Shortest Job First Algorithm in operating system

• Shortest job first a non-preemptive, pre-emptive scheduling algorithm.
• shortest possible waiting time strategy.
• In Batch systems where the necessary CPU time is known, implementation is easy.
• In interactive systems where the necessary CPU time is unknown, implementation is impossible
• The time required for the procedure should be known in advance by the processor.

Given Table process have Execution time and Arrival time,

Waiting time of process:

Average Wait Time: (0 + 4 + 12 + 5)/4 = 21 / 4 = 5.25

Characteristics of SJF Scheduling 

• The benefit of using Shortest Job First is that it has the shortest average waiting time out of all scheduling methods.
• This algorithm is greedy.
• It may cause starvation if shorter processes keep coming. This problem can be solved using the concept of ageing.
• OS may not know burst time and may not sort them. While it’s not possible to predict execution time. But several methods used t estimate execution time of job
• SJF used specialized environment where estimation of running time is available

Algorithm of SJF

• Sort every procedure in order of arrival time.
• Then choose the process with the shortest arrival time and burst time.
• Create a pool of processes that will arrive after the one that has just finished, and then choose the one from the pool that has the least amount of burst time.