Lec 2 Process Management

Process Concept

An operating system manages and executes programs, each running as a separate process. Early systems only allowed one program to run at a time, while modern systems support concurrent execution of multiple programs in memory

What is a Process?

Process: A program in execution, managed by the OS; process execution must progress in sequential fashion. NO parallel execution of instructions of a single process

Each process consists of multiple parts:

• Program code (text section)

• Current activity (program counter, processor registers)

• Stack (temporary data like function parameters, return addresses, and local variables)

• Data section (global variables)

• Heap (memory allocated dynamically during runtime).

• Program is passive entity stored on a disk (executable file)

• Process is active (Program becomes process when an executable file is loaded into memory)

• One programme can be several processes (consider multiple users executing the same program)

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Process in Memory

• Stack : Local variables and function parameters are stored in stack
• Heap : The Dynamic Memory Area, where memory is allocated on runtime
• Data : Static and global variables
• Text : The Executable Code of the program

Memory Layout of a C Porgram

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Process VS Program

Program	Process
A program is a set of instructions	A Process is a Program in Execution
A program is passive/static entity	Process is Active/dynamic Entity
Longer life span, stored in disk forever	Process has a limited life span. It is created when execution starts and terminated when execution finished
Require space on disk to store all instructions	A process contains various resources like : memory address ,disk ,printer as per requirement

Process State

1. New: The process is being created.
2. Ready: The process is ready and waiting to be assigned to the CPU.
3. Running: Instructions are being executed.
4. Waiting (Blocked): Process is waiting for some event to occur (e.g., I/O completion).
5. Terminated: The process has completed execution.

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Process Control Block (PCB) / Task Control Block

• Each process in an operating system is represented by a PCB
• The PCB is a data structure that stores all necessary information about a process. It includes:
  • Process Identifier : (PID)
  • Process State: Current status of the process.
  • Program Counter: Next instruction address.
  • CPU Registers: CPU-specific information.
  • Memory Limits: Memory allocated to the process.
  • Accounting Information: CPU usage, time elapsed, etc.
  • I/O Status: Resources like files, I/O devices.

Process Scheduling

The OS uses a scheduler to determine which process to run next based on scheduling policies (e.g., First-Come-First-Served, Round Robin).

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Scheduler Functions

1. Context Switching: Saves the current process state and restores the next process’s state.
2. Process Selection: Chooses the next process based on a scheduling policy.
3. Execution Continuation: Resumes the selected process’s execution.

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Types of Processes

Processes can be categorized as either independent or cooperating:

• Independent processes do not interact with other processes.
• Cooperating processes can work together, affecting or being affected by other processes.

The operating system allows cooperating processes to:

• Enable information sharing
• Allow access to resources
• Increase computation speed

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What are threads

• Threads are smaller units of a process that can run independently.
• Think of process as a big task, and a thread is smaller tasks within that big task

Why use Threads?

1. Efficient Resource Sharing: Threads within the same process share memory and resources.
2. Parallel Execution: Threads can run on different CPU cores.
3. Responsive Programs: If one thread is blocked, others can still execute.

Example: In a word processor:

• Thread 1 handles user input.
• Thread 2 checks spelling.
• Thread 3 saves work periodically.

Why not Multiple Processes?

• Memory Consumption
  • Running multiple processes (copying of the same program) uses a lot of memory because each process has its own memory space.
• Swapping overhead
  • Process often swap between main memory and secondary storage, which can slow down the execution
  • the swapping is called “overhead” and its inefficient for heavy processes.

Threads vs processes?

Aspect	Threads	Processes
Weight	Lightweight	Heavyweight
Memory Usage	Share the same memory space of a process	Have separate memory space
Context Switching	More efficient and faster context switching / Avoid Overhead	Swapping makes a process slow and inefficient

Summary about Threading

1. Threads are like smaller tasks within a big task (process).
2. They help make programs more efficient and responsive.
3. Threads use less memory and are faster than running multiple processes.
4. Swapping threads is quicker than swapping processes.
5. Multithreading allows multiple threads to share resources and run in parallel.

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