What is the SJF algorithm? What is its use in the operating system?
Operating systems use a set of different algorithms to manage processes. One of these algorithms is the SJF algorithm. Algorithms are also very diverse; some are suitable for the operating system, and some are for programming. For example, the spider algorithm in MATLAB and the Fibonacci algorithm are some of the algorithms. The SJF algorithm generally creates order in processes that require hardware resources, such as the CPU. In this article, we will first introduce the SJF algorithm and learn how to use it. Then, we will examine other issues related to the SJF algorithm.
What is the SJF algorithm?
The SJF algorithm is derived from the words “Shortest Job First.” This means that when working with this algorithm, the shortest job is always done first. As we said, this algorithm is used to schedule the hardware resources used, such as CPU, memory, and input and output modules. In general, the SJF algorithm works with two exclusive and non-exclusive strategies and reduces the average waiting time for activities to be completed.
This algorithm’s strong competitors used for scheduling processes include the Hunt Search (HUS), RR, FCFS, and HRRN algorithms. To function properly, this algorithm must correctly determine the completion time of processes to give tasks with less time to the operating system earlier. Since determining the time required to complete processes is difficult, this becomes one of the challenges of working with this algorithm. In the rest of this article, we have examined the methods for managing this challenge.
How is the completion time of processes calculated in the SJF algorithm?
The time it takes to complete a task or process cannot be obtained precisely; it can only be estimated or predicted. Obviously, the more accurate the time estimate, the more accurate the performance of this algorithm will be. Various methods can be used to estimate CPU time on processes. These methods are generally divided into two categories: static and dynamic, each with other subsets we will examine below.
Static Methods
Two techniques can be used with static methods, including the following.
Process Size
As the name suggests, this method uses process size. We estimate the time required to complete a task on a process using the execution time of an old process similar in size.
Process Type
There are several categories of processes, which are placed in various categories depending on their type. Depending on the process we are dealing with, we can also obtain an approximate time to complete the task. Some types of processes include background processes, user-related processes, operating system-related processes, etc.
Dynamic Methods
Using dynamic methods, we can also estimate the approximate time the operating system requires to perform different activities. Dynamic methods are divided into two subsets: simple averaging and exponential averaging.
What are the features of the SJF algorithm?
The SJF algorithm boasts extraordinary and unique features that set it apart from other scheduling algorithms. These features, which we will delve into, are sure to pique your interest.
The SJF scheduling algorithm has the shortest waiting time among its similar algorithms.
This algorithm is classified as a greedy algorithm.
As mentioned above, the operating system cannot calculate the completion time of processes accurately, but various methods can approximate these times, and with their help, the operating system can determine the order of processes.
This algorithm can be used in dedicated environments where accurate estimates of the completion time of various processes are available.
How does the SJF algorithm work in the operating system?
Various scheduling algorithms have been designed and provided to optimize and increase the efficiency of operating systems and can be used. Some of these algorithms are used to help schedule the execution of various processes in the operating system, and others are used to help better schedule writing and reading operations from the memory disk. If we want to explain how the SJF algorithm works in the operating system, we can divide it into the following three parts:
In the first step, all processes must be sorted based on their arrival and approximate completion times. The system can prioritize the processes that arrive earlier than the rest and have the shortest completion time.
In the second step, instead of checking one by one, we can use the interval tree data structure and sort the different processes based on the shortest completion time.
After finding a process eligible to be sent to the CPU, we use the “Arrival Time | AT” information to the queue and “Burst Time | BT” to calculate different times.
How are the different times related to the execution of each process calculated in the SJF algorithm?
In executing each process, we encounter different times, including operation completion time, activity execution time, and waiting time.
Operation completion time CT
Operation completion time is the time when the execution of operations ends. The start time and completion time of the process are added together.
Time required to perform the activity TAT
This time represents the difference between the process completion time and the time it reaches the execution queue.
Wait time WT
This time actually represents the difference between the time required to perform the activity and the time it takes to complete the process’s work.
What strategies does the SJF algorithm have in the operating system?
The SJF algorithm has two prevalent strategies, exclusive and non-exclusive approaches, which we will examine below.
Exclusive approach
In the exclusive approach, when the CPU starts a process, it does not stop until the process is completed unless a stop command is issued from outside and by the user.
Non-exclusive approach
In non-exclusive approaches, the shortest task is also entered into the CPU. Now, if during the process, a new process enters the operating system or is identified as having a shorter completion time, the process is stopped, and the process with a shorter time is replaced to be executed before other processes.
What are the advantages of the SJF algorithm?
The SJF algorithm offers numerous advantages that have contributed to its widespread use. These benefits, which we will explore, will surely convince you of its effectiveness.
The SJF algorithm has a shorter average waiting time than other scheduling algorithms, such as FCFS.
This algorithm can also be used for scheduling over a long time.
The SJF algorithm is suitable for processes sent in batches, whose execution time is specified in advance.
What are the disadvantages of the SJF algorithm?
Like any other algorithm, the SJF algorithm has its share of disadvantages, in addition to its many advantages. It’s important to be aware of these limitations, which we will discuss, to make informed decisions.
The SJF algorithm is designed to perform tasks faster with less completion time, and this can increase the completion time of time-consuming tasks and cause hunger in these activities.
This algorithm requires determining the time needed to complete its tasks, which can be challenging in many cases.
As we said, the time of activities can only be estimated and cannot be determined precisely, which is why this algorithm cannot be used for short-term scheduling in the CPU.
In the SJF algorithm, it is not possible to set a different priority for the execution of processes.
Final words
Various algorithms improve process execution in operating systems. One of the most widely used is the SJF algorithm. By estimating each process’s execution time, this algorithm executes processes with shorter execution times faster.
Like other algorithms, the SJF algorithm has various advantages and disadvantages. This algorithm can be used in two modes: exclusive and non-exclusive strategies.