Processes - Kent State University

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					Lecture 4: Process Management

            process
            process creation/termination
            context
            process control block (PCB)
            context switch
            5-state process model
            process scheduling
              • short/medium/long term
            Unix process model




                                            1
                     Process


   process (also called task, or job) is a program in
    execution
   process
     • program code (or text) sequence of instructions to
        be executed
          note: multiple processes may be running the
            same code (editor, web-browser, etc)
     • context (execution state)



    prompt% ps -efl
    UID PID PPID C STIME TTY              TIME COMMAND
    root     0    0   0 Jan 19 ?          10:53 swapper
    root     1    0   0 Jan 19 ?            0:17 init
    root     2    0   0 Jan 19 ?            7:39 telnetd
    …
    mikhail 1546 1321 0 15:38:45 pts/17    3:31   netscape   2
Unix Process Creation
   one process can create another process, perhaps to do
    some work for it
     • child – the new process
          an (almost) identical copy of parent (same code, same data, etc.)

     • parent – the original process
          parent can either wait for the child to complete, or continue
           executing in parallel (concurrently) with the child
          parent does not terminate until the child does

   In UNIX, a process creates a child process using the system call fork( )
     • In child process, fork() returns 0
     • In parent process, fork() returns process id of new child
     • child often uses exec( ) to start another completely different program




                                                                                3
Example of Unix Process Creation

  #include <sys/types.h>
  #include <stdio.h>
  int a = 6; /* global (external) variable */
  int main(void) {
    int b;             /* local variable */
    pid_t pid;         /* process id */
    b = 88;                                              example execution
    printf("..before fork\n");
                                                        prompt% forkprogram
                                                        ..before fork
      pid = fork();
                                                        ..after fork, a = 7, b = 89
      if (pid == 0) {   /* child */
                                                        ..after fork, a = 6, b = 88
         a++; b++;
      } else            /* parent */
         wait(pid);

      printf("..after fork, a = %d, b = %d\n", a, b);
      exit(0);
  }

                                                                                      4
Unix Process Tree
as one processes creates another, they form a tree




                                                     5
    Process Termination


   after completing work a process may execute exit()
    system call – asking the OS to delete it
     • parent is notified
     • process’ resources are deallocated by operating system
   parent may terminate execution of a child process - abort()
    system call
     • possible reasons
           task assigned to child is no longer required

           child exceeded allocated resources

     • if parent exits (some) OS require all children to terminate –
        cascading termination




                                                                       6
                        Context
   context – information about running process
     • program code (?)
     • static data (bss)
     • heap (dynamic data)
     • procedure call stack - contains temporary data -
        subroutine parameters, return addresses, local          program memory
        variables                                                  allocation
     • register contents
           general purpose registers

           program counter (PC) — address of next
            instruction to be executed
           stack pointer (SP)

           program status word (PSW) — interrupt status,
            condition codes, etc.
     • OS resources in use - open files, connections to other
        programs
     • accounting information
   note that for each process there are two contexts
     • for user mode and for kernel mode                                         7
          Process Control Block
   process control block (PCB) – data structure
    maintained by OS to keep track of process’ state
   contains
     • process id number
     • user id of the owner
     • process state (running, not-running, etc.)
     • PC, SP, PSW, general purpose registers
     • memory limits (static, dynamic) - base/limit register
        contents
     • list of open files
     • CPU scheduling information (e.g., priority)
     • I/O states, I/O in progress - list of I/O devices
        allocated to process, list of open files, etc. status
        of I/O requests
     • …



                                                                8
    Context Switch
  context switch - stopping
   one process and
   restarting another
sequence of actions
 OS takes control
   (through interrupt)
 saves old context in the
   process PCB
 reloads new context
   from the new process
   PCB
     • this amounts
       to changing
       the execution of processes (as soon as the PC is reloaded)
 returns control to app. program
 how is context switch different from mode switch? How are they similar?
   how many mode switches are there in a context switch?
 context switch requires several thousand instructions
     • time-sharing OS may do 100–1000 context switches a second            9
Interleaving of Process Execution in
Time-Sharing OS

   from the user’s standpoint
                                         in reality OS interleaves
   multiple processes
   are executed concurrently
                                         process execution


                                  time                         time
                                                process A
    process   process   process                 process B
      A         B         C                     process C
                                                process A
                                                process B
                                                process C
              process                           process A
                D                               process C
                                                process A
                                                process D
                                                process C
                                                process D
                                                process C


                                                                      10
    Five State Process Model
   OS maintains the state of a process.
   process is
     • new – when it is being created
     • running - executing on the CPU
     • waiting (blocked) – waiting on input/output even to occur
     • ready – ready to run on the CPU
     • terminated – finished execution, is being destroyed
   how many processes in the system can be in a particular state?




                                                                     11
         OS Process Queues
OS queues PCBs according to their state
 ready queue –
  processes in state
  ready (waiting to
  be run on CPU)
 for each device there
  is an I/O queue –
  processes in state
  waiting, blocked
  on particular device
  waiting for their
  request to be
  submitted
  to the device
  controller




                                          12
     Short Term
     Scheduler
   selects process
    from the ready
    queue to run on
    the CPU
   runs when
     • process is created
       or terminated
     • process switches
       from running to blocked
     • interrupt occurs
   goals:
     • minimize response time (e.g., program execution, character to screen)
     • minimize variance of average response time — predictability may be
       important
     • maximize throughput
          minimize overhead (OS overhead, context switching, etc.)

          efficient use of resources
                                                                               13
     • fairness — share CPU in an equitable fashion
Other Types
of Schedulers

   medium-term
     scheduler
      • temporarily
        swaps processes in/out of main memory
      • may suspend/resume processes
      • goal: balance load for better throughput
   long-term scheduler (job scheduler) – does not exist on time-sharing OS
      • selects job from the spool, and loads it into memory
      • executes infrequently, maybe only when process leaves system
      • controls degree of multiprogramming
      • goal: good mix of CPU-bound and I/O-bound processes
           I/O-bound process – spends more time doing I/O than
            computations, many short CPU bursts
           CPU-bound process – spends more time doing computations; few
            very long CPU bursts
                                                                              14
               Unix Process States


   enhanced 5-state model
   blocked and ready states has to be split depending on
    whether a process is swapped out on disk or in memory
   running state is also split depending on the mode: kernel or user
   Unix process states:
     • created - just created not yet ready to run
     • ready (memory) - ready as soon as kernel schedules it
     • ready (disk) - ready, but needs to be swapped to memory
     • asleep - blocked (memory) - waiting on event in memory
     • asleep - blocked (disk) - waiting on event on disk
     • running (kernel) - executing in kernel mode
     • running (user) - executing in user mode
     • zombie - process terminated but left a record for parent to collect


                                                                             15
       Unix Process Scheduling
   process is running in user mode until an interrupt occurs or
    it executes a system call
   if time slice expires the process is preempted and another is scheduled
   a process goes to sleep if it needs to wait for some event to occur and is woken
    up when this event occurs
   when process is created decision is made whether to put it in memory or disk
                                                              fork
user mode          kernel mode
                   interrupt                       enough   created       not enough
                                                   memory                 memory
                            kernel schedule                    swap out
          return           running              ready    swap in
                                                                               ready
                   interrupt,         preempt
  user
                   system call
running                                            wakeup                  wakeup
                               exit
                                       sleep
                                                              swap out
                           zombie               asleep                        asleep
                                                                          disk         16
              Lecture Review

   the execution of a certain task from OS standpoint is
    a process
      • OS maintains (creates/destroys) processes for user
      • OS schedules processes for user
           there are three types of schedulers: short/medium/long term

   OS saves process states in a process control block
   OS context switches between processes
   in 5-state process model a process can be in one of five states: created,
    running, ready, blocked, terminated
      • UNIX adds states for being swapped out to disk
      • depending on process state its PCB is attached to either ready queue
        or device queue




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