🧠 What is a CPU Scheduler?
The CPU Scheduler is a component of the OS that selects one process from the ready queue and allocates the CPU to it.
It decides which process runs next when:
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A process finishes or is preempted
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A process moves from waiting to ready
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A new process is created
🔁 When Does CPU Scheduling Happen?
Here are the scheduling decision points:
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✅ A process terminates
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⏳ A process switches from running to waiting (e.g., I/O)
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🔁 A process switches from running to ready (e.g., preemption)
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📥 A process switches from waiting to ready (e.g., I/O done)
Points 3 and 4 allow preemptive scheduling (Round Robin, Priority Preemptive)
Points 1 and 2 are enough for non-preemptive scheduling (FCFS, SJF Non-Preemptive)
📦 How the CPU Scheduler Works
[ Ready Queue ]
↓
[ CPU Scheduler ] —→ chooses → [ Running Process ]
It uses a scheduling algorithm (e.g., Round Robin, SJF, Priority)
to decide which process gets the CPU next.
🎯 Responsibilities of the CPU Scheduler
| Task | Description |
|---|---|
| 🧮 Choose process | Pick from ready queue |
| 🧠 Apply scheduling policy | FCFS, SJF, RR, Priority, etc. |
| 🔄 Initiate context switch | Save old process state, load new one |
| 🎯 Optimize goals | Maximize CPU use, minimize waiting, ensure fairness |
🚥 Scheduling Criteria
A good CPU scheduler tries to optimize:
| Metric | Goal |
|---|---|
| ⏳ Waiting Time | Keep as low as possible |
| 🔁 Turnaround Time | Time from submission to completion |
| 🕒 Response Time | For interactive tasks |
| ⚡ CPU Utilization | Keep CPU busy |
| 📈 Throughput | Max number of processes finished per unit time |
| ⚖️ Fairness | No starvation or over-prioritization |
🧰 Example: Round Robin Scheduler (Preemptive)
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All processes in ready queue
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CPU Scheduler picks one
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Assigns it CPU for time quantum (say 100 ms)
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After quantum, switches to next ready process
✅ Fair
✅ Responsive
⚠️ High context switch overhead if quantum is too short
🔄 CPU Scheduler vs Long-Term & Medium-Term Scheduler
| Scheduler | Purpose |
|---|---|
| Long-Term | Controls admission of jobs into system |
| Medium-Term | Swaps processes in/out of memory |
| ✅ Short-Term / CPU Scheduler | Chooses which process runs next on CPU |
🧠 Interview-Ready Definition:
The CPU Scheduler is the OS component responsible for selecting a process from the ready queue and assigning it to the CPU based on a scheduling algorithm. It’s critical for ensuring fairness, responsiveness, and efficient CPU utilization in multitasking systems.
⚙️ Real-World Analogy
Think of the CPU as a doctor, and the ready queue as a waiting room full of patients.
The CPU Scheduler is the receptionist who decides which patient sees the doctor next, based on urgency, arrival time, or other rules.