20. Scheduling Algorithms Overview (Preemptive vs Non-Preemptive)

Non-Preemptive Preemptive
CPU taken forcibly?	No	Yes
Response time	Worse	Better
Overhead	Low	Higher
Starvation risk	Higher	Lower
Use case	Batch systems	Interactive / real-time

Non-preemptive: once a process has the CPU, it keeps it until it finishes or blocks on I/O. Algorithms: FCFS, SJF (non-preemptive), Priority (non-preemptive). Simple, low overhead, but a long process can starve others (the "convoy effect").

Preemptive: the OS can interrupt a running process, typically via a timer interrupt. Algorithms: Round Robin, SRTF, Priority (preemptive). Better responsiveness, but needs synchronization to protect data a process might be preempted mid-update.

Key point: preemption is what makes time-sharing possible; modern OSes (Linux, Windows) are preemptive, at the cost of needing mutexes/semaphores everywhere shared state is touched.

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