CPU Scheduling in Operating Systems: Optimizing Resource Allocation for Efficient Execution

In the realm of operating systems, efficient resource allocation plays a crucial role in ensuring optimal performance and responsiveness. One of the key components involved in this process is CPU scheduling, which determines the order in which processes are executed by the central processing unit (CPU). This article aims to provide a comprehensive understanding of CPU scheduling, exploring its purpose, various algorithms, and their impact on system performance.

Table of Contents

1. Purpose of CPU Scheduling
2. Types of CPU Scheduling
   1. Preemptive Scheduling
      1. Round Robin Scheduling
      2. Shortest Job Next (SJN) Scheduling
      3. Priority Scheduling
   2. Non-Preemptive Scheduling
      1. First-Come, First-Served (FCFS) Scheduling
      2. Shortest Job First (SJF) Scheduling
      3. Priority Scheduling
3. Process States and Transitions
4. Scheduling Algorithms and Their Characteristics
   1. Round Robin Scheduling
   2. Shortest Job Next (SJN) Scheduling
   3. Priority Scheduling
   4. First-Come, First-Served (FCFS) Scheduling
   5. Shortest Job First (SJF) Scheduling
5. Factors Influencing CPU Scheduling Decisions
   1. Burst Time
   2. Priority
   3. Deadline
   4. Preemptive vs. Non-Preemptive
   5. Aging
6. CPU Scheduling in Modern Operating Systems
   1. Multilevel Queue Scheduling
   2. Multilevel Feedback Queue Scheduling
   3. Completely Fair Scheduler (CFS)
7. Impact of CPU Scheduling on System Performance
8. Conclusion

1. Purpose of CPU Scheduling

In an operating system, multiple processes compete for the limited resources of the CPU. CPU scheduling comes into play to manage and allocate these resources efficiently. Its primary objectives are:

• Maximizing CPU utilization: Ensuring that the CPU remains busy executing processes as much as possible.
• Fairness: Providing equal opportunities for all processes to execute and avoiding starvation.
• Responsiveness: Minimizing response time and latency to deliver a smooth user experience.
• Throughput: Maximizing the number of processes completed within a given time period.
• Turnaround time: Minimizing the time taken for a process to complete its execution.

2. Types of CPU Scheduling

CPU scheduling algorithms can be broadly classified into two categories: preemptive and non-preemptive scheduling.

2.1 Preemptive Scheduling

Preemptive scheduling allows the operating system to interrupt a running process and allocate the CPU to another process. This type of scheduling is suitable for time-sharing systems and real-time environments. Let's explore some popular preemptive scheduling algorithms:

2.1.1 Round Robin Scheduling

Round Robin (RR) scheduling assigns each process a fixed time slice, known as a time quantum or time slice. The CPU switches between processes in a circular fashion. If a process does not complete within its time quantum, it is preempted, and the next process in the queue is given a chance to execute.

• Advantages:
  • Fairness: Each process gets an equal share of the CPU's time.
  • Responsiveness: Shorter processes do not have to wait for long before execution.
• Disadvantages:
  • Overhead: Frequent context switches incur overhead due to saving and restoring process state.

2.1.2 Shortest Job Next (SJN) Scheduling

Shortest Job Next (SJN) scheduling selects the process with the shortest burst time for execution. It aims to minimize the waiting time of processes. This algorithm requires prior knowledge of the burst time, which may not be available in real-time scenarios.

• Advantages:
  • Minimizes waiting time and turnaround time for short processes.
• Disadvantages:
  • Requires accurate estimation or prediction of burst time.
  • Longer processes may face indefinite waiting.

2.1.3 Priority Scheduling

Priority scheduling assigns priorities to processes, and the CPU executes the process with the highest priority. In case two or more processes have the same priority, other scheduling algorithms, like round robin, can be used to determine the order.

• Advantages:
  • Allows for system-specific priorities based on criticality and importance.
• Disadvantages:
  • Can lead to starvation if a process with lower priority never gets a chance to execute.

2.2 Non-Preemptive Scheduling

Non-preemptive scheduling allows a process to continue executing until it completes, blocks, or voluntarily yields the CPU. The decision to allocate the CPU to another process is made only when the current process is no longer in the running state. Let's discuss some common non-preemptive scheduling algorithms:

2.2.1 First-Come, First-Served (FCFS) Scheduling

First-Come, First-Served (FCFS) scheduling is the simplest CPU scheduling algorithm. The CPU is assigned to the first process that arrives, and it continues execution until completion or until the process voluntarily releases the CPU.

• Advantages:
  • Simple and easy to implement.
• Disadvantages:
  • Poor performance in terms of average waiting and turnaround time.
  • No consideration for process priority.

2.2.2 Shortest Job First (SJF) Scheduling

Shortest Job First (SJF) scheduling selects the process with the shortest burst time for execution, similar to the preemptive SJN scheduling algorithm. However, in non-preemptive SJF, once a process starts executing, it continues until completion.

• Advantages:
  • Minimizes average waiting time and turnaround time.
• Disadvantages:
  • Requires accurate estimation or prediction of burst time.
  • Long processes may face indefinite waiting.

2.2.3 Priority Scheduling

Similar to preemptive scheduling, priority scheduling assigns priorities to processes. However, in the non-preemptive variant, the CPU is allocated to the highest priority process only when the current process completes or blocks.

• Advantages:
  • Allows for system-specific priorities based on criticality and importance.
• Disadvantages:
  • Can lead to starvation if a process with lower priority never gets a chance to execute.

3. Process States and Transitions

During the execution of a process, it undergoes various state transitions. These states can be broadly classified as follows:

• New: The process is being created or initialized.
• Ready: The process is waiting for execution and is eligible to run.
• Running: The process is currently being executed by the CPU.
• Waiting: The process is waiting for an event or resource, such as I/O completion.
• Terminated: The process has finished execution or has been explicitly terminated.

The transitions between these states depend on the scheduling decisions made by the operating system and the events occurring during the process's lifetime.

4. Scheduling Algorithms and Their Characteristics

Different scheduling algorithms have varying characteristics, which influence the performance and behavior of the system. Let's delve into the details of some common scheduling algorithms:

4.1 Round Robin Scheduling

• Time quantum: Each process is allocated a fixed time quantum for execution.
• Advantages:
  • Fairness: Ensures equal distribution of CPU time among processes.
  • Responsiveness: Shorter processes get quick execution.
• Disadvantages:
  • Overhead: Frequent context switches incur overhead.
  • High waiting time for longer processes.

4.2 Shortest Job Next (SJN) Scheduling

• Criteria: Selects the process with the shortest burst time.
• Advantages:
  • Minimizes waiting time and turnaround time for short processes.
• Disadvantages:
  • Requires accurate estimation or prediction of burst time.
  • Indefinite waiting for longer processes.

4.3 Priority Scheduling

• Criteria: Assigns priorities to processes.
• Advantages:
  • Allows for system-specific priorities.
• Disadvantages:
  • Can lead to starvation if lower priority processes are never executed.

4.4 First-Come, First-Served (FCFS) Scheduling

• Criteria: Allocates the CPU to the first arrived process.
• Advantages:
  • Simple and easy to implement.
• Disadvantages:
  • Poor performance in terms of waiting and turnaround time.
  • No consideration for process priority.

4.5 Shortest Job First (SJF) Scheduling

• Criteria: Selects the process with the shortest burst time.
• Advantages:
  • Minimizes average waiting time and turnaround time.
• Disadvantages:
  • Requires accurate estimation or prediction of burst time.
  • Indefinite waiting for longer processes.

5. Factors Influencing CPU Scheduling Decisions

Several factors come into play when making CPU scheduling decisions. These factors help determine the most suitable process for execution. The major factors include:

5.1 Burst Time

Burst time refers to the amount of time required by a process to complete its execution. Accurate estimation or prediction of burst time is critical for scheduling algorithms that prioritize shorter processes.

5.2 Priority

Priority indicates the relative importance or urgency of a process. Processes with higher priorities are scheduled for execution before those with lower priorities.

5.3 Deadline

Some real-time systems require processes to meet specific deadlines. Scheduling algorithms may consider the deadline associated with a process to ensure timely execution.

5.4 Preemptive vs. Non-Preemptive

The decision to allow preemption or not affects the scheduling algorithm's behavior. Preemptive scheduling allows the CPU to be taken away from a running process, while non-preemptive scheduling allows a process to continue execution until it completes or voluntarily releases the CPU.

5.5 Aging

Aging is a technique employed in priority scheduling to avoid starvation. It gradually increases the priority of waiting processes to ensure fairness and prevent lower-priority processes from being indefinitely delayed.

6. CPU Scheduling in Modern Operating Systems

Modern operating systems often employ more advanced CPU scheduling techniques to cater to the diverse requirements of different applications and workloads. Let's explore some notable approaches:

6.1 Multilevel Queue Scheduling

Multilevel queue scheduling categorizes processes into multiple queues based on priority. Each queue may use its own scheduling algorithm, and the queues are ordered hierarchically. Processes move between queues based on priority or other criteria.

6.2 Multilevel Feedback Queue Scheduling

Multilevel feedback queue scheduling expands on multilevel queue scheduling by allowing processes to move between queues based on their behavior and resource requirements. This approach provides flexibility and adaptability to varying workloads.

6.3 Completely Fair Scheduler (CFS)

The Completely Fair Scheduler (CFS) is a scheduling algorithm used in the Linux kernel. It uses a red-black tree to keep track of processes and ensures fair allocation of CPU time based on process priorities and resource requirements.

7. Impact of CPU Scheduling on System Performance

Efficient CPU scheduling can significantly impact the overall performance of an operating system. Properly designed scheduling algorithms can lead to improved resource utilization, reduced waiting and response times, enhanced fairness, and increased throughput. Conversely, poor scheduling decisions can result in high latency, increased context switching overhead, and potential performance bottlenecks.

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