##### SJF scheduling

Shortest-Job-First(SJF) Scheduling

• Best approach to minimize waiting time.
• Actual time taken by the process is already known to processor.
• Impossible to implement.
• example:- In Preemptive Shortest Job First Scheduling, jobs are put into ready queue as they arrive, but as a process with short burst time arrives, the existing process is preempted.

##### Shortest Job Remaining Time First

Shortest Remaining Time First(SRTF) algorithm: This is preemptive version of shortest job first algorithm.  It’s minimize average turn arround time. If all processes are arrived at same time, then it works as SJF.

Example: Consider the following table:

 Process no. Arrival Time Burst Time P1 0 6 P2 2 1 P3 4 4 P4 5 3

Find the average waiting time and average turn arround time using SRTF(preemptive) algorithm?

Solution: Using SRTF(non-preemptive) algorithm, gantt chart is: Therefore,

 Waiting Time Turn around Time Completion Time Process No. Arrival Time Burst Time 7 - 6 = 1 7 - 0 = 7 7 P1 0 6 1 - 1 = 0 3 - 2 = 1 3 P2 2 1 10 - 4 = 5 14 - 4 = 10 14 P3 4 4 5 - 3 = 2 10 - 5= 5 10 P4 5 3

So,
Average Turn arround time  = (7 + 1 + 10 +5) / (4) = 5.75

Average Waiting time = (1 + 0 + 5 + 2) / (4) =2

##### 1Comment
Sathya Dhinesh
1 Jun 2019 11:25 pm
Superb sir