Context Switching in CPU Scheduling

Context Switching

• Involves switching the CPU from one process to another, ensuring smooth execution of multiple processes.
• Happens when a high-priority process comes to a ready state, an interrupt occurs, user and kernel-mode switch, or preemptive CPU scheduling is used.

Threads in Operating Systems

• Definition: The smallest unit of a process that can be scheduled and executed by the CPU.
• Importance: Allows parallelism within a process, leading to more efficient and faster execution.
• Types of Threads:
  • User-Level Threads (ULTs): Managed by user-level libraries, with fast context switch and no OS modification required.
  • Kernel-Level Threads (KLTs): Managed by the operating system kernel, with the OS managing and scheduling threads independently.

Advantages of Using Threads

• Responsiveness: Allows a program to continue running even if part of it is blocked.
• Resource Sharing: Threads of the same process share memory and resources.
• Economy: Creating and managing threads is less costly than processes.
• Scalability: Threads can run on different processors in a multiprocessor system.

Thread Lifecycle

• New: Thread is created.
• Runnable: Thread is ready to run.
• Running: CPU is executing the thread.
• Blocked: Thread is waiting for resources.
• Terminated: Thread has finished execution.

Process Scheduling in Operating Systems

• Definition: The method by which the operating system decides which process runs at any given time.
• Importance: Ensures efficient execution of processes, optimal CPU utilization, and overall system performance.

Categories of Scheduling

• Non-preemptive: A process's resource cannot be taken before the process has finished running.
• Preemptive: The OS assigns resources to a process for a predetermined period, and the process switches from running state to ready state or from waiting state to ready state during resource allocation.

Types of Schedulers

• Long-term Scheduler (Job Scheduler): Determines which processes are to be admitted into the system and brings the new process to the 'Ready State'.
• Short-term Scheduler (CPU Scheduler): Selects which process should be executed next and brings process from the ready state for scheduling it on the running state.
• Medium-term Scheduler: Manages swapping of processes in and out of memory to balance load.

Scheduling Queues

• Job Queue: All processes in the system.
• Ready Queue: Processes that are ready to run.
• Device Queue: Processes waiting for devices (I/O).

Scheduling Algorithms Overview

• Two types of scheduling methods:
  • Preemptive Scheduling: Used when a process switches from running state to ready state or from waiting state to ready state.
  • Non-Preemptive Scheduling: Used when a process terminates, or when a process switches from running state to waiting state.

CPU Scheduling Algorithms

• First-Come, First-Served (FCFS): Processes are executed in the order they arrive.
• Shortest Job Next (SJN): Processes are executed in the order of their burst time.
• Priority Scheduling: Processes are assigned priority numbers, and the highest priority process is selected next.
• Round Robin (RR): Each process is allocated a fixed time slot (time quantum), and if a process does not finish execution within its time quantum, it is preempted and placed at the end of the ready queue.

Key Concepts in CPU Scheduling

• Arrival Time: The moment in time when a process enters the ready queue and is awaiting execution by the CPU.
• Burst Time: The amount of CPU time the process requires to complete its execution.
• Completion Time: The summation of the arrival, waiting, and burst times.
• Turnaround Time: The time elapsed between the arrival of a process and its completion.
• Waiting Time: A process's duration in the ready queue before it begins executing.
• Response Time: The duration between the arrival of a process and the first time it runs.

This quiz covers the concept of context switching, which involves switching the CPU from one process to another, ensuring smooth execution of multiple processes.