CPU Scheduling

CPU Scheduling is a crucial aspect of operating systems that determines how the CPU is allocated to different processes.

Additionally, it involves efficiently managing tasks and determining the order in which the CPU executes them.

Types of CPU Scheduling

Operating systems consider CPU scheduling as an essential aspect.

Furthermore, they categorize it into two main types: preemptive and non-preemptive scheduling.

Both have their advantages and disadvantages, and the choice between them depends on the specific requirements of the system.

Preemptive CPU Scheduling

Preemptive CPU scheduling is one of the types of CPU scheduling techniques used in operating systems.

It allows the operating system to interrupt a running process and allocate the CPU to another process with higher priority.

It ensures that the CPU can preempt a process and execute the most important process first on it.

Real-time systems, where tasks have strict deadlines and require timely execution, particularly benefit from preemptive scheduling.

By allowing the operating system to interrupt processes, preemptive scheduling helps achieve better system responsiveness and efficient resource utilization.

Non-preemptive CPU Scheduling

Non-preemptive CPU scheduling is one of the types of CPU scheduling algorithms used in operating systems.

In it, a process continues to execute on the CPU until it completes or voluntarily releases it

The process retains control of the CPU until it completes its execution or explicitly relinquishes the CPU.

This approach ensures that a process can run without interruption, allowing it to complete its tasks efficiently.

However, it may result in longer waiting times for other processes in the ready queue, as they must wait for the currently running process to finish before the CPU can be allocated to them.

CPU Scheduling Algorithms

By implementing the following various scheduling algorithms, the operating system ensures fair and efficient execution of processes.

It plays a vital role in optimizing system performance, reducing response time, and maximizing overall throughput.

It is a fundamental concept that allows multiple processes to share the CPU's resources effectively.

First-Come-First-Serve (FCFS)

Operating on a simple principle, the CPU scheduling algorithm known as First-Come-First-Serve (FCFS) serves the first process that arrives.

Consequently, this implies that the processes are executed in the order they arrive, without considering their priority or execution time.

FCFS is a non-preemptive algorithm, meaning a process continues executing until completion or voluntarily giving up the CPU.

FCFS is easy to implement, but it can result in poor waiting time, particularly when long processes arrive first.

Shortest Job Next (SJN) or Shortest Job First (SJF)

The Shortest Job Next CPU scheduling algorithm minimizes process waiting time by prioritizing the execution of the shortest job.

Additionally, it selects the process with the smallest burst time, as it requires a small time to complete its execution.

Prioritizing shorter jobs, the Shortest Job Next algorithm ensures the execution of processes with shorter burst times first, thereby reducing waiting times and enhancing overall system performance

Round Robin (RR)

Operating systems use the round-robin CPU scheduling algorithm to allocate CPU time to multiple processes in a balanced manner.

In it, each process executes within a fixed time slice or quantum, typically ranging from a zero to hundred milliseconds.

Once a process's time expires, the system preempts it and allows the next process to execute.

It continues until all processes execute and ensures that no process occupies the CPU for an extended period.

Many people use this algorithm because it is simple and allows all processes to run equally.

Priority Scheduling

Priority scheduling is a method used to determine the order in which processes are executed by the CPU.

This algorithm assigns a priority value to each process, with higher priority values indicating a greater urgency or importance.

The process with the highest priority gets access to the CPU first, followed by the process with the next highest priority, and so on.

This ensures the execution of processes with higher priority before those with lower priority, allowing for efficient utilization of the CPU's resources.

Priority Scheduling is a popular algorithm used in operating systems to manage the execution of processes and ensure that critical tasks are given precedence.

Criteria for CPU Scheduling in OS

CPU scheduling criteria are the set of guidelines and principles that determine how the CPU should allocate its resources to different processes.

The criteria CPU utilization, Throughput, Response Time, and Fairness, play a crucial role in determining the efficiency and fairness of the scheduling algorithm.

CPU Utilization

One of the key criteria used in CPU scheduling is the CPU utilization. It refers to the percentage of time that the CPU is busy executing tasks.

A high CPU utilization indicates effective utilization, with the CPU not being idle for long periods.

Conversely, low CPU utilization suggests that it is not fully utilized and remains idle for a significant amount of time.

CPU scheduling algorithms aim to maximize CPU utilization by efficiently allocating tasks to the CPU, ensuring that it remains busy and productive.

Throughput

Throughput, referring to the number of processes completed in a given time, is one of the important criteria for CPU scheduling.

It measures the efficiency of the CPU in terms of how many tasks it can handle and complete within a specific timeframe.

A higher throughput indicates that the CPU can process a larger number of processes, resulting in better overall system performance.

This criterion is particularly important in scenarios where multiple processes are competing for CPU resources, as it helps determine how efficiently the CPU can handle and complete these tasks.

Response Time

CPU scheduling is a crucial aspect of operating systems, and one of the key criteria used to evaluate the efficiency of a scheduling algorithm is response time.

Response time refers to the time it takes for a process to start executing after it has been submitted to the CPU.

It is an important metric as it directly impacts the overall performance and user experience of a system.

A scheduling algorithm that prioritizes processes with shorter response times can ensure faster execution and reduce waiting times for users.

By optimizing response time, operating systems can enhance the overall efficiency and responsiveness of the system, leading to improved user satisfaction.

Fairness

One of the key criteria for CPU scheduling is fairness.

Fairness refers to treating all processes in the system equally and providing them with a fair share of the CPU's processing time.

This ensures that no process is favoured over others and that each process gets a fair chance to execute its tasks.

Fairness is important in CPU scheduling as it helps prevent any particular process from monopolizing the CPU and ensures that all processes have an equal opportunity to run and complete their tasks on time.

By implementing fair scheduling algorithms, the operating system can efficiently distribute the CPU's resources and ensure fair treatment for all processes.

CPU Scheduling Example

In this example of CPU scheduling, we have three processes named P1, P2, and P3.

These processes have different arrival times, with P1 arriving at time 0, P2 at time 1, and P3 at time 2.

Each process also has a corresponding burst time, which represents the amount of time it requires to complete its execution.

For P1, the burst time is 8 units, for P2 it is 4 units, and for P3 it is 6 units.

FCFS Scheduling

• P1 executes first (0-8).
• P2 starts after P1 completes (8-12).
• P3 starts after P2 completes (12-18).

Round Robin (Time Slice = 4)

• P1 (0-4), P2 (4-8), P3 (8-12), P1 (12-16), P2 (16-18).

Shortest Job Next (SJN)

• P1 (0-8), P2 (8-12), P3 (12-18).

The CPU scheduling algorithm determines the order in which the CPU will execute these processes.

The arrival time and burst time of each process play a crucial role in this decision-making process.

By considering these factors, the scheduler can allocate CPU time to each process effectively, ensuring optimal utilization of system resources.

In this particular example, the scheduler will need to consider the arrival times and burst times of P1, P2, and P3 to make informed decisions about the order in which they will be executed.

Conclusion

CPU scheduling is a critical aspect of operating systems, aiming to maximize CPU utilization, and throughput, and minimize turnaround time and waiting time.

Various algorithms offer different trade-offs, and their effectiveness depends on the system's characteristics and workload.

Understanding these concepts is essential for designing efficient and responsive operating systems.