Concept of Process

• Process


  A program in execution. The unit of work in most systems.
• Process


  • Needs resources like CPU time, memory, files and I/O devices to accomplish its task
  • Resources are typically allocated to the process while it is executing
• Modern operating systems


  • Support processes having multiple threads of control
  • On systems with multiple hardware processing cores, these threads can run in parallel
• Program counter


  Specifies the next instruction to execute
• Memory layout of a process

  • Text section: executable code
  • Data section: global variables
  • Heap section: dynamically allocated during program run time
  • Stack section: temporary data storage when invoking functions
• Program


  A passive entity, such as a file containing a list of instructions stored on disk
• Process


  An active entity, with a program counter specifying the next instruction to execute and a set of associated resources
• A program becomes a process when an executable file is loaded into memory
• Process states


  • New
  • Running
  • Waiting
  • Ready
  • Terminated
• Process Control Block (PCB)

  Represents a process in the operating system and serves as the repository for all the data needed to start, or restart, a process, along with some accounting

  
• Contents of the PCB

  • Process state
  • Program counter
  • CPU registers
  • CPU-scheduling information
  • Memory-management information
  • Accounting information
  • I/O status information

  
• Thread


  Allows a process to have multiple threads of execution and thus to perform more than one task at a time
• Multiprogramming


  Objective is to have some process running at all times so as to maximize CPU utilization
• Time sharing


  Objective is to switch CPU core among processes so frequently that users can interact with each program while it is running
• Process scheduler


  Selects an available process for program to execute on a core
• A system with a single CPU core can only run one process at a time whereas a multicore system can run multiple processes at one time
• Degree of multiprogramming


  The number of processes currently in memory
• Types of processes


  • I/O-bound process
  • CPU-bound process
• I/O-bound process


  Spends more of its time doing I/O than it spends doing computations
• CPU-bound process


  Generates I/O requests infrequently, using more of its time doing computations
• Scheduling queue


  1. When processes enter the system, they are put into a ready queue

  2. When a process is allocated a CPU core, it executes for a while and eventually terminates, is interrupted, or waits for the occurrence of a particular event, such as the completion of an I/O request

  3. Waiting processes are placed in the waiting queue

  
• CPU scheduler


  Responsible to select from among the processes that are in the ready queue and allocate a CPU core to one of them

  
• Swapping


  An intermediate form of scheduling where a process is removed from memory for active contention of the CPU and later reintroduced into memory, and its execution can be continued where it left off

  
• Context switch


  The task of Switching the CPU core to another process by performing a state save of the current process and a state restore of a different process

  
• Process creation


  1. During the course of execution, a process may create several new processes

  2. The creating process is called a parent process, and the new processes are called the children of that process

  3. A child process may be able to obtain its resources directly from the operating system, or it may be constrained to a subset of the resources of the parent process

  
• Process termination


  1. A process terminates when it finishes executing its final statement and asks the operating system to delete it by using the exit() system call

  2. A parent process may terminate the execution of one of its children

  3. When a process terminates, its resources are deallocated by the operating system

  4. A terminating child process must have its entry in the process table until the parent calls wait()

  5. A process that has terminated, but whose parent has not yet called wait(), is known as a zombie process

  6. A situation where a parent did not invoke wait() but instead terminate, leaves its child processes as orphans process
• The new process consists of a copy of the address space of the original process when a new process is created by the fork() system call
• After a fork() system call, one of the two processes typically uses the exec() system call to replace the process's memory space with a new program