Process Management

What is a Process?

• A process is defined as the execution of a program that performs the actions specified in the program.
• It represents a program in execution, consisting of the program code, current activity, and allocated resources.

Components of a Process in Memory

• Text Section: Contains the compiled program code, brought in from non-volatile storage during execution.
• Data Section: Stores global and static variables, initialized before the execution of the main function.
• Heap: Used for dynamic memory allocation (managed via new, delete, malloc, free, etc.).
• Stack: Used for local variables.
• Memory Layout: The stack and heap start at opposite ends of the process's memory space and grow towards each other to avoid conflicts (stack overflow or allocation failures).

What is Process Management?

• Process management involves various tasks including:
  • Creation of processes
  • Scheduling of processes
  • Termination of processes
  • Handling deadlocks
• The operating system is responsible for resource allocation, allowing processes to share and exchange information efficiently.

Process States

• A process state describes the current condition of a process at a specific time.
• Five States of a Process:
  • New: The process is being created.
  • Ready: The process is loaded in memory, waiting to be executed.
  • Running: The process is being executed.
  • Waiting: The process is waiting for resources or events (e.g., I/O operations, inter-process messages).
  • Terminated/Halted: The process has completed execution.

Context Switching

• Context switching involves saving the state of a currently running process and loading the state of another process.
• It occurs when:
  • A high-priority process enters the ready state.
  • An interrupt is triggered.
  • A user/kernel mode switch occurs (not mandatory).
  • Preemptive CPU scheduling is implemented.

Context Switch vs Mode Switch

• Context Switch:

  • Saves and restores the state of a CPU so multiple processes can share a single CPU resource.
  • Managed by the OS scheduler based on scheduling policies.

• Mode Switch:

  • Refers to changing the CPU's privilege level (e.g., system call).
  • Occurs with system calls or faults.

Process Creation

• Events leading to process creation include:
  • User request
  • System initialization
  • Batch job initialization
  • Execution of process creation system calls (e.g., fork(), spawn()).
• Parent and Child Processes:
  • A process that creates another is the parent; the created process is the child.
  • fork() duplicates the current process in Unix-based systems; spawn() creates new processes at a higher level of abstraction.

Steps for Process Creation

1. Initialization: Prepare for process creation.
2. Allocation of Process ID (PID): A unique identifier for the new process.
3. Memory Allocation: Allocate memory for code, data, heap, and stack.
4. Creation of Process Control Block (PCB): Initialize the data structure managing the process.
5. Scheduling: Schedule the process for execution based on OS policy.
6. Execution: The process begins executing its instructions.

Process Termination

• Occurs when the exit system call is invoked.

• Resources such as memory and I/O devices are deallocated upon termination.

• Causes of Termination:

  • The parent process terminating leads to child termination.
  • Memory scarcity (more memory needed than allocated).
  • Unauthorized resource access.
  • Resource allocation failures within the time limit.

Process Threads

• A thread is a segment of a process. Multiple threads can exist within a single process.
• Threads are like workers in a factory: each can perform tasks simultaneously under the supervision of the process.
• Multithreading: The ability for a program to perform multiple threads concurrently, optimizing resource usage.

OS as a Scheduler

• Types of schedulers:
  • Long-term Scheduler (Job Scheduler): Selects processes from the queue for memory allocation.
  • Short-term Scheduler (CPU Scheduler): Manages processes that are ready to execute and allocates CPU time.
  • Medium-term Scheduler (Swapper): Manages swapping of processes between main memory and secondary memory.

Process Scheduling

• Process scheduling involves selecting which process to run and managing the removal of running processes from the CPU.
• Goals include:
  • Keeping the CPU occupied.
  • Minimizing response time.

Scheduling Categories

• Non-preemptive Scheduling: Once a process starts execution, it runs to completion.
• Preemptive Scheduling: Allows a running process to be interrupted and moved back to the ready state.

Process Times

• Arrival Time (AT): When the process enters the ready queue.
• Completion Time: When a process finishes execution.
• Burst Time (BT): Time required for CPU execution.
• Turn Around Time (TT or TaT): TT = Completion Time - Arrival Time.
• Waiting Time (WT): WT = Turn Around Time - Burst Time.

Process Scheduling Algorithms

• Non-Preemptive Algorithms:

  • First Come First Serve (FCFS)
  • Shortest Job First (SJF)
  • Non-Preemptive Priority (NPP)

• Preemptive Algorithms:

  • Round Robin (RR)
  • Shortest Remaining Time First (SRTF)
  • Preemptive Priority (PP)