In ECET 2026 Computer Science, Operating System (OS) questions are highly scoring. Among them, Scheduling Algorithms are always expected in the exam. The two most important and easy-to-learn ones are First Come First Serve (FCFS) and Shortest Job First (SJF). By mastering these, students can confidently answer 1–2 direct questions in ECET exams.
📘 Concept Notes – Scheduling Algorithms
⚙️ What is CPU Scheduling?
In a multitasking system, multiple processes wait for CPU execution. CPU Scheduling algorithms decide the order in which processes get CPU time, aiming to minimize waiting time and turnaround time.
1️⃣ First Come First Serve (FCFS):
- Processes are executed in the order of arrival (like a queue).
- Non-preemptive: Once a process starts, it cannot be stopped until finished.
- Example: Ticket counter queue.
- Key Metric: Simple, but average waiting time may be high.
Formulae:
- Waiting Time (WT) = Turnaround Time (TAT) – Burst Time (BT)
- Turnaround Time (TAT) = Completion Time (CT) – Arrival Time (AT)
2️⃣ Shortest Job First (SJF):
- Process with the smallest burst time executes first.
- Can be Non-preemptive (once started, it runs till completion).
- Can be Preemptive (Shortest Remaining Time First – SRTF).
- Efficient: Minimizes average waiting time.
- Limitation: May cause starvation for long processes.
💡 Comparison Table – FCFS vs SJF
| Feature | FCFS | SJF |
|---|---|---|
| Scheduling Type | Non-preemptive | Preemptive/Non-preemptive |
| Basis | Arrival time | Burst time |
| Waiting Time | May be high (convoy effect) | Usually minimized |
| Example | Ticket queue | Hospital emergency (short cases) |
🔟 10 Most Expected MCQs – ECET 2026 [OS Scheduling]
Q1. Which scheduling algorithm executes processes in the order of arrival?
A) SJF
B) Priority
C) FCFS
D) Round Robin
Q2. In SJF, the process selected is the one with:
A) Highest priority
B) Longest burst time
C) Shortest burst time
D) Earliest deadline
Q3. FCFS scheduling is:
A) Preemptive
B) Non-preemptive
C) Priority-based
D) Round-based
Q4. Which algorithm may cause starvation?
A) FCFS
B) SJF
C) Round Robin
D) None
Q5. Turnaround Time = ?
A) CT – AT
B) WT + BT
C) Both A & B
D) None
Q6. Which scheduling algorithm is like a ticket counter queue?
A) SJF
B) FCFS
C) Priority
D) Multilevel Queue
Q7. Which is better for reducing average waiting time?
A) FCFS
B) SJF
C) Round Robin
D) Priority
Q8. SRTF is a type of:
A) FCFS
B) Preemptive SJF
C) Priority Scheduling
D) Multilevel Queue
Q9. Convoy effect occurs in:
A) SJF
B) FCFS
C) Priority
D) Round Robin
Q10. Which of the following is non-preemptive?
A) FCFS
B) Non-preemptive SJF
C) Both A & B
D) None
✅ Answer Key Table
| Q.No | Answer |
|---|---|
| Q1 | C |
| Q2 | C |
| Q3 | B |
| Q4 | B |
| Q5 | C |
| Q6 | B |
| Q7 | B |
| Q8 | B |
| Q9 | B |
| Q10 | C |
🧠 Explanations of All Answers
- Q1 → C: FCFS executes processes in order of arrival.
- Q2 → C: SJF selects the process with the shortest burst time.
- Q3 → B: FCFS is always non-preemptive.
- Q4 → B: In SJF, long processes may starve if short ones keep arriving.
- Q5 → C: TAT can be expressed as both CT–AT or WT+BT.
- Q6 → B: FCFS works like a queue at ticket counters.
- Q7 → B: SJF minimizes average waiting time.
- Q8 → B: Shortest Remaining Time First is a preemptive version of SJF.
- Q9 → B: Convoy effect (long process delaying others) occurs in FCFS.
- Q10 → C: Both FCFS and Non-preemptive SJF are non-preemptive.
🎯 Why This Practice Matters for ECET 2026
OS Scheduling Algorithms are repeated every year in ECET papers. Questions are mostly direct and formula-based. By practicing FCFS & SJF, students can secure 2–3 marks easily in the Operating Systems section. These algorithms are also vital in real-world OS concepts and job interviews.
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