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    Question

    Which CPU scheduling algorithm is considered optimal

    for minimizing the average waiting time in cases where process execution times are known in advance?
    A First-Come-First-Served (FCFS) Correct Answer Incorrect Answer
    B Shortest Job First (SJF) Correct Answer Incorrect Answer
    C Round Robin (RR) Correct Answer Incorrect Answer
    D Priority Scheduling Correct Answer Incorrect Answer
    E Multi-Level Queue Scheduling Correct Answer Incorrect Answer

    Solution

    Shortest Job First (SJF) is widely considered optimal for minimizing the average waiting time in cases where process execution times (burst times) are known in advance. This scheduling algorithm selects the process with the smallest execution time for execution next, ensuring that shorter tasks do not have to wait behind longer ones. In detail:

    1. How SJF Works: It computes the burst times for each process and executes the one with the minimum burst time. Once a process is completed, it selects the next shortest burst time process. This preemptive or non-preemptive approach prevents long processes from unnecessarily delaying short ones.
    2. Why it Minimizes Waiting Time: SJF prioritizes processes with minimal execution times, thereby reducing the cumulative waiting time for subsequent processes. This strategy significantly impacts the overall performance of systems with predictable job sizes.
    3. Drawbacks: Despite its efficiency, SJF is impractical for real-time systems or when execution times are not predictable. It may lead to the starvation of longer processes in preemptive mode.
    Example:
    • Processes: P1 (8ms), P2 (4ms), P3 (1ms)
    • Execution Order (SJF): P3 → P2 → P1
    • Average Waiting Time: (0+1+5)/3 = 2ms
    Why Other Options Are Incorrect 1.    First-Come-First-Served (FCFS):
      • Processes are executed in the order of arrival, leading to the convoy effect , where longer processes delay shorter ones. Average waiting time is higher compared to SJF.
      • Example: For the same processes (P1, P2, P3), FCFS gives a waiting time of (0+8+12)/3=6.67ms(0 + 8 + 12)/3 = 6.67ms ( 0 + 8 + 12 ) /3 = 6.67ms , which is worse than SJF.
    2.    Round Robin (RR):
      • RR uses time slices (quantum) for execution, cycling through processes. While fair, it does not minimize waiting time due to frequent context switching and lack of consideration for process burst times.
    3.    Priority Scheduling:
      • While this algorithm selects processes based on priority, it does not inherently optimize for waiting time. Processes with lower priorities may suffer from starvation.
    4.    Multi-Level Queue Scheduling:
      • Used for systems with distinct process categories, but it does not focus on minimizing waiting time.

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