Which statement best describes preemptive CPU scheduling?

• A. It always runs processes to completion, no interruption.
• B. It can interrupt running processes to improve responsiveness; non-preemptive runs until completion or block.
• C. It ignores fairness and overhead.
• D. It never yields CPU.

Answer: (B)

Preemptive CPU scheduling allows the operating system to interrupt a currently running process so another process can run, typically driven by a timer interrupt or a higher-priority event. This design lets the system respond quickly to interactive tasks and share CPU time fairly among many processes. The timer provides a time slice, and when it expires the scheduler may switch to a different task based on the policy (priority, fairness, aging). In contrast, non-preemptive scheduling lets the running process continue until it finishes or blocks on I/O, which can cause long tasks to hog the CPU and reduce responsiveness. So the best description is that preemptive scheduling can interrupt running processes to improve responsiveness, while non-preemptive runs until completion or block. Preemption introduces some overhead due to context switching, but it’s essential for multitasking and responsive systems.