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SJF (Shortest Job First) in Operating Systems - Cheat Sheet & Quick Revision

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Recall & Review
beginner
What does SJF stand for in operating systems?
SJF stands for Shortest Job First, a scheduling algorithm that selects the process with the smallest execution time to run next.
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beginner
How does SJF scheduling decide which process to run?
SJF chooses the process that requires the least amount of CPU time among the available processes.
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intermediate
What is the main advantage of SJF scheduling?
It minimizes the average waiting time for processes, making the system more efficient in handling tasks quickly.
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intermediate
What is a major drawback of SJF scheduling?
It can cause starvation, where longer processes may wait indefinitely if shorter jobs keep arriving.
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advanced
Is SJF scheduling preemptive or non-preemptive?
SJF can be both: non-preemptive SJF runs a process to completion once started; preemptive SJF (also called Shortest Remaining Time First) can interrupt a running process if a shorter job arrives.
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What does SJF scheduling prioritize?
AProcess with the highest priority number
BProcess that arrived first
CProcess with the shortest execution time
DProcess with the longest execution time
Which problem can SJF scheduling cause?
AStarvation of longer processes
BDeadlock
CHigh CPU utilization
DFairness to all processes
What is the main goal of SJF scheduling?
AMinimize average waiting time
BMaximize throughput
CEnsure fairness
DMaximize CPU idle time
In preemptive SJF, what happens if a new shorter job arrives while another is running?
AThe running job continues until completion
BThe running job is interrupted and the shorter job runs
CBoth jobs run simultaneously
DThe new job waits until the running job finishes
Which of these is NOT true about SJF scheduling?
AIt can be preemptive or non-preemptive
BIt can cause starvation
CIt always reduces average waiting time
DIt is easy to know exact job lengths in practice
Explain how the Shortest Job First scheduling algorithm works and its main benefits.
Think about how choosing the smallest task first affects waiting times.
You got /3 concepts.
    Describe the potential problem of starvation in SJF scheduling and why it happens.
    Consider what happens if short tasks keep coming before longer ones get CPU time.
    You got /3 concepts.

      Practice

      (1/5)
      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