SJF (Shortest Job First) in Operating Systems - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to identify the process with the shortest burst time.

Operating Systems

shortest_process = min(processes, key=lambda p: p[1])

Drag options to blanks, or click blank then click option'

Aprocess_id

Barrival_time

Cpriority

Dburst_time

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing arrival time instead of burst time

Using priority which is unrelated to SJF

The SJF algorithm selects the process with the shortest burst time to execute next. Here, burst_time is used as the key to find the minimum.

2fill in blank

medium

Complete the code to sort processes by their burst time for SJF scheduling.

Operating Systems

sorted_processes = sorted(processes, key=lambda p: p[1])

Drag options to blanks, or click blank then click option'

Aburst_time

Barrival_time

Cpriority

Dprocess_id

Attempts:

3 left

💡 Hint

Common Mistakes

Sorting by arrival time or priority instead of burst time

Sorting by burst_time arranges processes from shortest to longest execution time, which is essential for SJF scheduling.

3fill in blank

hard

Fix the error in the condition to select the next process with the shortest burst time.

Operating Systems

if current_process.burst_time > [1].burst_time:

Drag options to blanks, or click blank then click option'

Awaiting_process

Bprevious_process

Cnext_process

Drunning_process

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing with previous or running process which is incorrect in this context

The comparison should be with the next_process to decide if the current process should be preempted in SJF.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that maps process IDs to their burst times for processes with burst time less than 10.

Operating Systems

{p[1]: p.burst_time for p in processes if p.burst_time [2] 10}

Drag options to blanks, or click blank then click option'

A.process_id

B>

C<

D.id

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' instead of '<' in the condition

Using incorrect attribute for process ID

The dictionary keys are process IDs accessed by .process_id. The condition filters processes with burst time less than 10 using <.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that maps process names in uppercase to their burst times for processes with burst time greater than 5.

Operating Systems

{p[1]: p[2] for p in processes if p.burst_time [3] 5}

Drag options to blanks, or click blank then click option'

A.name.upper()

B.burst_time

C>

D.id

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '>' in the condition

Using incorrect attributes for keys or values

The dictionary keys are process names converted to uppercase using .name.upper(). The values are burst times accessed by .burst_time. The condition filters processes with burst time greater than 5 using >.

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