Shortest job next

Shortest job first

Shortest Job Next (SJN), also known as Shortest Job First (SJF), is a scheduling algorithm used in computer science and operating systems to manage the execution of processes. This algorithm selects the process with the smallest execution time to run next, thereby minimizing the average waiting time for all processes in the queue.

Overview

The main principle behind SJN is to prioritize processes based on their execution time, with the aim of improving overall system efficiency and process throughput. It is a non-preemptive scheduling algorithm, meaning that once a process starts executing, it runs to completion without being interrupted by a shorter process that might arrive.

Algorithm

The SJN algorithm involves the following steps:

1. All incoming processes are added to a queue.
2. The process with the shortest execution time is selected from the queue for execution.
3. Once the selected process completes, it is removed from the queue, and the next shortest job is selected for execution.
4. This process repeats until all processes have been executed.

Advantages

• Efficiency: By minimizing the average waiting time, SJN can significantly improve the efficiency of the system.
• Simplicity: The algorithm is straightforward to implement, requiring only basic queue management and sorting based on execution time.

Disadvantages

• Starvation: Longer processes may suffer from starvation, as shorter processes continuously jump ahead in the queue.
• Predictability: It requires prior knowledge of the execution time of all processes, which may not always be feasible in real-time systems.

Applications

SJN is primarily used in batch processing systems where execution times are known in advance. It is less suitable for real-time systems where process execution times cannot be predicted accurately.

Comparison with Other Scheduling Algorithms

SJN is often compared with other scheduling algorithms such as First-Come, First-Served (FCFS) and Round-Robin (RR) scheduling. While FCFS is simpler and does not suffer from starvation, it generally has a longer average waiting time. RR, on the other hand, is more suitable for real-time applications but may lead to higher overhead due to context switching.

See Also

• Process Scheduling
• Preemptive Scheduling
• Non-Preemptive Scheduling
• Round-Robin Scheduling
• First-Come, First-Served

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