SJF (Shortest Job First) in Operating Systems - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding SJF Scheduling Principle

Which statement best describes the main principle of the Shortest Job First (SJF) scheduling algorithm?

A It schedules the process with the shortest burst time next, minimizing average waiting time.

B It randomly selects a process to run next to ensure fairness.

C It schedules the process with the longest burst time first to maximize CPU utilization.

D It schedules processes in the order they arrive, regardless of their burst time.

💡 Hint

Think about which process gets priority in SJF to reduce waiting time.

SJF always picks the process with the shortest burst time next. This reduces the average waiting time compared to other algorithms like FCFS.

📋 Factual

intermediate

Preemptive vs Non-preemptive SJF

What is the key difference between preemptive and non-preemptive SJF scheduling?

A Non-preemptive SJF can interrupt a running process if a shorter job arrives; preemptive SJF runs the current process to completion.

B Both preemptive and non-preemptive SJF always run processes to completion without interruption.

C Preemptive SJF can interrupt a running process if a shorter job arrives; non-preemptive SJF runs the current process to completion.

D Preemptive SJF schedules processes randomly; non-preemptive SJF schedules by arrival time.

💡 Hint

Consider if the CPU can switch to a new process before the current one finishes.

Preemptive SJF (also called Shortest Remaining Time First) allows interruption if a new process with a shorter burst time arrives. Non-preemptive SJF waits for the current process to finish.

🚀 Application

advanced

Calculating Average Waiting Time in SJF

Given these processes with arrival times and burst times, what is the average waiting time using non-preemptive SJF?

Process | Arrival Time | Burst Time
P1      | 0            | 6
P2      | 1            | 8
P3      | 2            | 7
P4      | 3            | 3

A 6.25 units

B 5.0 units

C 3.5 units

D 4.5 units

💡 Hint

Schedule processes by shortest burst time available at each step and calculate waiting times.

Schedule: P1 runs 0-6 (wait=0). At 6, available: P2(8), P3(7), P4(3); pick P4 6-9 (wait=6-3=3). Then P3 9-16 (wait=9-2=7). Then P2 16-24 (wait=16-1=15). Average wait = (0+3+7+15)/4 = 6.25 units.

🔍 Analysis

advanced

SJF Starvation Problem

Why can the SJF scheduling algorithm cause starvation for some processes?

A Because SJF always runs processes in arrival order, ignoring burst time.

B Because long processes may wait indefinitely if shorter processes keep arriving.

C Because all processes are treated equally regardless of burst time.

D Because SJF randomly selects processes, causing unfairness.

💡 Hint

Think about what happens to long processes when many short ones arrive.

SJF favors short jobs, so if short jobs keep arriving, longer jobs may never get CPU time, causing starvation.

❓ Reasoning

expert

Choosing SJF Over Other Scheduling Algorithms

In which scenario is using SJF scheduling most beneficial compared to First-Come, First-Served (FCFS)?

A When the system must guarantee no process waits longer than a fixed time.

B When all processes have similar burst times and arrive simultaneously.

C When process arrival times are unpredictable and fairness is the highest priority.

D When the system has many short processes mixed with few long ones, and minimizing average waiting time is critical.

💡 Hint

Consider when minimizing average waiting time is most effective.

SJF reduces average waiting time especially when many short jobs exist among longer ones. FCFS does not optimize waiting time in such cases.

Practice

1. What is the main goal of the SJF (Shortest Job First) scheduling algorithm?

easy

A. To schedule the shortest job next to minimize average waiting time

B. To schedule jobs in the order they arrive

C. To schedule the longest job first to maximize CPU usage

D. To schedule jobs randomly without any priority

Solution

1. Step 1: Understand SJF scheduling principle

   SJF always picks the job with the shortest execution time next to run.

2. Step 2: Identify the goal of SJF

   This approach reduces the average waiting time for all jobs in the queue.

3. Final Answer:

   To schedule the shortest job next to minimize average waiting time -> Option A

4. Quick Check:

   SJF = shortest job first, reduces waiting time [OK]

Hint: SJF picks shortest job first to reduce waiting time [OK]

Common Mistakes:

• Confusing SJF with FCFS (First Come First Serve)
• Thinking SJF schedules longest jobs first
• Assuming SJF schedules jobs randomly

2. Which of the following is the correct way to describe the SJF scheduling algorithm?

easy

A. Schedules jobs based on their arrival time only

B. Schedules the job with the shortest burst time next

C. Schedules jobs in a round-robin fashion

D. Schedules jobs randomly without considering job length

Solution

1. Step 1: Recall SJF scheduling criteria

   SJF selects the job with the shortest burst (execution) time next.

2. Step 2: Match the description to options

   Only Schedules the job with the shortest burst time next correctly states scheduling by shortest burst time.

3. Final Answer:

   Schedules the job with the shortest burst time next -> Option B

4. Quick Check:

   SJF = shortest burst time scheduling [OK]

Hint: SJF = shortest burst time next, not arrival time [OK]

Common Mistakes:

• Confusing SJF with FCFS which uses arrival time
• Mixing SJF with round-robin scheduling
• Ignoring job length in scheduling decision

3. Given the following jobs with their burst times:
Job A: 6 units, Job B: 2 units, Job C: 8 units, Job D: 3 units
What is the average waiting time using non-preemptive SJF scheduling?

medium

A. 5.0 units

B. 3.5 units

C. 4.5 units

D. 6.0 units

Solution

1. Step 1: Order jobs by burst time for SJF

   Order: Job B (2), Job D (3), Job A (6), Job C (8).

2. Step 2: Calculate waiting times for each job

   Waiting times: B=0, D=2, A=5 (2+3), C=11 (2+3+6).

3. Step 3: Compute average waiting time

   Average = (0 + 2 + 5 + 11) / 4 = 18 / 4 = 4.5 units.

4. Final Answer:

   4.5 units -> Option C

5. Quick Check:

   Average waiting time = 4.5 units [OK]

Hint: Sort jobs by burst time, sum waiting times, divide by count [OK]

Common Mistakes:

• Not sorting jobs by burst time
• Calculating waiting time incorrectly by mixing completion times
• Forgetting to start first job waiting time at zero

4. Consider this non-preemptive SJF schedule with jobs and burst times:
Job X: 4 units, Job Y: 3 units, Job Z: 5 units
If the scheduler mistakenly picks Job Z first, what is the main error?

medium

A. Scheduling jobs randomly

B. Scheduling jobs based on arrival time

C. Using preemptive instead of non-preemptive scheduling

D. Ignoring the shortest job first rule

Solution

1. Step 1: Identify correct SJF behavior

   SJF should pick the job with the shortest burst time first, which is Job Y (3 units).

2. Step 2: Analyze the mistake

   Picking Job Z (5 units) first ignores the shortest job first rule.

3. Final Answer:

   Ignoring the shortest job first rule -> Option D

4. Quick Check:

   Picking longer job first breaks SJF rule [OK]

Hint: SJF must pick shortest job first, not longer ones [OK]

Common Mistakes:

• Confusing arrival time with burst time priority
• Mixing preemptive and non-preemptive concepts
• Assuming random scheduling is allowed in SJF

5. In a system using preemptive SJF (Shortest Remaining Time First), if a new job arrives with a burst time shorter than the remaining time of the current job, what happens?

hard

A. The new job preempts the current job immediately

B. The current job continues until completion

C. The new job waits until the current job finishes

D. Both jobs run simultaneously

Solution

1. Step 1: Understand preemptive SJF behavior

   Preemptive SJF (Shortest Remaining Time First) allows interruption if a shorter job arrives.

2. Step 2: Apply rule to scenario

   If new job's burst time is less than current job's remaining time, it preempts immediately.

3. Final Answer:

   The new job preempts the current job immediately -> Option A

4. Quick Check:

   Preemptive SJF switches to shortest remaining job [OK]

Hint: New shorter job preempts current in preemptive SJF [OK]

Common Mistakes:

• Assuming current job always runs to completion
• Confusing preemptive with non-preemptive SJF
• Thinking jobs run in parallel