os

• Process


  A program in execution or the unit of work in a modern time-sharing system
• Process


  • Needs resources such as CPU time, memory, files and I/O devices to accomplish its task
  • Resources are allocated to the process either when it is created or while it is executing
• Process structure


  Text section (program code), Data section (global variables), Heap (dynamically allocated memory), Stack (temporary data)

  
• A program becomes a process when an executable file is loaded into memory
• Two processes may be associated with the same program but are considered separate execution sequences
• Process states


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


  Repository for information that may vary from process to process
• Information in PCB


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


  A single thread of execution within a process
• In a single processor system, a process has a single thread of execution
• In a multicore/multiprocessor system, a process can have multiple threads of execution running in parallel
• Process Scheduling


  Selecting an available process for program execution on the CPU

  
• Scheduling Queues


  • Job Queue
  • Ready Queue
  • Device Queue
• Long Term Scheduler


  Selects processes from the job queue and loads them into main memory for execution
• Short Term Scheduler


  Selects from among the processes that are ready to execute and allocates the CPU to one of them
• Medium Term Scheduler


  Performs swapping (removing a process from main memory and storing it into secondary storage) and moves processes between CPU, I/O waiting queue and ready queue

  
• Process types


  • I/O bound (spends more time doing I/O)
  • CPU bound (spends more time doing computations)
• Context Switch


  Switching the CPU from one process to another by saving the context of the old process and loading the context of the new process
• Process Creation


  1. Parent process creates child process

  2. Child process inherits resources from parent

  3. Parent can wait for child to terminate or continue concurrently
• Process Identifier (PID)


  Unique integer number identifying each process
• Linux process tree


  • init (pid 1)
  • kthreadd
  • sshd
• Process System Calls in Unix/Linux


  1. fork()

  2. exec()

  3. wait()

  4. exit()
• The fork() system call creates a new child process as a copy of the parent process
• The exec() system call replaces the process's memory space with a new program
• The wait() system call allows the parent to wait for the child process to terminate
• C program showing UNIX system calls fork, exec, wait


  1. Fork child process

  2. If fork fails, print error and return 1

  3. If child process, execute /bin/ls

  4. If parent process, wait for child to complete

  5. Print "Child Complete"

  6. Return 0
• Two different processes are running copies of the same program
• Child process


  Value of pid is zero
• Parent process


  Value of pid is an integer value greater than zero (the actual pid of the child process)
• Child process inherits privileges, scheduling attributes, and certain resources such as open files from the parent
• Child process


  Overlays its address space with the UNIX command /bin/ls using the execlp() system call
• Parent process


  Waits for the child process to complete with the wait() system call
• When the child process completes, the parent process resumes from the call to wait() and completes using the exit() system call
• Process termination


  A process terminates when it finishes executing its final statement and asks the operating system to delete it by using the exit() system call
• The process may return a status value to its parent process via the wait() system call
• All the resources of the process including physical and virtual memory, open files and I/O buffers are deallocated by the operating system
• Reasons a parent may terminate the execution of one of its children


  • The child has exceeded its usage of some of the resources that it has been allocated
  • The task assigned to the child is no longer required
  • The parent is exiting and the operating system does not allow a child to continue if its parent terminates
• Cascading Termination


  If a parent process terminates either normally or abnormally then all its children must also be terminated
• Terminating a process in Linux/UNIX


  Use the exit() system call providing an exit status as a parameter
• Under normal termination, exit() may be called either directly (i.e. exit(1)) or indirectly (i.e. by a return statement in main())