Consider a scheduling approach which is non pre-emptive similar to shortest job next in nature. The priority of each job is dependent on its estimated run time, and also the amount of time it has spent waiting. Jobs gain higher priority the longer they wait, which prevents indefinite postponement. The jobs that have spent a long time waiting compete against those estimated to have short run times. The priority can be computed as : Using the data given below compute the waiting time and turnaround time for each process and average waiting time and average turnaround time.
Answers
Shortest Job First scheduling works on the process with the shortest burst time or duration first.
This is the best approach to minimize waiting time.
This is used in Batch Systems.
It is of two types:
Non Pre-emptive
Pre-emptive
To successfully implement it, the burst time/duration time of the processes should be known to the processor in advance, which is practically not feasible all the time.
This scheduling algorithm is optimal if all the jobs/processes are available at the same time. (either Arrival time is 0 for all, or Arrival time is same for all)
Non Pre-emptive Shortest Job First
Consider the below processes available in the ready queue for execution, with arrival time as 0 for all and given burst times.
Non Pre-emptive Shortest Job First Scheduling
As you can see in the GANTT chart above, the process P4 will be picked up first as it has the shortest burst time, then P2, followed by P3 and at last P1.
We scheduled the same set of processes using the First come first serve algorithm in the previous tutorial, and got average waiting time to be 18.75 ms, whereas with SJF, the average waiting time comes out 4.5 ms.Shortest Job First scheduling works on the process with the shortest burst time or duration first.
This is the best approach to minimize waiting time.
This is used in Batch Systems.
It is of two types:
Non Pre-emptive
Pre-emptive
To successfully implement it, the burst time/duration time of the processes should be known to the processor in advance, which is practically not feasible all the time.
This scheduling algorithm is optimal if all the jobs/processes are available at the same time. (either Arrival time is 0 for all, or Arrival time is same for all)
Non Pre-emptive Shortest Job First
Consider the below processes available in the ready queue for execution, with arrival time as 0 for all and given burst times.
Non Pre-emptive Shortest Job First Scheduling
As you can see in the GANTT chart above, the process P4 will be picked up first as it has the shortest burst time, then P2, followed by P3 and at last P1.
We scheduled the same set of processes using the First come first serve algorithm in the previous tutorial, and got average waiting time to be 18.75 ms, whereas with SJF, the average waiting time comes out 4.5 ms.
Answer:
Hey mate!!
Your answer is
Shortest job first is a scheduling algorithm in which the process with the smallest execution time is selected for execution next. Shortest job first can be either preemptive or non-preemptive. Owing to its simple nature, shortest job first is considered optimal. It also reduces the average waiting time for other processes awaiting execution.
Shortest job first is also known as shortest job next (SJN) and shortest process next (SPN).
Techopedia explains Shortest Job First (SJF)
Shortest job first depends on the average running time of the processes. The accurate estimates of these measures help in the implementation of the shortest job first in an environment, which otherwise makes the same nearly impossible to implement. This is because often the execution burst of processes does not happen beforehand. It can be used in interactive environments where past patterns are available to determine the average time between the waiting time and the commands. Although it is disadvantageous to use the shortest-job-first concept in short-term CPU scheduling, it is considered highly advantageous in long-term CPU scheduling. Moreover, the throughput is high in the case of shortest job first.
Shortest job first also has its share of disadvantages. For one, it can cause process starvation for longer jobs if there are a large number of shorter processes. Another is the need to know the execution time for each process beforehand. Often, this is almost impossible in many environments.
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