Process Scheduling and Execution

Process Scheduling

• Definition of Process Scheduling

  • The method by which an operating system decides the order in which processes are executed by the CPU.

• CPU Execution Requirements

  • For a CPU to start executing a process:

    • The process must be in a ready state.

    • Multiple processes may be ready, causing a need for a selection mechanism.

The Ready Queue

• Description

  • A ready queue is a data structure that holds all processes that are ready to run but are waiting for CPU allocation.

  • Visual representation:

    • A diagram shows multiple entries in the ready queue and one process currently using the CPU.

CPU Scheduler

• Function of CPU Scheduler

  • The CPU scheduler is a critical component of the operating system. Its roles include:

    • Managing how processes utilize CPU resources.

    • Deciding which of the ready processes is dispatched to the CPU for execution.

    • Determining the time quantum or how long a process is allowed to run before being preempted.

Preemption

• Definition of Preemption

  • An operation in which the operating system interrupts an executing process to save its current state (context).

• Preemption Mechanism

  • Required for effective CPU management, allowing the OS to manage multiple processes efficiently and fairly.

Scheduling Algorithm

• Description

  • After a process is preempted, the CPU scheduler runs a scheduling algorithm to select the next process from the ready queue.

Dispatching Processes

• Process Dispatching

  • Once a process is selected by the scheduler, the OS must:

    • Dispatch or transfer control of the CPU to the selected process.

    • Switch into the context of the selected process to begin execution.

Efficiency of the Operating System

• Importance of Efficiency

  • CPU resources are limited and must be utilized effectively:

    • Minimize the time spent executing scheduling algorithms and other OS operations.

    • Focus on running processes rather than overhead related to management tasks.

• Design and Implementation

  • Emphasizes the need for:

    • Efficient designs of scheduling algorithms.

    • Efficient implementations to reduce the overhead during scheduling processes.

    • Effective data structures for representing the state of waiting processes.

Information for Scheduling Decisions

• Key Information Used in Scheduling

  • Priority of Processes

    • Processes may have different priority levels that affect scheduling decisions.

  • Historical Data

    • Records about how long each process has run in the past can influence future scheduling choices.

  • Other Relevant Information

    • Additional metrics or data deemed useful for making informed scheduling decisions may also be utilized.