CPU Scheduling in Operating Systems (Os) with Examples

What is CPU Scheduling?

• CPU scheduling is a crucial aspect of modern operating systems.
• CPU scheduling involves the management of processes in a system and determines which process gets access to the central processing unit (CPU) at any given time.

What are Threads in Os?

• Processes are independent program units that run concurrently in a computer system.
• Processes can contain multiple execution units called threads that share the same memory and resources but execute different parts of the code.
• CPU scheduling primarily deals with processes and their threads.

What is Context Switching?

• A context switch is the process of saving the state of one process or thread and loading the saved state of another.
• This is necessary when a running process is interrupted and must yield the CPU to another process.
• Context switching introduces overhead and must be minimized for efficient CPU scheduling.

let's understand with an example

• Imagine you're using a computer with a web browser and an email application open.
• Web Browsing: You're reading a webpage, and the CPU is dedicated to the web browser.
• Email Interruption: You receive a new email, interrupting the web browser's execution.
• Context Switch: The operating system saves the web browser's state and loads the email application.
• Email Response: You read and respond to the email, with the CPU now dedicated to the email application.
• Returning to Web Browsing: You go back to the web browser, triggering another context switch.
  Smooth Transition:
• The web browser's state is loaded, and you continue reading the webpage seamlessly.
• Context switching is the behind-the-scenes process that allows your computer to smoothly transition between different tasks,

Types of CPU Scheduling

• Preemptive
• Non Preemptive

Preemptive Scheduling

• In preemptive scheduling, the operating system has the capability to interrupt a currently running process and temporarily suspend its execution to start or resume another process.
• The decision to switch between processes can occur at any time, typically based on factors like priority or a predefined time quantum.
  

Key Characteristics:

• Allows for dynamic prioritization based on system conditions.
• Frequent context switches between processes.
  Responsive to changes in process states.
  

Non-Preemptive Scheduling

• In non-preemptive scheduling, a running process cannot be interrupted until it completes its execution or enters a waiting state voluntarily.
• The operating system makes scheduling decisions only when a process terminates or moves to the waiting state.
  
  

Key Characteristics:

• Simpler to implement and manage.
• Generally results in longer CPU bursts for running processes.
• Less frequent context switches compared to preemptive scheduling.
  

Preemptive vs Non Preemptive

Context Switching:

• Preemptive: Involves frequent context switches, as processes can be interrupted at any time.
• Non-Preemptive: Context switches occur only when a process voluntarily gives up the CPU or completes its execution.
  

Responsiveness

• Preemptive: Highly responsive to changes in system conditions or priorities.
• Non-Preemptive: Less responsive during the execution of a process, as decisions are made only at certain points.

Complexity

• Preemptive: More complex due to the need for real-time decision-making during process execution.
• Non-Preemptive: Simpler, as scheduling decisions are made less frequently.
  

Examples

• Preemptive: Round Robin with time slicing, Priority Scheduling with preemption.
• Non-Preemptive: FCFS (First-Come, First-Served), SJF (Shortest Job First), Priority Scheduling without preemption.

CPU Scheduling Criteria

• CPU Utilization
• Throughput
• Turnaround Time
• Waiting Time
• Response Time

CPU Utilization

• One of the primary objectives of CPU scheduling is to maximize CPU utilization.
• This means keeping the CPU busy as much as possible to ensure that it is not idle.
• Higher CPU utilization indicates efficient resource utilization.

Throughput

• Throughput refers to the number of processes completed in a unit of time.
• Efficient scheduling should aim to maximize throughput by completing more processes in less time.

Turnaround Time

• Turnaround time is the total time taken for a process to enter the system, execute, and finish.
• Lower turnaround times are desirable as they indicate faster process completion.

Waiting Time

• Waiting time is the cumulative amount of time a process spends waiting in the ready queue before being executed.
• Reducing waiting time enhances overall system performance.

Response Time

• Response time is the time it takes for a system to respond to an external event or a user request.
• Quick response times lead to a more interactive and responsive system.

What is the Need for CPU Scheduling Algorithm?

• In a computer system, multiple processes compete for the CPU's attention. A CPU scheduling algorithm is needed to:
• Maximize CPU Utilization: Without scheduling, a powerful CPU might be underutilized if processes are not efficiently managed.
• Optimize Throughput: By efficiently managing the order of process execution, the system can complete more tasks in less time.
• Minimize Waiting Time: Without scheduling, processes might wait longer in the queue, leading to delays. Scheduling minimizes waiting times.
• Quick Response Time: In interactive systems, scheduling ensures quick responses to user inputs, making the system more responsive.

CPU Scheduling Algorithms

First-Come, First-Served (FCFS)

• In the First-Come, First-Served (FCFS) scheduling algorithm, processes are executed in the order they arrive in the ready queue.
• It's like waiting in a queue, and the first process to enter the queue is the first to be served.
• However, FCFS doesn't consider the execution time of processes.

Example:

• Think of a queue at a ticket counter.
• The person who arrives first gets the ticket first, regardless of how long the ticketing process will take.
• This can lead to situations where a very long task is ahead of shorter ones, causing delays.

Shortest Job First (SJF)

• The Shortest Job Next (SJN) or Shortest Job First (SJF) scheduling algorithm prioritizes the process with the shortest execution time.
• This minimizes waiting times and is based on the assumption that shorter tasks should be completed first to improve overall efficiency.

Example:

• Imagine a fast-food restaurant.
• They prioritize preparing and serving smaller, quicker-to-cook orders (shortest jobs) ahead of larger, time-consuming orders (longest jobs) to reduce waiting times for customers.

Round Robin

• In Round Robin scheduling, processes are assigned a fixed time slice (quantum) in a circular manner.
• Each process gets a turn to execute for a fixed time, and then the CPU switches to the next process in the queue.
• This ensures that all processes get a fair share of the CPU.

Example:

• Think of a round-robin discussion in a group meeting.
• Each person gets a fixed time to speak, and when their turn is up, the discussion moves to the next person.
• This way, everyone has an equal opportunity to express their ideas.

Priority Scheduling

• Priority Scheduling assigns a priority to each process, and the scheduler selects the highest-priority process for execution.
• This allows important or time-sensitive tasks to be handled before lower-priority tasks.

Example:

• In a hospital emergency room, patients are prioritized based on the severity of their condition.
• Those with life-threatening issues (high priority) are treated before patients with less urgent needs (lower priority).

Multilevel Queue

• In the Multilevel Queue scheduling algorithm, processes are divided into multiple queues, each with different priorities.
• Each queue can have its own scheduling algorithm.
• This approach is used to manage processes with varying importance or characteristics effectively.

Example:

• Consider a customer support center with multiple teams.
• High-priority support requests (e.g., system outages) are handled by the "Urgent" team, while less critical issues (e.g., user questions) go to the "General" team.
• Each team may use different strategies to handle their workload.

Conclusion