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.