22516 Operating System Unit 4 Notes PDF

                              CPU scheduling is a fundamental concept in operating systems. It determines which processes will run on the CPU and for how long. The goal is to use CPU resources efficiently, ensuring fair use and minimizing waiting times. Here's an overview:

CPU Scheduling:

                              CPU scheduling is the process of determining the sequence in which the processes access the CPU. Since multiple processes are often in memory simultaneously, the operating system needs a strategy to decide which one gets the CPU when.

Criteria for CPU Scheduling:

1. CPU Utilization: Keep the CPU as busy as possible.
2. Throughput: Number of processes that complete their execution per time unit.
3. Turnaround Time: Total time taken to execute the process, from submission to completion.
4. Waiting Time: Total time a process has been in the ready queue waiting for the CPU.
5. Response Time: Time it takes from the submission of a request until the first response is produced.

Scheduling Algorithms:

Various algorithms are used for CPU scheduling. Each has its advantages and drawbacks:

1. First-Come, First-Served (FCFS):
• Processes are scheduled in the order they arrive.
• Pros: Simple and easy to understand.
• Cons: Can lead to the "convoy effect", where short processes wait for long processes to finish, leading to inefficient CPU utilization.
2. Shortest Job Next (SJN) or Shortest Job First (SJF):
• The process with the smallest execution time is the next to execute.
• Pros: Minimizes waiting time for short processes.
• Cons: Difficult to predict the next CPU burst's length; can lead to process starvation.
3. Priority Scheduling:
• Each process is assigned a priority. The highest priority process runs next.
• Pros: Useful for systems that have jobs with different priority levels.
• Cons: Can lead to starvation if low-priority processes are continually pre-empted by high-priority processes.
4. Round Robin (RR):
• Each process is assigned a fixed time slice or quantum.
• Processes are dispatched in a circular order, with each allowed to run for a quantum or until completion.
• Pros: Fair and simple; suitable for time-sharing systems.
• Cons: Choice of time quantum is critical and can impact performance.
5. Multilevel Queue Scheduling:
• Processes are divided into multiple queues, each having its own scheduling algorithm.
• Processes are assigned to a queue based on their characteristics.
• Pros: Offers flexibility and can accommodate various types of processes.
• Cons: Determining which process goes into which queue can be complex.
6. Multilevel Feedback Queue:
• A variation of the multilevel queue scheduling.
• Allows processes to move between queues based on their behavior and estimation of CPU bursts.
• Pros: Adaptable to a process's needs and behavior.
• Cons: More complex to implement.
7. Fair Share Scheduling:
• Used in systems where each user is allocated a portion of the CPU time.
• Ensures that users get a fair share of CPU resources.

Pre-emptive vs. Non-pre-emptive Scheduling:

• Pre-emptive: In this mode, the OS can remove a process from the CPU and replace it with another process. This can happen if a higher-priority process arrives or after a time quantum expires in Round Robin scheduling.
• Non-pre-emptive: Once a process is in the running state, it continues until it completes or yields the CPU, typically used in systems where interruption could be problematic or inefficient.

Conclusion:

CPU scheduling is vital for ensuring system responsiveness, fairness, and effective utilization of CPU resources. The choice of scheduling algorithm can affect system performance, and often operating systems may use a combination of algorithms based on the system's requirements and the types of processes it handles.