🧠 What is Non-Preemptive Scheduling?
In Non-Preemptive Scheduling, once a process is given the CPU, it keeps it until it finishes or voluntarily gives it up (e.g., by waiting for I/O).
🛑 The OS never interrupts a running process, even if a higher-priority process arrives.
🧩 How It Works
Process P1 starts running →
P2 arrives (higher priority) →
P1 keeps running →
Only after P1 finishes, P2 gets CPU
There’s no forced context switch. The OS simply waits.
🕰️ When Is the CPU Released?
-
The running process terminates
-
The process moves to waiting state (e.g., waiting for I/O)
✅ Advantages of Non-Preemptive Scheduling
| ✅ Benefit | 📘 Why It Matters |
|---|---|
| 🧠 Simple to implement | No need for complex interrupt handling |
| 🔁 Low overhead | Fewer context switches (less CPU time wasted) |
| 💾 Predictable | Good for batch systems and embedded environments |
| 🔒 Safer for shared resources | No risk of being preempted while holding a lock |
❌ Disadvantages
| ❌ Issue | 📘 Explanation |
|---|---|
| 🕒 Poor responsiveness | Slow for interactive or real-time systems |
| 🧊 CPU hogging | Long processes block shorter, urgent ones |
| ⚠️ Starvation possible | Important tasks may be delayed indefinitely |
| 📉 Low CPU utilization | If a process blocks on I/O, CPU may sit idle |
📊 Common Non-Preemptive Scheduling Algorithms
| Algorithm | How It Works |
|---|---|
| First-Come First-Served (FCFS) | Run processes in order of arrival |
| Shortest Job First (SJF) | Run the shortest total burst time process first |
| Priority Scheduling (Non-Preemptive) | Run the highest-priority ready process — but don’t preempt current one |
🧠 Interview-Ready Definition:
Non-Preemptive Scheduling is a CPU scheduling approach where once a process starts executing, it runs to completion or until it voluntarily releases the CPU. It’s simple and predictable, but less responsive — making it unsuitable for real-time or interactive systems.
🖥️ Real-World Analogy
Imagine a railway crossing where once a train starts crossing, it must finish completely before another is allowed — even if an ambulance is waiting.
That’s non-preemption.
🔄 Summary Table: Preemptive vs Non-Preemptive
| Feature | Preemptive | Non-Preemptive |
|---|---|---|
| 🧠 Interruptible | ✅ Yes | ❌ No |
| 🕒 Response Time | ✅ Fast | ❌ Slow |
| 🔁 Context Switch Overhead | High | Low |
| ⚖️ Fairness | Better | Can lead to starvation |
| 🧰 Use Case | Interactive/real-time | Batch/embedded systems |