Question

Name and describe three CPUscheduling algorithms.

Short answer

Describe First-Come, First-Served, Shortest Job Next, and Round Robin scheduling algorithms.

Step-by-step solution

Understand the Problem

We need to describe three different algorithms used in CPU scheduling, which is a crucial part of operating systems to manage the execution of processes.

First-Come, First-Served (FCFS) Scheduling

In the FCFS algorithm, the first process that arrives is the first to be executed. It's akin to a queue at a ticket counter where the person who arrives first gets served first. This can lead to a problem called the 'convoy effect,' where short processes get stuck waiting behind long ones.

Shortest Job Next (SJN) Scheduling

SJN, also known as Shortest Job First (SJF), selects the process with the smallest execution time to execute next. This algorithm can optimize throughput, but it requires precise knowledge of how long a process will take, which may not always be possible. It can cause 'starvation' as shorter processes continually bypass longer ones.

Round Robin (RR) Scheduling

The Round Robin algorithm assigns a fixed time unit per process and cycles through them. Each process gets executed for a short time, and if it isn't completed, it goes back to the queue. This method is fair for all processes, reducing wait times, but can introduce overhead because of frequent context switches.

Key concepts

First-Come First-Served (FCFS)

The First-Come First-Served (FCFS) scheduling algorithm is one of the simplest types of CPU scheduling. It works just like a queue at a grocery store. The process that arrives first gets processed first, hence the name, "first come, first served." This straightforward approach makes it easy to implement, but not without challenges.

One major downside to using FCFS is the potential for the 'convoy effect'. This occurs when shorter tasks are forced to wait for longer tasks to finish, just because the longer task arrived first. This means that if a long process shows up at the beginning of the queue, all subsequent processes—no matter how short—must wait for it to finish. This can significantly increase the average waiting time for processes.

Despite its simplicity, FCFS doesn't work well in a time-sharing environment where quick response time is crucial. Understanding these drawbacks is important when considering FCFS for any real-world applications. Overall, while it's easy to understand and use, FCFS might not be the best choice when efficiency and fairness are priorities.

Shortest Job Next (SJN)

The Shortest Job Next (SJN), also known as Shortest Job First (SJF), is a CPU scheduling algorithm that aims to optimize the overall throughput. This algorithm selects the process with the smallest burst or execution time to run next.

SJN can drastically reduce the average waiting time for processes, since shorter-length tasks are given priority. As a result, processes are completed quickly, freeing up system resources for other tasks. However, this requires the scheduler to accurately predict the execution time of all tasks, which might not always be feasible.

One downfall of the SJN algorithm is the possibility of 'starvation'. This happens when longer processes are indefinitely postponed because there are always shorter jobs in the queue. If the influx of short jobs is continuous, longer jobs might never get processed, stagnating the queue.

Ultimately, SJN can be extremely efficient if the execution times of processes are known in advance and consistent. It excels in environments where processes are predictable, but its vulnerabilities appear in dynamic situations where predictability is not guaranteed.

Round Robin (RR)

The Round Robin (RR) scheduling algorithm is designed to ensure fairness among processes. Each process in the queue gets a fixed slice of time, known as a "time quantum", during which it can execute. Once a process's time quantum expires, the next process in line receives its chance.

Round Robin is particularly suitable for time-sharing environments where processes must receive fair allocation of CPU time. Since all processes get equal time intervals to execute, the average waiting time is reduced, mitigating the 'convoy effect' commonly observed in FCFS.

However, the efficiency of the Round Robin scheduling depends heavily on the size of the time quantum. A time quantum that's too large can lead to behavior similar to FCFS, while one that's too small can induce a high number of context switches, thereby reducing overall system throughput due to overhead.

Overall, Round Robin maintains a balance between fairness and performance. Its implementation ensures that all processes are continually given CPU time, making it ideal for multiuser systems where tasks need to be juggled efficiently.