Intro to Processes

Notes

• This is chapter 3 of the book.
• Jobs, processes, threads
  • Before multitasking a program to run was called a job
  • Eventually we renamed them to be processes
  • Finally we have threads, or lightweight processes
  • We will look more at threads in the future
  • These terms are not equivalent, but ...
• A process has a state
  • Roughly these are
    • New, starting, being created
    • Running: currently in the CPU executing]
    • Ready: has everything it needs, but is not in the CPU
    • Waiting: is blocked while a request is being processed (I/O)
    • Exiting:
  • In a linux you can see the processes with
    • top
      • The state column: R- running, S-Sleeping, D-Deep Sleep, - T stopped, Z -ZOMBIE!, I -idle, R
    • ps -ef
    • ps -eo pid,user,stat,comm
      • The Stat field

        First Letter	Meaning
        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

        Second Letter	Meaning
        <	high-priority (not nice to other users)
        N	low-priority (nice to other users)
        L	has pages locked into memory (for real-time and custom IO)
        s	is a session leader
        l	is multi-threaded (using CLONE_THREAD, like NPTL pthreads do)
        +	is in the foreground process group