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    CPU Scheduling Notes for GATE CS

    CPU Scheduling notes for GATE CS: 42 study cards covering concepts, formulas, shortcuts and exam traps, plus solved practice questions.

    cpu scheduling notes

    Chapter Roadmap: CPU Scheduling

    The CPU Scheduling Journey
    Foundations & Metrics: Arrival, Burst, Completion, Turnaround, Waiting, Response.
    SJF & SRTF: Optimal algorithms for minimizing average waiting time.
    Scheduling Properties: Preemption overhead, starvation, convoy effect.
    Round Robin (RR): Time-slicing, quantum selection, context switches.
    Priority & Multiprocessor: Diverse workloads and multi-CPU distribution.
    Goal: Build flawless Gantt charts and calculate metrics with 100% accuracy.

    Topic Hero: The Core Philosophy of SJF and SRTF

    The primary objective of any scheduling algorithm is to optimize system metrics. For batch systems, the most critical metric is average waiting time.
    The Mathematical Truth: To minimize the sum of waiting times, you must execute the shortest jobs first. If you execute a long job first, all subsequent short jobs are forced to wait for the entire duration of that long job.
    • Shortest Job First (SJF): Non-preemptive. Once a process gets the CPU, it runs to completion.
    • Shortest Remaining Time First (SRTF): Preemptive. If a newly arrived process has a burst time strictly less than the remaining time of the currently running process, the CPU is snatched away.
    Note: Both algorithms are mathematically proven to give the minimum average waiting time for a given set of processes.

    Executing Non-Preemptive SJF

    Core Rule: No Preemption

    Once a process starts, it runs to completion.

    1. Identify current time .
    2. Filter processes where .
    3. Select process with minimum .
    4. Tie-breaker: If same , pick earliest (FCFS).
    5. Allocate CPU. New time . Record .
    6. Repeat until all completed.
    CPU Idle? If no processes have arrived (), CPU remains idle until the earliest remaining .

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    CPU Scheduling Notes for GATE CS

    CPU Scheduling notes for GATE CS: 42 study cards covering concepts, formulas, shortcuts and exam traps, plus solved practice questions.

    Chapter Roadmap: CPU Scheduling

    The CPU Scheduling Journey
    Foundations & Metrics: Arrival, Burst, Completion, Turnaround, Waiting, Response.
    SJF & SRTF: Optimal algorithms for minimizing average waiting time.
    Scheduling Properties: Preemption overhead, starvation, convoy effect.
    Round Robin (RR): Time-slicing, quantum selection, context switches.
    Priority & Multiprocessor: Diverse workloads and multi-CPU distribution.
    Goal: Build flawless Gantt charts and calculate metrics with 100% accuracy.

    Topic Hero: The Core Philosophy of SJF and SRTF

    The primary objective of any scheduling algorithm is to optimize system metrics. For batch systems, the most critical metric is average waiting time.
    The Mathematical Truth: To minimize the sum of waiting times, you must execute the shortest jobs first. If you execute a long job first, all subsequent short jobs are forced to wait for the entire duration of that long job.
    • Shortest Job First (SJF): Non-preemptive. Once a process gets the CPU, it runs to completion.
    • Shortest Remaining Time First (SRTF): Preemptive. If a newly arrived process has a burst time strictly less than the remaining time of the currently running process, the CPU is snatched away.
    Note: Both algorithms are mathematically proven to give the minimum average waiting time for a given set of processes.

    Executing Non-Preemptive SJF

    Core Rule: No Preemption

    Once a process starts, it runs to completion.

    1. Identify current time .
    2. Filter processes where .
    3. Select process with minimum .
    4. Tie-breaker: If same , pick earliest (FCFS).
    5. Allocate CPU. New time . Record .
    6. Repeat until all completed.
    CPU Idle? If no processes have arrived (), CPU remains idle until the earliest remaining .

    Executing Preemptive SRTF

    Core Rule: Preemption on Strictly Smaller RBT
    1. Maintain Remaining Burst Time (RBT) table.
    2. At time , filter processes with and .
    3. Select process with minimum RBT.
    4. Tie-breaker: If RBTs equal, earliest continues (No preemption).
    5. Run for 1 unit (or until next arrival). Decrease RBT by 1.
    6. If , record . Check new arrivals.
    Crucial Condition: Preempt ONLY if . If equal, do NOT preempt.

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