Process

Definition:

• A process is the fundamental unit of work in an operating system.
  • Process management includes creating and deleting processes and providing mechanisms for processes to communicate and synchronize with each other.
• has pid (process identifier)
• Executable program becomes process when it is loaded into memory
• Types of processes:
  • System process
  • User process
  • Foreground process
  • Background process
• Independent vs cooperating process:
  • Independent process: not affected by and doesnt affect others (no shared data)
  • Cooperating process: affected by other (shared data)
    • Needs Inter-Process Communication
    • share: code, data, files
    • not shared: registers or stacks

Process ID

Process signal:

• .
• Three ways of ending processes are
  • SIGTERM (-15): graceful exit
  • SIGKILL (-9): kill
  • SIGHUP: a process exit when the terminal running the process closes. It’s assumed that if the terminal closes, the user is no longer interested in the results of the command
    • HANGup
    • tells the app to exit when shell closes

Process State

• 
• Ready: waiting to be assigned to CPU
• Running: in CPU
• List:
  • D uninterruptible sleep (usually IO)
  • I Idle kernel thread
  • R running or runnable (on run queue)
  • S interruptible sleep (waiting for an event to complete)
  • T stopped by job control signal
  • t stopped by debugger during the tracing
  • W paging (not valid since the 2.6.xx kernel)
  • X dead (should never be seen)
  • Z defunct (“zombie”) process, terminated but not reaped by its parent

Process Control Block (PCB)

• Found in memory
• PCB manages information associated with each process:
  • Process state: running, waiting, etc.
  • Program counter: location of instruction to next execute
  • CPU registers: contents of all process-centric registers
  • CPU scheduling: priorities, scheduling queue pointers
  • Memory-management information – memory allocated
  • Accounting information: CPU used, time elapsed since start
  • I/O information: I/O devices allocated, list of open files
  • …

Process scheduling:

• 
• 
• Ready queue: processes residing in main memory waiting to execute
• Wait queue: processes waiting for an event (ie I/O)
• A context switch occurs when the CPU switches from one process to another.
  • Pure overhead
  • Hardware-dependent

Process creation:

• OSs provide a create-process function to create a new process
  • Parent process is the one called the create-process function (folk())
  • Child process is created after the create-process function is called
• Execution options:
  • Parent and children execute concurrently
  • Parent waits until children terminate
• Parent process links to many child process below like a tree
• If parent process is terminated, all children are also killed

Process termination:

• If it executes last statement and call exit() to notify OS to delete it
• Its parent calls abort() or kill()
  1. Exceed allocated resources
  2. Task assigned is no longer required
  3. The parent is terminated