Shortest Remaining Time First in Operating Systems

 

Shortest Job First (SJF) is a scheduling algorithm used in operating systems to allocate CPU time to processes. The basic idea behind SJF is to prioritize processes with shorter burst times, giving them a higher priority for CPU execution. The goal of SJF is to minimize the average waiting time for all processes, ensuring that the CPU is used efficiently.

SJF operates by sorting the processes based on their burst time and assigning the process with the shortest burst time to the CPU first. The process runs until completion, and the next process with the shortest burst time is assigned to the CPU. This process continues until all processes have completed execution.

There are two types of SJF scheduling algorithms:

1. Non-preemptive SJF

2. Preemptive SJF Non-preemptive SJF: In non-preemptive SJF, once a process is assigned to the CPU, it runs to completion. Other processes must wait until the current process has completed execution before being assigned to the CPU.

        Consider the following example to illustrate the operation of non-preemptive SJF:

• * A system has four processes with burst times 10, 5, 8, and 12 units of time.
• * The processes are sorted based on their burst time in increasing order, resulting in the order (5, 8, 10, 12).
• * The first process (5) is assigned to the CPU and runs for 5 units of time.
• * The next process (8) is assigned to the CPU and runs for 8 units of time.
• * The third process (10) is assigned to the CPU and runs for 10 units of time.
• * The last process (12) is assigned to the CPU and runs for 12 units of time.

In this example, the average waiting time for all processes is calculated to be 5.25 units of time, which is significantly lower compared to other scheduling algorithms such as First-Come First-Served (FCFS).

Advantages of non-preemptive SJF:

• * Minimizes the average waiting time for all processes, ensuring that the CPU is used efficiently.
• * Simple and easy to implement.
• * Does not suffer from starvation, where a process may wait indefinitely for the CPU.

Disadvantages of non-preemptive SJF:

• * Non-preemptive SJF may lead to poor response time for long processes, as they may have to wait for several shorter processes to complete before being assigned to the CPU.
• * May result in a long waiting time for short processes, as they may have to wait for long processes to complete before being assigned to the CPU.
• * May not be suitable for real-time systems, where deadlines are critical and a long process may block the CPU, preventing real-time processes from being executed on time.
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• Preemptive SJF: In preemptive SJF, a process can be interrupted and replaced by a shorter process. The process with the shortest remaining time is assigned to the CPU. This ensures that the CPU is used efficiently, even if the process with the shortest burst time is not available at the time of scheduling.

Consider the following example to illustrate the operation of preemptive SJF:

• 1. A system has four processes with burst times 10, 5, 8, and 12 units of time.
• The processes are sorted based on their burst time in increasing order, resulting in the order (5, 8, 10, 12). 2. The first process (5) is assigned to the CPU and runs for 2 units of time.
• 3. The next process (8) arrives and is assigned to the CPU, as it has a shorter burst time compared to the current process (5). The process (5) is moved to the waiting queue and the process (8) runs for 8 units of time.
• 4. The next process (10) arrives and is assigned to the CPU, as it has a shorter burst time compared to the current process (5). The process (5) remains in the waiting queue and the process (10) runs for 10 units of time.
• 5. The last process (12) arrives and is assigned to the CPU, as it has the shortest remaining burst time. The process (5) is assigned to the CPU after the process (12) has completed execution.

In this example, the average waiting time for all processes is calculated to be 4 units of time, which is even lower compared to non-preemptive SJF.

Advantages of preemptive SJF:

• 1. Minimizes the average waiting time for all processes, ensuring that the CPU is used efficiently.
• 2. Prevents long processes from blocking the CPU and impacting the response time of other processes.
• 3. Suitable for real-time systems, where deadlines are critical and a long process may block the CPU, preventing real-time processes from being executed on time.

Disadvantages of preemptive SJF:

• 1. May result in higher overhead and complexity compared to non-preemptive SJF, as processes must be constantly monitored and rescheduled based on their remaining burst time.
• 2. May lead to starvation, where a process may wait indefinitely for the CPU if shorter processes keep arriving and preempting the current process.

In conclusion, SJF is an efficient scheduling algorithm that prioritizes processes based on their burst time. Non-preemptive SJF is simple and easy to implement, while preemptive SJF provides better response time and is suitable for real-time systems. Both algorithms have their advantages and disadvantages, and the choice of algorithm depends on the requirements of the operating system and the system being designed.