Process Management: Operating System

Guide to Process Management

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Introduction to Process Management

Process management is one of the critical functions of an operating system. A process is a program in execution. It is the active entity that utilizes the CPU, memory, I/O devices, and other resources to perform tasks. The operating system is responsible for managing all processes running on a computer, ensuring efficient execution, coordination, and proper resource utilization.

Processes can be created, executed, and terminated by the operating system, making it vital to have robust mechanisms for tracking and managing them.

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Understanding Processes

What is a Process?

A process is a dynamic execution instance of a program. While a program is a static set of instructions, a process is the active, running version that requires resources to execute.

Components of a Process:

1. Code Section (Text): The executable code of the program.
2. Program Counter: Keeps track of the instruction currently being executed.
3. Stack: Holds temporary data like function parameters and local variables.
4. Data Section: Stores global and static variables.
5. Heap: Memory for dynamically allocated data.

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Process States

A process transitions through several states during its lifecycle. These states are:

1. New: The process is being created.
2. Ready: The process is waiting to be assigned to the CPU.
3. Running: The process is actively executing instructions.
4. Waiting (Blocked): The process is waiting for an event, such as I/O completion.
5. Terminated: The process has finished execution.

State Transition Diagram:

• New → Ready: Process creation completes.
• Ready → Running: Scheduler assigns CPU to the process.
• Running → Waiting: Process requests I/O or a resource.
• Waiting → Ready: Requested resource or event becomes available.
• Running → Terminated: Process execution finishes or is aborted.

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Process Control Block (PCB)

The Process Control Block (PCB) is a data structure used by the operating system to store all relevant information about a process. It is crucial for context switching and process management.

Information in PCB:

1. Process State: Current state of the process (e.g., Ready, Running).
2. Process ID (PID): Unique identifier for the process.
3. Program Counter: Address of the next instruction.
4. CPU Registers: Current values of registers.
5. Memory Management Information: Details of allocated memory (e.g., base and limit registers).
6. I/O Status: List of I/O devices allocated to the process.
7. Accounting Information: CPU usage, time limits, and priority.

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Operations on Processes

1. Process Creation

• Parent Process: Creates child processes.
• Example: fork() in UNIX creates a new process.
• Child inherits some resources from the parent (e.g., open files, environment variables).

2. Process Termination

• A process is terminated using system calls like exit().
• Reasons for Termination:
  • Normal completion.
  • Errors (e.g., division by zero).
  • User intervention (e.g., kill command).

3. Process Suspension and Resumption

• Suspension: A process is temporarily paused (moved to disk from memory).
• Resumption: A suspended process is brought back to active execution.

4. Context Switching

• When the CPU switches from one process to another, the OS saves the current process state in its PCB and loads the state of the next process.
• Overhead: Context switching is computationally expensive but essential for multitasking.

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Cooperating vs. Independent Processes

Cooperating Processes:

• Processes that share data and resources with other processes.
• Examples:
  • Producer-consumer model.
  • Inter-process communication (IPC) using shared memory or message passing.
• Advantages:
  • Task modularity and efficiency.
  • Enhanced resource sharing.

Independent Processes:

• Processes that do not share data or resources with others.
• Characteristics:
  • No dependency on other processes.
  • Easier to manage and debug.

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Process Management in UNIX

UNIX provides robust mechanisms for process management:

1. Process Creation

• The fork() system call creates a new process by duplicating the parent process.
• Example:

  #include <stdio.h>
  #include <unistd.h>

  int main() {
      pid_t pid = fork();

      if (pid == 0) {
          printf("Child process with PID: %d\n", getpid());
      } else {
          printf("Parent process with PID: %d\n", getpid());
      }
      return 0;
  }

2. Process Termination

• The exit() system call terminates a process.
• Parent processes can use wait() to collect the exit status of child processes.

3. Process Monitoring

• Commands like ps, top, and htop provide information about running processes.
• Example:

  ps -aux

4. Signals

• UNIX processes can communicate using signals.
• Example:
  • SIGKILL to terminate a process.
  • SIGSTOP to pause a process.

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Process Termination: Zombie and Orphan Processes

When a process completes its execution, it enters a termination phase, during which resources allocated to the process are released. However, improper handling during this phase can result in zombie or orphan processes.

Zombie Process

• A zombie process is a terminated process that still has an entry in the process table because its parent has not yet retrieved its termination status using the wait() system call.
• Key Characteristics:
  • Does not consume system resources (e.g., memory or CPU).
  • Occupies a process table slot.
• Resolution:
  • The parent process must call wait() to clean up the zombie entry.
  • If the parent does not handle it, the init process (PID 1) reclaims it.

Orphan Process

• An orphan process is a child process whose parent has terminated before it.
• Key Characteristics:
  • The orphan process is automatically adopted by the init process.
  • Continues to run independently until it completes or is terminated.
• Resolution:
  • The operating system ensures that orphans are correctly managed by reassigning their parent to init.

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Threads

A thread is the smallest unit of execution within a process. Threads enable parallel execution of tasks within the same process.

Difference Between Processes and Threads:

Aspect	Process	Thread
Definition	Independent executing program.	Lightweight unit within a process.
Memory	Separate memory space.	Shared memory space.
Overhead	High (due to context switching).	Low (less context switching).
Communication	Inter-process communication.	Shared memory within the process.

Benefits of Threads:

1. Concurrency: Multiple threads can execute simultaneously.
2. Resource Sharing: Threads within a process share resources like memory and files.
3. Reduced Overhead: Threads are faster to create and manage than processes.

Thread Models:

1. User-Level Threads:
   • Managed by the user, not visible to the kernel.
   • Example: Java threads.
2. Kernel-Level Threads:
   • Managed directly by the OS.
   • Example: POSIX threads (pthreads).

Thread Creation Example in C (POSIX):

#include <pthread.h>
#include <stdio.h>

void* thread_function(void* arg) {
    printf("Thread is running.\n");
    return NULL;
}

int main() {
    pthread_t thread_id;
    pthread_create(&thread_id, NULL, thread_function, NULL);
    pthread_join(thread_id, NULL);
    return 0;
}

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This comprehensive guide covers all aspects of process management. Let me know if you'd like a deeper dive into a specific section, or if we should extend this to scheduling and synchronization concepts!

Guide to Process Scheduling

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Introduction to Process Scheduling

Process scheduling is a fundamental aspect of an operating system, determining how processes access the CPU and other resources. Efficient scheduling is critical for maximizing CPU utilization, ensuring fairness, and maintaining system responsiveness.

The Process Scheduler is responsible for deciding the order in which processes are executed and managing transitions between different states.

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Goals of Process Scheduling

1. Maximize CPU Utilization: Keep the CPU as busy as possible.
2. Fairness: Ensure all processes get a fair share of CPU time.
3. Throughput: Maximize the number of processes completed in a given time.
4. Turnaround Time: Minimize the total time taken for process execution (submission to completion).
5. Response Time: Reduce the time taken to start responding to a user process.
6. Waiting Time: Minimize the time a process spends waiting in the ready queue.

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Process Scheduling Queues

An operating system maintains several queues for efficient process scheduling:

1. Job Queue:

   • Contains all processes in the system.
   • Newly created processes are added here.

2. Ready Queue:

   • Holds processes that are ready to execute but are waiting for CPU allocation.
   • Typically managed as a linked list or a priority queue.

3. Device (I/O) Queue:

   • Contains processes waiting for specific I/O devices to become available.

4. Suspended Queue:

   • Holds processes that have been swapped out of memory and await resumption.

Queue Transitions:

Processes move between these queues as they transition between states:

• Job Queue → Ready Queue: When a process is admitted into memory.
• Ready Queue → Running State: When the CPU is allocated to a process.
• Running → I/O Queue: When a process performs an I/O operation.
• Running → Ready Queue: When preempted by a higher-priority process or when its time quantum expires.

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Process Scheduler

The process scheduler determines which process gets to execute and for how long. It operates in different levels:

Types of Schedulers

1. Long-Term Scheduler (Job Scheduler):

   • Determines which processes are admitted into the system for processing.
   • Controls the Degree of Multiprogramming:
     • The degree of multiprogramming refers to the number of processes in memory.
     • The long-term scheduler regulates this to prevent overloading memory and maintain system stability.
   • Frequency: Runs less frequently than other schedulers.
   • Goal: Maintain a balanced mix of I/O-bound and CPU-bound processes.
   • Example: Adding jobs from the job queue to the ready queue.

2. Short-Term Scheduler (CPU Scheduler):

   • Allocates the CPU to a process from the ready queue.
   • Frequency: Runs frequently (after every context switch or quantum expiration).
   • Goal: Maximize CPU utilization and minimize response time.

3. Medium-Term Scheduler:

   • Swaps processes in and out of memory to manage the degree of multiprogramming.
   • Goal: Ensure memory is efficiently utilized and prevent overloading.

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Scheduling Criteria

Key metrics used to evaluate scheduling algorithms include:

1. CPU Utilization: Percentage of time the CPU is actively executing processes.
2. Throughput: Number of processes completed per unit time.
3. Turnaround Time: Total time taken for a process (from submission to completion).
4. Waiting Time: Time spent by a process in the ready queue.
5. Response Time: Time from request submission to the first response.

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Types of Scheduling Algorithms

1. Non-Preemptive Scheduling:

Once a process is given the CPU, it cannot be preempted until it completes execution or moves to the waiting state.

• First-Come, First-Served (FCFS):

  • Processes are executed in the order they arrive.
  • Advantages: Simple and easy to implement.
  • Disadvantages: Poor performance for long processes (convoy effect).

• Shortest Job Next (SJN):

  • Executes processes with the shortest burst time first.
  • Advantages: Minimizes average waiting time.
  • Disadvantages: Requires knowledge of burst times (not always feasible).

2. Preemptive Scheduling:

Processes can be interrupted and moved back to the ready queue to allow another process to execute.

• Round Robin (RR):

  • Each process is given a fixed time slice (quantum).
  • Advantages: Ensures fairness.
  • Disadvantages: Poor performance for processes requiring frequent I/O.

• Priority Scheduling:

  • Processes are executed based on priority.
  • Advantages: High-priority tasks are executed first.
  • Disadvantages: Risk of starvation for low-priority processes.

• Shortest Remaining Time (SRT):

  • Preemptive version of SJN; executes the process with the shortest remaining burst time.
  • Advantages: Optimizes waiting time.
  • Disadvantages: Overhead due to frequent context switching.

3. Multilevel Queue Scheduling:

• Divides processes into multiple queues based on characteristics (e.g., foreground, background).
• Advantages: Specialization for different process types.
• Disadvantages: Rigid queue boundaries.

4. Multilevel Feedback Queue Scheduling:

• Similar to multilevel queues but allows processes to move between queues based on their behavior.
• Advantages: Combines benefits of various algorithms.
• Disadvantages: Complex to implement.

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Context Switching

Definition:

Context switching refers to the process of saving the state of a currently running process and restoring the state of the next scheduled process. This enables multitasking and process scheduling.

Steps in Context Switching:

1. Save the current process's state (e.g., program counter, registers) in its Process Control Block (PCB).
2. Select the next process to execute from the ready queue.
3. Load the selected process's saved state from its PCB.
4. Update the CPU to resume execution of the selected process.

Overhead:

Context switching incurs overhead as no productive work is done during the switch.

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This guide provides a complete view of process scheduling, its types, and its implementation in modern systems. Let me know if you’d like an in-depth discussion of specific algorithms or further elaboration on related topics like deadlocks or real-time scheduling!

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Interprocess Communication (IPC)

Interprocess Communication (IPC) is a mechanism that allows processes to exchange data and coordinate their activities. It is essential in multitasking environments to enable collaboration between processes, which may run concurrently or in a distributed system.

Goals of IPC

1. Data Sharing: Exchange information between processes.
2. Resource Sharing: Coordinate access to shared resources to avoid conflicts.
3. Synchronization: Ensure processes execute in the correct sequence to avoid race conditions.
4. Modularity: Facilitate structured and modular application design.

Types of IPC Mechanisms

1. Shared Memory

• Description: Processes share a segment of memory for communication.
• How It Works:
  • One process writes data to the shared memory region, and the other reads it.
  • Requires synchronization (e.g., using semaphores) to prevent race conditions.
• Advantages:
  • Fast communication, as no data copying between processes is needed.
• Disadvantages:
  • Requires explicit synchronization.
• Example: POSIX shared memory in UNIX systems.

2. Message Passing

• Description: Processes communicate by sending and receiving messages.
• How It Works:
  • Data is encapsulated in messages and exchanged via system calls like send() and receive().
  • Can be implemented using message queues or sockets.
• Advantages:
  • Simplifies synchronization.
  • Suitable for distributed systems.
• Disadvantages:
  • Slower compared to shared memory due to message copying.
• Example: POSIX message queues, sockets.

IPC Mechanisms in UNIX

1. Pipes:

   • Named Pipes (FIFOs): Allow communication between unrelated processes.
   • Anonymous Pipes: Used for communication between parent and child processes.
   • Unidirectional by default.

2. Message Queues:

   • Provide a structured way to exchange messages with priorities.
   • Persist until explicitly deleted.

3. Shared Memory:

   • Allows multiple processes to access the same memory region.
   • Fastest IPC mechanism.

4. Semaphores:

   • Used for process synchronization.
   • Prevents concurrent access to critical sections.

5. Sockets:

   • Enable communication between processes on different systems over a network.
   • Support TCP/IP and UDP protocols.

Synchronization in IPC

IPC mechanisms often require synchronization to ensure data consistency and avoid issues like race conditions or deadlocks. Common synchronization tools include:

1. Mutexes: Lock mechanisms ensuring mutual exclusion.
2. Semaphores: Counters that manage access to shared resources.
3. Monitors: Higher-level synchronization constructs.

Use Cases of IPC

1. Multithreaded Applications: Coordination between threads.
2. Client-Server Communication: Data exchange in distributed systems.
3. Parallel Processing: Sharing intermediate results among processes.
4. Device Communication: Interaction with hardware drivers.

IPC ensures efficient communication and synchronization between processes, enabling robust, scalable, and modular application design.

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