🧠 What is Preemptive Scheduling?
Preemptive Scheduling is a CPU scheduling method where the operating system forcibly takes the CPU away from the currently running process and gives it to another higher-priority or waiting process.
So, even if a process hasn’t finished — the OS can say:
🛑 “You’re done for now. Someone else needs the CPU.”
🔁 When Does Preemption Happen?
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A higher-priority process becomes ready
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The running process exceeds its time slice (quantum)
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A process I/O completes, and it rejoins the ready queue
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System interrupts or signals occur
🔄 Example:
Round Robin (with time quantum = 100 ms)
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P1 runs for 100 ms ⏱️
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OS preempts it and gives CPU to P2
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Even if P1 isn’t done — it’s forced to yield
🔩 Key Features
| Feature | Description |
|---|---|
| ⏱️ Time-limited execution | Each process runs only for a short time quantum |
| ⚡ Responsive | Good for interactive and real-time systems |
| ⚖️ Fair | Prevents CPU hogging |
| 🔄 Context Switch | Frequent — leads to more overhead |
✅ Advantages of Preemptive Scheduling
| ✅ Benefit | 📘 Why It Matters |
|---|---|
| 💨 Faster responsiveness | For real-time and UI apps (e.g., mouse clicks, keypresses) |
| 🔁 Fair CPU usage | Long-running processes can’t block others |
| 📈 Higher throughput | Shorter jobs don’t get stuck behind long ones |
| 🧠 Efficient priority handling | High-priority tasks preempt low ones immediately |
❌ Disadvantages
| ❌ Issue | 📘 Explanation |
|---|---|
| 🔁 High context switch overhead | Frequent task switching uses CPU time |
| 🧵 Race conditions | If threads are preempted mid-update |
| ⚠️ Starvation risk | Low-priority processes may be delayed indefinitely |
| 💥 Complex | Requires fine-tuned time management + interrupt handling |
📊 Common Preemptive Scheduling Algorithms
| Algorithm | How It’s Preemptive |
|---|---|
| Round Robin | Preempts on time slice expiry |
| Shortest Remaining Time First (SRTF) | Preempts if a shorter job arrives |
| Priority Scheduling (Preemptive) | Preempts when higher-priority process arrives |
| Multilevel Feedback Queue (MLFQ) | Dynamically adjusts priorities and can preempt |
🧠 Interview-Ready Definition:
Preemptive Scheduling allows the OS to forcibly remove the CPU from a running process to assign it to another, usually based on priority or time-slicing. It ensures fairness, responsiveness, and better resource utilization in multi-user, real-time systems — but comes at the cost of complexity and overhead.
🖥️ Real-World Analogy
You’re working in an office (CPU), and your boss (OS) says,
“You’re doing great, but this VIP just walked in — give them your chair for now.”
That’s preemption — you get up, and they get to work.