Shortest job first Scheduling (SJF)
- 3. When the CPU is available, it is assigned to the process that has the smallest CPU burst. Ready queue is treated as a priority queue based on smallest CPU time requirement Priorities are assigned in inverse order of time needed for completion of the entire job If equal time of completion, then FCFS is used for assigning priority. Dispatcher selects shortest job (1st in queue) and runs to completion
- 4. Gives minimum average waiting time for a given set of processes. Minimizes average turnaround time. It cannot be implemented at the level of short term CPU scheduling. Store estimated value in PCB for the current burst, and compare with actual value. Exponential averaging is used to estimate the process’ burst duration.
- 5. Minimizes average waiting time. Provably optimal with respect to average turnaround time. Throughput is high.
- 6. In general, cannot be implemented. Requires future knowledge In practice, can’t actually predict the length of next burst Can lead to unfairness or starvation Doesn’t always minimize average turnaround time Elapsed time (i.e., execution-completed-time) must be recorded, it results an additional overhead on the processor.
- 7. Criteria- Burst time Mode:- Non-Preemptive Process No. Arrival Time Burst Time 1 1 7 2 2 5 3 3 1 4 4 2 5 5 8
- 8. Process No. Arrival Time Burst Time 1 1 7 2 2 5 3 3 1 4 4 2 5 5 8 Gantt Chart:- 0 1 8 P1 9 P3 P4 11 P2 16 P5 24