SJF (Shortest Job First) in Operating Systems - Mini Project: Build & Apply

Understanding SJF (Shortest Job First) Scheduling

📖 Scenario: You are learning how operating systems decide which task to run next. One common method is called Shortest Job First (SJF), where the system picks the task that needs the least time to finish.Imagine you have several tasks waiting, each with a different time needed to complete. Your goal is to organize these tasks so the shortest ones run first.

🎯 Goal: Build a simple list of tasks with their durations, then sort them using the SJF method to see which task runs first.

📋 What You'll Learn

Create a list of tasks with their exact names and durations

Add a variable to hold the number of tasks

Sort the tasks by their duration using SJF logic

Display the sorted list of tasks in order of execution

💡 Why This Matters

🌍 Real World

Operating systems use SJF scheduling to improve efficiency by running shorter tasks first, reducing waiting time for many tasks.

💼 Career

Understanding SJF helps in roles like system administration, software development, and performance optimization where task scheduling matters.

1

Create the list of tasks

Create a list called tasks with these exact tuples: ("Task1", 6), ("Task2", 2), ("Task3", 8), ("Task4", 3) where each tuple has the task name and its duration.

Operating Systems

# Create the list of tasks with their durations
# Your code here

Hint

Use a list of tuples. Each tuple has a string and a number.

2

Add a variable for the number of tasks

Create a variable called num_tasks and set it to the length of the tasks list.

Operating Systems

tasks = [("Task1", 6), ("Task2", 2), ("Task3", 8), ("Task4", 3)]
# Create num_tasks as the count of tasks
# Your code here

Hint

Use the len() function to count items in the list.

3

Sort tasks by duration using SJF logic

Sort the tasks list by the second item in each tuple (the duration) using the sort() method with a key argument.

Operating Systems

tasks = [("Task1", 6), ("Task2", 2), ("Task3", 8), ("Task4", 3)]
num_tasks = len(tasks)
# Sort tasks by duration (second item in tuple)
# Your code here

Hint

Use tasks.sort(key=lambda task: task[1]) to sort by duration.

4

Display the sorted tasks in order

Create a list called execution_order that contains only the task names from the sorted tasks list using a list comprehension.

Operating Systems

tasks = [("Task1", 6), ("Task2", 2), ("Task3", 8), ("Task4", 3)]
num_tasks = len(tasks)
tasks.sort(key=lambda task: task[1])
# Create execution_order list with task names only
# Your code here

Hint

Use a list comprehension to get the first item of each tuple.

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