process

1
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

2
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)

3
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

4
Example of Unix Process Creation

• include ltsys/types.hgt
• include ltstdio.hgt
• int a 6 / global (external) variable /
• int main(void)
• int b / local variable /
• pid_t pid / process id /
• b 88
• printf("..before fork\n")
• pid fork()
• if (pid 0) / child /
• a b
• else / parent /
• wait(pid)
• printf("..after fork, a d, b d\n", a,
  b)
• exit(0)

example execution
prompt forkprogram ..before fork ..after fork, a
7, b 89 ..after fork, a 6, b 88
5
Unix Process Tree

• as one processes creates another, they form a tree

6
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

7
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
  variables
• 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

program memory allocation
8
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

9
Context Switch

• context switch - stopping one process and
  restarting another
• sequence of actions
• OS takes control (through interrupt)
• saves old context in theprocess PCB
• reloads new contextfrom the new process PCB
• this amountsto changingthe 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 1001000 context switches
  a second

10
Interleaving of Process Execution in Time-Sharing
OS
from the users standpoint multiple processes are
executed concurrently
in reality OS interleaves process execution
11
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?

12
OS Process Queues

• OS queues PCBs according to their state
• ready queue processes in stateready (waiting
  tobe run on CPU)
• for each device thereis an I/O queue processes
  in statewaiting, blockedon particular
  devicewaiting for theirrequest to be
  submittedto the devicecontroller

13
Short Term Scheduler

• selects processfrom the readyqueue 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
• fairness share CPU in an equitable fashion

14
Other Types of Schedulers

• medium-term scheduler
• temporarilyswaps 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

15
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

16
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

17
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