makes efficient use of hardware, and treats tasks fairly. It works with the ready queue, where waiting processes line up, and with the running state, when a task is actually executing. When a process waits for I/O, the scheduler hands the CPU to another candidate.

Key terms help here. A process can contain one or more threads. The CPU spends short bursts of time before it needs input or output again. Before a new process runs, the system performs a context switch, saving the old state and loading the new one. This switch has a tiny cost, so fast decisions matter.

There are three levels of scheduling. The long-term scheduler picks which processes enter the system. The medium-term scheduler may swap processes out to free memory. The short-term scheduler, also called the CPU scheduler, runs very often and must be fast. Its decisions shape how quickly tasks respond and how busy the CPU stays.

Common policies guide these decisions. First Come First Served (FCFS) is simple but can create long waits. Shortest Job First (SJF) favors quick tasks but may starve longer ones; its preemptive version (SRTF) can improve responsiveness. Round Robin splits time into slices (a time quantum) so every task gets attention, but a short quantum increases context switching. Priority scheduling runs tasks by importance and can use aging to prevent starvation. Multilevel feedback queues adaptively move work between queues based on behavior.

Trade-offs guide choices. Higher fairness can reduce peak throughput. Very aggressive scheduling keeps the CPU busy but may hurt interactivity. Real systems often blend rules and add safeguards, like avoiding too much context switching or ensuring a responsive foreground task in a desktop environment.

Example: a video editor executing a long render (heavy CPU burst) and a chat app with small, frequent messages. A round-robin or multilevel queue setup helps both stay usable without freezing the screen.

Real-time needs add another layer. Some tasks must finish by deadlines; in those cases, the OS uses specialized real-time scheduling to guarantee timing, not just average performance.

In short, scheduling is a balancing act. It blends speed, fairness, and predictability to keep computers feel snappy and reliable.

Key Takeaways

• Scheduling selects the next task and handles transitions between waiting, running, and ready states.
• Different policies trade off responsiveness, throughput, and fairness; no one policy fits every workload.
• Understanding these ideas helps explain why your computer feels smooth during some tasks and slower during others.