Shortest Job Next SJN - TarisMajor/5143-OpSystems GitHub Wiki

SJF

Shortest Job Next (SJN), also known as Shortest Job First (SJF), is a CPU scheduling algorithm that selects the process with the shortest expected execution time for execution next. This approach aims to minimize the average waiting time for processes and improve overall system performance.

Key Characteristics of SJN Scheduling

  1. Selection Based on Execution Time: The process with the shortest estimated execution time is selected to run next. This decision is made based on the estimated or known duration of each process.
  2. Non-Preemptive Nature: In its basic form, SJN is a non-preemptive algorithm. Once a process starts executing, it runs to completion before the next process is selected. However, a preemptive version, known as Shortest Remaining Time First (SRTF), allows interruptions based on remaining execution time.

Advantages of SJN Scheduling

  1. Minimized Average Waiting Time: By always selecting the shortest job next, SJN minimizes the total waiting time for all processes, leading to improved efficiency.
  2. Improved Throughput: The algorithm increases throughput by reducing the time processes spend in the ready queue, allowing more processes to be completed in a given time frame.
  3. Optimality: SJN is optimal in that it produces the lowest average waiting time for a set of processes compared to other scheduling algorithms.

Disadvantages of SJN Scheduling

  1. Predictability Issues: Accurate knowledge of the execution time of each process is required, which is often difficult to obtain in practice.
  2. Potential Starvation: Long processes may experience starvation if shorter processes keep arriving, as they are continuously given priority.
  3. Lack of Preemption: In its non-preemptive form, SJN cannot adapt to changes in process priorities or accommodate high-priority processes that arrive while a longer process is running.

Use Cases for SJN Scheduling

  1. Batch Processing Systems: Suitable for batch processing environments where processes with known execution times can be scheduled efficiently.
  2. Non-Interactive Systems: Ideal for non-interactive systems where processes do not require immediate response times, such as certain scientific and data analysis applications.
  3. Environments with Predictable Processes: Effective in environments where the execution times of processes are predictable and can be accurately estimated.

Example of SJN Scheduling

Consider three processes with the following burst times:

  • Process P1: Burst Time = 6 units
  • Process P2: Burst Time = 8 units
  • Process P3: Burst Time = 7 units

The SJN schedule would be:

  1. Process P1 (6 units)
  2. Process P3 (7 units)
  3. Process P2 (8 units)

The average waiting time can be calculated as:

  • P1: 0 units (starts immediately)
  • P3: 6 units (waits for P1 to finish)
  • P2: 13 units (waits for P1 and P3 to finish)

Average waiting time = (0 + 6 + 13) / 3 = 6.33 units

Sources for Further Reading