Shortest Job First Scheduling Algorithm

Hello OS lovers! It is time to jump into the universe of process scheduling algorithms.

Here, I will walk you through the nuances of the shortest job first scheduling algorithm. It is one of the most interesting and productive algorithms when it comes to process scheduling in OS. The SJF scheduling algorithm in OS also forms the foundation for other more advanced scheduling methods.

So, let’s begin our journey into the world of shortest job first scheduling algorithm, and understand SJF scheduling algorithm, SJF preemptive scheduling example, and also look at the shortest job first scheduling program in OS.

Let’s start!

What is the Shortest Job First Scheduling Algorithm?

You are a chef in a busy kitchen, handling many orders at the same time. How do you determine which order to prepare first? Well, if you prefer efficiency, it could be that you decide to deal with the easiest orders initially so customers can get their food quickly. This is the primary concept of the shortest job first scheduling algorithm in operating systems, also known as the SJF scheduling algorithm in OS.

The SJF scheduling algorithm is a non-preemptive algorithm. It picks the process having the shortest burst time to execute next. For an analogy, imagine a clever chef who always picks the fastest dishes to cook. This reduces the average waiting time for all orders.

The SJF scheduling algorithm in OS, therefore, works by minimizing the average waiting time and turnaround time of processes. This works by highlighting shorter processes first which keeps the system responsive and effective, reducing overall time spent on queue wait for processes.

Advantages of SJF Scheduling Algorithm in OS

The SJF scheduling algorithm has several advantages that make it an attractive choice for process scheduling in operating systems. Let's explore a few of them:

1. Minimal Average Waiting Time: By prioritizing shorter processes, SJF minimizes the average waiting time for all processes in the system. This means that processes spend less time waiting in the queue, resulting in faster overall execution.
   

2. Shorter Process Time: The SJF algorithm gives priority to processes with shorter time required for completion. This can lessen the average waiting time of all processes in the queue.
   

3. Better Resource Utilization: In SJF, the CPU is always busy with shorter processes when they arrive. This lessens the CPU's idle time and enhances system resource utilization efficiency.
   

4. Reduced Turnaround Time: The turnaround time, that is the sum of time from submitting a process to its completion, is frequently made smaller with SJF. By running shorter processes first, SJF aids in decreasing the total turnaround time for all processes.

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Disadvantages of SJF Scheduling Algorithm in OS

Though the SJF scheduling algorithm in OS has its plus points, it also has some downsides. Let's see its disadvantages:

1. Starving of Longer Processes: A big issue with the SJF scheduling algorithm in OS is the possibility for longer processes to starve. When a steady flow of shorter processes keeps coming, it can keep pushing back the longer ones in the queue and make them wait indefinitely.
   

2. Burst Time Prediction: SJF requires a good prediction of burst time for processes. However, it can be difficult to predict the exact time a process will take, especially with processes that have variable or unknown behavior. If predictions are wrong, this may cause less-than-ideal scheduling choices.
   

3. No Priority Consideration: SJF doesn't think about the priority or significance of processes. It only concentrates on burst time, but this might not always match with system's or user's priorities. Sometimes, we may delay critical processes having longer burst times to favor shorter ones that are less important.
   

4. Context Switching Overhead: In the non-preemptive version of SJF, where a new process can only start after the current one finishes, there are fewer context switches. However, in the preemptive version, where a newly arriving process with a shorter burst time can take over from the currently executing one - frequent context switches might occur. This could result in more overhead and affect system performance overall.
   

Shortest Job First Scheduling Example

To grasp the functioning of the SJF scheduling algorithm, let's contemplate on a straightforward illustration. Imagine we possess these processes along with their burst times:

• Process A: Burst Time = 5 units
• Process B: Burst Time = 2 units
• Process C: Burst Time = 8 units
• Process D: Burst Time = 3 units

Using the SJF algorithm, the execution order would be as follows:

1. Process B (Burst Time = 2 units)
2. Process D (Burst Time = 3 units)
3. Process A (Burst Time = 5 units)
4. Process C (Burst Time = 8 units)

From the burst times, we can see that the order of processing starts with the shortest process (Process B), then proceeds to the next shortest (Process D), and so on. By doing this, the average waiting time is reduced and it guarantees quicker completion for shorter processes.

Shortest Job First Scheduling Program in OS

In the operating system, we can write a program to hold and manage a queue of processes. It will choose the process with the smallest burst time for execution. Here is an example of simplified pseudocode for the shortest job first scheduling program in OS:

function SJF_Scheduling(processes):
while not isEmpty(processes):
shortestProcess = findShortestProcess(processes)
execute(shortestProcess)
remove(shortestProcess, processes)
end while
end function

function findShortestProcess(processes):
shortestProcess = processes[0]
for each process in processes:
if process.burstTime < shortestProcess.burstTime:
shortestProcess = process
end if
end for
return shortestProcess
end function

For this shortest job first scheduling algorithm pseudocode, the SJF_Scheduling function repeats the process of finding the shortest burst time using the findShortestProcess function, executing it, and removing it from the list of processes until all processes are finished.

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Concluding Remarks

You have arrived at the conclusion of this thrilling adventure through the domain of Shortest Job First (SJF) scheduling algorithm! We have examined basic aspects about SJF, its strong points and limitations. Moreover, we observed an illustration on how it functions in reality.

I, who have great interest in the field of operating systems, am quite fascinated by the elegance and effectiveness of SJF. The main goal of SJF is to give priority to shorter processes which helps in reducing average waiting time and enhancing system's response. Nevertheless, we must not forget its drawbacks like the possibility for long processes to get starved and difficulties with predicting burst times accurately.

Understanding the ideas of process scheduling is very important for anyone who wants to be an expert in operating systems. It creates basic knowledge about how operating systems handle and improve the running of processes, making sure they work well and effectively.

For those who are enthusiastic about operating systems and want to study additional interesting subjects such as memory management, file systems, and synchronization, upGrad has a broad selection of courses. From Computer Science to Software Engineering and more, upGrad gives complete learning routes that can assist you in becoming an actual expert in operating systems.

Lastly, keep searching, keep gaining knowledge and above all else, continue to become absorbed in the amazing realm of operating systems. You may end up creating the next revolutionary scheduling method one day!

Happy coding, and may your processes always find the shortest path to execution!

FAQs

1. What is the briefest job first process planning algorithm?

The shortest job first (SJF) process scheduling algorithm is a non-preemptive method. It chooses the process with the smallest burst time (time of execution) to be executed next. This algorithm tries to reduce the average waiting time and enhance the system's responsiveness.

2. What is the SJF algorithm in C?

The SJF algorithm, which stands for Shortest Job First, can be put into action with the help of C. You need to keep a process queue and choose the shortest burst time among them for execution. This programming logic might be written utilizing arrays or different data structures that portray processes and their characteristics.

The initial scheduling algorithm is first come, first served (FCFS). This algorithm is usually seen as the easiest and earliest scheduling method. It runs processes based on their arrival sequence without taking burst times or priorities into account.

3. How does SJF scheduling differ from priority scheduling?

In SJF scheduling, processes are chosen based on their burst times, and the one with the shortest execution time is given priority. But in priority scheduling, priorities are assigned to processes, and the process with the highest priority is selected for execution even if its burst time is longer.

4. Is SJF preemptive?

It can be both non-preemptive and preemptive. In the non-preemptive form, once a process starts executing it carries on until finished. In the preemptive version, which is also called shortest remaining time first (SRTF), a new coming process having a shorter burst time can interrupt the presently running process.

5. What is the best scheduling algorithm?

There's no one "best" scheduling algorithm. The selection depends on particular demands and features of the system, with each having its own advantages and disadvantages. The selection of an algorithm for a certain scenario is influenced by factors such as fairness, efficiency, response time and priority requirements.

6. Is SJF a greedy algorithm?

Yes, the shortest job first (SJF) can be viewed as a greedy algorithm. This is because, at each step, the local best choice is made by picking the process with the minimum burst time. The main objective of SJF is to reduce average waiting time without taking into account how this selection may affect other processes in the long run.

7. What is SJF good for in operating systems?

In operating systems, SJF can be beneficial because it offers the least average wait time. This method also leads to a higher system reaction speed and better use of resources, along with lesser turnaround time by focusing on shorter processes.

8. What are the advantages of SJF in operating systems?

Advantages of SJF include minimal average waiting time, shorter process time, better resource utilization, and reduced turnaround time.

9. What are the disadvantages of SJF?

The disadvantages of SJF include possible starvation for longer processes, uncertainty in predicting burst times, no priority consideration and the overhead connected with context switching in its preemptive version.

10. Which is better, SJF or FCFS?

The decision to use either SJF or FCFS depends on the needs of the system. In general, SJF offers lower average waiting times and improved system responsiveness when compared to FCFS. However, FCFS has its own benefits. It's simple, easy to understand and implement. Processes are executed in the exact order they came in, so it guarantees fairness and stops any starvation issue. Determining which method is better depends on many things, such as the type of workload, the need for priority settings, and what trade-offs between fairness and efficiency are acceptable.