c program for sjf preemptive scheduling algorithm

It can be represented in the gantt chart as follow: waiting time for process P3 will be calculated as 3 + 4 = 7 (burst time of P5 and P2).

Next turn is for process P3 with shortest burst time 5 milliseconds.

After that process P2 has the smallest CPU burst so P2 will get next tern after completion of P1 so process P2 has to wait for 3 milliseconds which is the completion time for process P5.

As per the rule of Shortest Job First CPU will be allocated to the process with the shortest CPU burst time so in our example, process P5 has the shortest CPU burst time so obviously process P5 will get the CPU first for execution thus waiting time for P1 will be zero.Les us see how this policy works for the scheduling of processes with the help of an example.Ĭonsider the following processes and their CPU burst time (in millis.) and find out average waiting time and average turnaround time using non-preemptive SJF technique.

Preemptive SJF scheduling is also referred to as Shortest Remaining Time Next (SRTN) scheduling.

When a new process arrives having a short CPU burst time than the currently executing process preemptive SJF algorithm will preempt the currently executing process, and allocates the CPU to the newly arrived process where as a non-preemptive SJF algorithm will allow the current running process to finish its CPU burst.

SJF algorithm may be either preemptive or non-preemptive.

In fact more appropriate term for this algorithm would be the “Next Shortest CPU Burst Time First” because it allocates the CPU by examining the length of the next CPU burst of a process rather than its total length of a burst time.

If two processes have the same length of CPU burst then FCFS policy is used to solve the tie.

When the CPU is available it is assign to the process that has the smallest next CPU burst.

In this algorithm, the process that has the shortest CPU burst time is selected first for the execution.

C program for sjf preemptive scheduling algorithm