Shortest job First (SJF)

  

Out of all available (waiting) processes, it selects the processes, it selects the process with the smallest burst time to execute next.

• This algorithm for both Non-preemptive and Preemptive (SRTF- Shortest Remaining Time First).
• If two processors the burst time is same then we will go with the first come first serve.
• Here Completion time= Gantt chart values.
•   Turn Around time = Completion time -  Arrival time.
•  Waiting Time = Turn Around time -  Burst time.
• Response time = Gantt chart values arrival - Arrival time.
• It is preemption running process here in Gantt chart we check for processor for each and every one unit.
• After all the elements in the ready queue it will work  as Non- Preemption.

Advantage of Shortest job First

• Min average waiting time & min average Turn Around time.
• Better Response time than FCFS.
• Max Throughput.
• Provides a standard for other algorithms in case of average waiting time.

Disadvantage

• Can not be implemented.
• Starvation problem with process having larger Burst time.

Shortest job First (SJF) with prediction of Burst Time in OS

• Process Size.
• Process type.
• Simple Averaging (τ(n+1) = (1/n) ∑ T(i) )
• Exponential average (Aging) – Τn+1 = αtn + (1 - α)Τn 

                         α = smoothing factor  and 0 <= α <= 1
                         tn = actual burst time of nth process
                          Τn = predicted burst time of nth process.