Operating Systems

Unit 3: CPU Scheduling

From FCFS to Multilevel Feedback Queues — master every scheduling algorithm with fully worked numerical problems, Gantt charts, and step-by-step solutions.

⏱️ 8 hrs theory + 6 hrs lab | 💰 Earning Potential: ₹5,000–₹20,000/month | 📝 30 MCQs (Bloom's Mapped)

💼 Jobs this unlocks: Performance Engineer (₹6–10 LPA) | SRE — Site Reliability Engineer (₹8–15 LPA) | Systems Programmer (₹7–12 LPA)

Section A

Opening Hook — The 60-Second Window That Breaks India's Internet

🚂 IRCTC Tatkal Booking — 12 Lakh Users, 60 Seconds Each

Every morning at 10:00 AM sharp, over 12 lakh (1.2 million) users simultaneously hit the IRCTC servers trying to book Tatkal tickets. Each user gets roughly a 60-second window before their session times out. The server must decide: Who gets CPU time first? How long? What if someone's request is more urgent?

This is CPU scheduling in action. IRCTC uses a variant of Round Robin scheduling — each user request gets a fixed time quantum, ensuring no single request hogs the server. Priority queues ensure Tatkal requests (higher revenue) get processed before general waitlist queries.

The result? On a good day, 15,000 tickets are booked per minute. On a bad day? The entire system crashes because the scheduler couldn't handle the load. The difference between success and failure is the scheduling algorithm.

Could YOU have designed a better scheduler? By the end of this chapter, you'll know exactly how — and you'll be able to prove it with numbers.

🇮🇳 IRCTC🇮🇳 Zerodha🇮🇳 FlipkartGoogleLinux KernelMicrosoft Windows

The Linux kernel's CFS (Completely Fair Scheduler) handles CPU scheduling for 96.3% of the world's top 500 supercomputers, every Android phone, and most cloud servers. It was written by Ingo Molnár in 2007 and uses a red-black tree to guarantee O(log n) scheduling decisions. You're learning the foundations of the same algorithms today.

Section B

Learning Outcomes — Bloom's Taxonomy Mapped

Bloom's Level	Learning Outcome
🔵 Remember	Define scheduling criteria (CPU utilization, throughput, turnaround time, waiting time, response time) and list all 7 scheduling algorithms
🔵 Understand	Explain the difference between preemptive and non-preemptive scheduling, and describe the convoy effect, starvation, and aging
🟢 Apply	Solve numerical problems for FCFS, SJF, SRTF, Priority, and Round Robin — compute Gantt charts, waiting time, and turnaround time
🟢 Analyze	Compare scheduling algorithms on the basis of average waiting time, throughput, and fairness using computed results
🟠 Evaluate	Evaluate which scheduling algorithm is optimal for given real-world scenarios (e.g., IRCTC, trading systems, batch processing)
🟠 Create	Design a multilevel feedback queue scheduling policy for a given workload mix and implement a CPU scheduler simulator in Python