OS: Process Management

Program

A set of instructions written in a programming language that can be executed by a computer to perform a specific task.

Process

An instance of a program running in a computer, including its code, data, and system resources.

Thread

It is the smallest unit of execution within a process that allows for concurrent execution of multiple

tasks.

States of a Process: New

The process is created but not yet started the execution.

States of a Process: Ready

The process is prepared to run but is waiting for CPU allocation.

States of a Process: Running

The process is actively being executed by the CPU

States of a Process: Waiting

The process is paused, waiting for an external event to proceed.

States of a Process: Terminated

The process has completed execution and exit the system.

Process Control Block

• It is a data structure maintained by the operating system that contains all the information required to manage a specific process. It simply defines as “IDENTITY CARD” of the process.

• This information is used by the operating system to track the execution status of each process.

PCB: Process ID

A unique identifier for the process.

PCB: Process State

The current state of the process

PCB: Process Counter

A pointer to the next instruction to be executed by the process.

PCB: CPU Registers

The values of the CPU registers at the time of the process's context switch.

PCB: Memory Limits

Information about the process’s memory allocation.

PCB: Open Files

A list of file descriptors or handles the process is using.

Process Scheduling

It refers to the mechanism that determines which process gets to use the CPU at any given time.

It aims to optimize resource utilization and minimize waiting times.

Process Scheduler

It is a component within an operating system that determines which process should be running on the CPU at a given time. This particular component decides which available process should be prioritized by the CPU.

Scheduling Queues: Job Queue

The queue contains all the processes in the system that are waiting to be processed.

Scheduling Queues: Ready Queue

The queue keeps a set of all processes residing in main memory, ready and waiting to execute.

Scheduling Queues: Ready Queue

The processes which are blocked due to the unavailability of an I/O device are added to this queue.

Types of Process Schedulers: Long-Term Job Scheduler

It determines which programs are admitted to the system for processing. It selects processes from the queue and loads them into memory for execution.

Types of Process Schedulers: Short-Term Scheduler (CPU Scheduler)

This scheduler selects a process among the processes that are ready to execute and allocates CPU to one of them.

Types of Process Schedulers: Medium-Term Scheduler (Process Swapping Scheduler)

This scheduler is in-charge of handling the swapped out-processes.

Categories of Scheduling: Non-Preemptive

In this type of scheduling, a running process cannot be interrupted until it has completed it’s execution or voluntarily moves to the waiting state.

Categories of Scheduling: Pre-emptive

In this type of scheduling, current processes are allowed to be interrupted and moved back to the ready state so that another process with high-priority can run.

Preemptive Scheduling Algorithms: Round Robin

Each process is assigned a fixed time slice or quantum. When a process uses up its time slice, it is preempted and placed back into the ready queue.

Preemptive Scheduling Algorithms: Shortest Remaining Time First

The process with the shortest remaining execution time is given the CPU next.

Preemptive Scheduling Algorithms: Priority Scheduling

allows processes to be executed based on their priority, and if a higher-priority process arrives while another is running, the lower-priority process can be interrupted and resumed later.

Non-preemptive Scheduling Algorithms: First Come First Serve

Processes are executed in the order they arrive in the ready queue. No preemption occurs, so the first process that arrives gets executed to completion before the next process starts.

Non-preemptive Scheduling Algorithms: Shortest Job First

The process with the shortest burst time (execution time) is selected for execution first.

Non-preemptive Scheduling Algorithms: Priority Scheduling

Each process is assigned a priority, and the process with the highest priority (lowest numerical value) gets executed first.