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SJF PREEMPTIVE SCHEDULING PROGRAM C:
2024-01-19 05:59:32

SJF PREEMPTIVE SCHEDULING PROGRAM C:

Introduction

The procedure that has to be completed with the least amount of time is chosen to run first in the Shortest Remaining Time First (SRTF) scheduling method. Processes always continue until they finish or a new process is started that takes less time, as the process that is presently running is the one that by definition has the least amount of time left and since that time should only decrease as execution advances.

In this tutorial, we will discuss the Shortest Job First (SJF) pre-emptive Process Scheduling algorithm and also write a program for the Shortest Job First (SJF) pre-emptive Process Scheduling algorithm. In the Shortest Job First (SJF) algorithm, if the CPU is available, it is assigned to the process that has the minimum next CPU burst. If the subsequent CPU bursts of twoprocesses become the same, then FCFS scheduling is used to break the tie.

The shortest job scheduling is what?

"The shortest tasks are given top priority as one of the CPU scheduling techniques. It is an algorithm whose burst processing is time-dependent. Alternatively said, a process with a shorter burst time runs first. The phrase "shortest job first" is sometimes known as "shortest job next" (SJN)."

With burst times of 5, 7, 6, and 2, for example, we have four processes: P1, P2, P3, and P4. Now that Process P4 has a shorter burst duration, Process P4 will be executed first. Then, the P1, P3, and P2 processes will be completed in that sequence.

Programming scheduling in C is done using SJF (shortest job first) or SJN (shortest job next). It is one of the CPU scheduling software. The SJF is described as the scheduling procedure or scheduling policy in the algorithm that dictates the waiting time process into the subsequent execution time.

CPU scheduling treats with the issues of deciding which of the processes in the ready queue needs to be allocated to the CPU. There are several different CPU scheduling algorithms used nowadays within an operating system

We will use C++ to write this algorithm due to the standard template library support. Hence, we will write the program of the Shortest Job First algorithm in C++, although, it's very similar to C.

INPUT: The first line contains the word "processes," or "n."

The process name (pname), arrival time (atime), and burst time (btime) are the three variables present in the next n lines.

Output:

Print the matrix containing the following information: process name, arrival time, burst time, completion time, turnaround time, waiting time, and response time.

Types of sjf:

  1. Non-pre-emptive
  2. Pre-emptive
  3. SJF, also known as shortest job next, is a scheduling strategy that chooses the waiting process with the lowest execution time to run first. SJN, commonly referred to as the Shortest Job Next (SJN), can be either pre-emptive or non-pre-emptive.

Scheduling for SJF features:

  • The benefit of using Shortest Job First is that it has the shortest average waiting time out of all scheduling methods.
  • This algorithm is greedy.
  • If shorter processes continue to be produced, hunger might result. The idea of ageing can be used to overcome this issue.
  • Since the operating system could not be aware of burst timings, it is almost impossible to categorise them. Although execution time cannot be predicted, it may be estimated using a variety of techniques, such as a weighted average of earlier execution times.
  • SJF can be employed in specialised settings when precise running time estimations are available.

Algorithm:

  • Sort every procedure in order of arrival time. Then choose the method with the shortest arrival time and burst time.
  • Create a pool of processes that come after the one that has just finished and choose the one from the pool that has the shortest burst time after the process has finished.
  • Process execution is complete when a process reaches its completion time.
  • Turnaround time is the interval between the completion and the arrival times.
  • Turn Around Time is calculated as Completion Time - Arrival Time.
  • Waiting Time (W.T): The interval of time between the turn-around and burst times.
  • Waiting time is Burst time minus Turn around time.

Problem description:

To find the typical waiting time and typical turnaround time given n processes and their burst times, create a C SJF scheduling programme.

  • SJF Scheduling Algorithm in C

The CPU scheduling method Shortest Job First (SJF) distributes the CPU to the processes based on the processes with the shortest execution times.

SJF employs both pre-emptive and non-pre-emptive scheduling. Shortest Remaining Time First (SRTF) is the pre-emptive variation of SJF. Here, we'll talk about SJF, or non-pre-emptive scheduling.

  • In a scheduling technique called "Shortest Job First," the process with the shortest execution time is carried out first. SJF greatly decreases the average wait time for other procedures.
  • types of algorithms for the shortest job first
  • These are the two different SJF algorithms:
  • 1. Non-pre-emptive: A process keeps the CPU in use until it either enters the waiting state or ends.

EXAMPLE: Take into account the following five procedures:

  • Step 1: P4 arrives at time = 0 and begins executing.
  • Step 2: Process P3 enters the queue at time = 1, but P4 keeps running because it only has to finish two units.
  • Step 3: P1 enters the waiting line at time = 2 while P4 keeps running.
  • Step 4: P4 ends running at time = 3 and P1 begins running since it has the shortest burst time.
  • Step 5: While P1 executes at time = 4, P5 enters the waiting queue.
  • Step 6: Process P2 joins the waiting queue at time = 5 and arrives. P1 will carry on running.
  • Step 7: P1 completes execution at time = 9 and, after P3, P5, and P2 have all had their burst times compared, P2 with the shortest burst time begins to execute.
  • Step 8: While P3 and P5 are in the waiting queue at time = 10, P2 continues to run.
  • Step 9: P2 completes its execution at time = 11 and P5 begins.
  • Step 10: P5 completes execution at time = 15.
  • Step 11: P3 completes its execution at time = 23.
  • Step 12: The typical wait period is:
P4 = 0 - 0 = 0


P1 = 3 - 2 + 6 = 7


P2 = 5 - 5 = 0


P5 = 4 - 4 + 2 = 2 P3 = 15 - 1 = 14


The waiting time is (0 + 7 + 0 + 2 + 14)/5 = 23/5 = 4.6 on average.

Advantages of sjf:

The benefits of SJF are as follows:

  1. used for lengthy scheduling.
  2. reduces the typical wait time.
  3. useful in batch processing situations when runtimes are known in advance.
  4. We can get a burst time estimate from the job description for long-term scheduling.
  5. The value of the next burst period must be anticipated for short-term scheduling.
  6. In terms of turnaround time average, ideal.

Disadvantages of sjf:

  1. Negative aspects of SJF
  2. The following are SJF's drawbacks:
  3. The task completion time must be known in advance because it is difficult to anticipate.
  4. It is used for batch system long-term scheduling.
  5. We are unable to forecast the duration of the impending CPU burst, hence we are unable to immediately implement this approach for CPU scheduling.
  6. It causes hunger or exceedingly extended turnaround times.
  7. It is vital to be aware of a process's runtime duration.
  8. It is challenging to forecast how long the next CPU request will last.
  9. The processor's overhead increases as a result of recording the elapsed time.