SJF (Shortest Job First) Scheduling Algorithm

The SJF scheduling algorithm uses a fore knowledge of each task execution time and then executives the tasks in the order of their runtime with the short task first. The SJF scheduling algorithm is fair in the sense that it reduces the time in which shorter tasks have to wait in the system and leaves longer tasks to wait longer in the system. Although the issue of the order in which the processes arrived is not considered in SJF, it still tries to be fair by ensuring that all the processes are eventually completed. But in the case where a job waits for ever because a shorter job gets called so often thereby preventing its execution even though it arrived first, SJF can be considered unfair.

The following differences were identified between the SJF and FCFS scheduling algorithm.
 FCFS executes processes based on the order in which they arrive the processor, the process that requests the CPU first is allocated the CPU first while SJF executes the processes based on the order of the time it takes to execute each process with the shortest task first.
 In SJF, the duration time of all the tasks needs to be identified in advance while this is not required in FCFS
 The average waiting time is higher in FCFS than in SJF because in FCFS, shorter tasks could be made to wait for longer time if they arrive the CPU late, thereby increasing the overall average wait time. In SJF, the shortest tasks are completed before the longer ones, thereby reducing the average wait time.