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Opera&ng Systems ECE344 Ding Yuan Lecture 10: Scheduling

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Page 1: Operang)Systems ) ECE344) · 2014-03-31 · Priority)Scheduling) • Each)job)is)assigned)apriority) • FCFS)within)each)priority)level) • Selecthighestpriority)job)over)lower)ones)

Opera&ng  Systems  ECE344  

 

Ding  Yuan  

Lecture  10:  Scheduling  

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Scheduling  Overview  

•  In  discussing  process  management  and  synchroniza&on,  we  talked  about  context  switching  among  processes/threads  on  the  ready  queue  

•  But  we  have  glossed  over  the  details  of  exactly  which  thread  is  chosen  from  the  ready  queue  

•  Making  this  decision  is  called  scheduling  •  In  this  lecture,  we’ll  look  at:  –  The  goals  of  scheduling  –  Various  well-­‐known  scheduling  algorithms  –  Standard  Unix  scheduling  algorithm  

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Mul&programming  

•  In  a  mul&programming  system,  we  try  to  increase  CPU  u&liza&on  and  job  throughput  by  overlapping  I/O  and  CPU  ac&vi&es  –  Doing  this  requires  a  combina&on  of  mechanisms  and  policy  

•  We  have  covered  the  mechanisms  –  Context  switching,  how  it  happens  –  Process  queues  and  process  states  

•  Now  we’ll  look  at  the  policies  – Which  process  (thread)  to  run,  for  how  long,  etc.  

•  We’ll  refer  to  schedulable  en&&es  as  jobs  (standard  usage)  –  could  be  processes,  threads,  people,  etc.  

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Scheduling  

•  Deciding  which  process/thread  should  occupy  the  resource  (CPU,  disk,  etc.)  

 

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When  to  schedule?  

•  A  new  job  starts  •  The  running  job  exits  •  The  running  job  is  blocked  •  I/O  interrupt  (some  processes  will  be  ready)  •  Timer  interrupt    – Every  10  milliseconds  (Linux  2.4)  – Every  1  millisecond  (Linux  2.6)  – Why  is  the  change?  

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What  are  the  scheduling  objec&ves?  

•  Anyone?  

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Scheduling  Objec&ves  

•  Fair  (nobody  cries)  •  Priority  (lady  first)  •  Efficiency  (make  best  use  of  equipment)  •  Encourage  good  behavior  (good  boy/girl)  •  Support  heavy  load  (degrade  gracefully)  •  Adapt  to  different  environment  (interac&ve,  real-­‐&me,  mul&-­‐media,  etc.)  

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Performance  Criteria  

•  Throughput:  #  of  jobs  that  complete  in  unit  &me  

•  Turnaround  &me  (also  called  elapse  &me)  – Amount  of  &me  to  execute  a  par&cular  process  from  the  &me  it  entered  

•  Wai&ng  &me  –  amount  of  &me  process  has  been  wai&ng  in  ready  queue  

•  Mee&ng  deadlines:  avoid  bad  consequences  

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Different  Systems,  Different  Focuses  

•  Batch  Systems  (e.g.,  billing,  accounts  receivable,  accounts  payable,  etc.)  – Max  throughput,  max  CPU  u&liza&on  

•  Interac&ve  Systems  (e.g.,  our  PC)  – Min.  response  &me  

•  Real-­‐&me  system  (e.g.,  airplane)  – Priority,  mee&ng  deadlines  

•  Example:  on  airplane,  Flight  Control  has  strictly  higher  priority  than  Environmental  Control    

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Program  Behaviors  Considered  in  Scheduling  

•  Is  it  I/O  bound?  Example?  •  Is  it  CPU  bound?  Example?  •  Batch  or  interac&ve  environment  •  Priority    •  Frequency  of  page  fault  •  Frequency  of  I/O  

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Preemp&ve  vs.  Non-­‐preemp&ve  

•  Non-­‐preemp&ve  scheduling  – The  running  process  keeps  the  CPU  un&l  it  voluntarily  gives  up  the  CPU  •  Process  exits  •  Switch  to  blocked  state  •  1  and  4  only  (no  3  unless            calls  yield)  

•  Preemp&ve  scheduling  – The  running  process  can  be  interrupted  and  must  release  the  CPU  

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Scheduling  Algorithms  

•  First  Come  First  Serve  (FCFS)  •  Short  Job  First  (SJF)  •  Priority  Scheduling  •  Round  Robin  •  Mul&-­‐Queue  &  Mul&-­‐Level  Feedback  •  Earliest  Deadline  First  Scheduling  

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Batch    Systems  

Interac4ve    Systems  

Real-­‐4me  Systems  

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First  Come  First  Serve  (FCFS)  

•  Also  called  first-­‐in  first-­‐out  (FIFO)  –  Jobs  are  scheduled  in  order  of  arrival  to  ready  queue  

– “Real-­‐world”  scheduling  of  people  in  lines  (e.g.,  supermarket)  

– Typically  non-­‐preemp&ve  (no  context  switching  at  market)  

–  Jobs  treated  equally,  no  starva&on    

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FCFS  Example  

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Problems  with  FCFS  

•  Average  wai&ng  &me  can  be  large  if  small  jobs  wait  behind  long  ones  (high  turnaround  &me)  – Non-­‐preemp&ve  –  You  have  a  basket,  but  you’re  stuck  behind  someone  with  a  cart  

•  Solu4on?  –  Express  lane  (12  items  or  less)  

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Shortest  Job  First  (SJF)  

•  Shortest  Job  First  (SJF)  –  Choose  the  job  with  the  smallest  expected  dura&on  first  

•  Person  with  smallest  number  of  items  to  buy  –  Requirement:  the  job  dura&on  needs  to  be  known  in  advance  

–  Used  in  Batch  Systems  –  Op&mal  for  Average  Wai&ng  Time  if  all  jobs  are  available  simultaneously  (provable).  Why?  

–  Real  life  analogy?  •  Express  lane  in  supermarket  •  Shortest  important  task  first                                              -­‐-­‐  The  7  Habits  of  Highly  Effec4ve  People  

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Non-­‐preemp&ve  SJF:  Example  

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0  

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Comparing  to  FCFS  

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0  

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SJF  is  not  always  op&mal  

•  Is  SJF  op&mal  if  not  all  the  jobs  are  available  simultaneously?  

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0  

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Preemp&ve  SJF  

•  Also  called  Shortest  Remaining  Time  First  – Schedule  the  job  with  the  shortest  remaining  &me  required  to  complete  

•  Requirement:  again,  the  dura&on  needs  to  be  known  in  advance  

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Preemp&ve  SJF:  Same  Example  

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A  Problem  with  SJF  •  Starva&on  –  In  some  condi&on,  a  job  is  wai&ng  forever  – Example:  •  Process  A  with  dura&on  of  1  hour,  arrives  at  &me  0  •  But  every  1  minute,  a  short  process  with  dura&on  of  2  minutes  arrive  

•  Result  of  SJF:  A  never  gets  to  run  

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Scheduling  Algorithms  

•  First  Come  First  Serve  (FCFS)  •  Short  Job  First  (SJF)  •  Priority  Scheduling  •  Round  Robin  •  Mul&-­‐Queue  &  Mul&-­‐Level  Feedback  •  Earliest  Deadline  First  Scheduling  

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Batch    Systems  

Interac4ve    Systems  

Real-­‐4me  Systems  

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Priority  Scheduling  

•  Each  job  is  assigned  a  priority  •  FCFS  within  each  priority  level  •  Select  highest  priority  job  over  lower  ones  •  Ra&onale:  higher  priority  jobs  are  more  mission-­‐cri&cal  –  Example:  DVD  movie  player  vs.  send  email  

•  Real  life  analogy?  –  Boarding  at  airports  

•  Problems:  – May  not  give  the  best  AWT  –  indefinite  blocking  or  starving  a  process  

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Set  Priority  

•  Two  approaches  – Sta&c  (for  systems  with  well-­‐known  and  regular  applica&on  behaviors)  

– Dynamic  (otherwise)  

•  Priority  may  be  based  on:  –  Importance  – Percentage  of  CPU  &me  used  in  last  X  hours  

•  Should  a  job  have  higher  priority  if  it  used  more  CPU  in  the  past?  Why?  

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Priority  Schedulring:  Example  

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0  

(worse  than  SJF)  

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Priority  in  Unix  

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Nobody  wants  to  Be  “nice”  on  Unix  

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More  on  Priority  Scheduling  •  For  real-­‐&me  (predictable)  systems,  priority  is  onen  used  to  isolate  a  process  from  those  with  lower  priority.  Priority  inversion:  high  priority  task  is  indirectly  preempted  by  medium/low  priority  tasks  – A  solu&on:  priority  inheritance  

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low  priority  job  

high  priority  job  

medium  priority  job  

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Round-­‐robin  •  One  of  the  oldest,  simplest,  most  commonly  used  scheduling  algorithm  

•  Select  process/thread  from  ready  queue  in  a  round-­‐robin  fashion  (take  turns)  

•  Real  life  analogy?  

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Problem:    •   Do  not  consider  priority  •   Context  switch  overhead  

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Round-­‐Robin:  example  

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Time  Quantum    

•  Time  slice  too  large  –  FIFO  behavior  –  Poor  response  &me  

•  Time  slice  too  small  –  Too  many  context  switches  (overheads)  –  Inefficient  CPU  u&liza&on  

•  Heuris&cs:  70-­‐80%  of  jobs  block  within  &me-­‐slice  •  Typical  &me-­‐slice:  5  –  100  ms  – Wait:  isn’t  4mer-­‐interrupt  frequency  1ms  on  Linux  2.6?  

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Combining  Algorithms  •  Scheduling  algorithms  can  be  combined  

–  Have  mul&ple  queues  –  Use  a  different  algorithm  for  each  queue  – Move  processes  among  queues  

•  Example:  Mul&ple-­‐level  feedback  queues  (MLFQ)  – Mul&ple  queues  represen&ng  different  job  types  

•  Interac&ve,  CPU-­‐bound,  batch,  etc.  –  Queues  have  priori&es  –  Jobs  can  move  among  queues  based  upon  execu&on  history  •  Feedback:  switch  from  interac&ve  to  CPU-­‐bound  behavior  

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Example  

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Unix  Scheduler  

•  The  Unix  scheduler  uses  a  MLFQ  –  ~170  priority  levels  

•  Priority  scheduling  across  queues,  RR  within  a  queue  –  The  process  with  the  highest  priority  always  runs  –  Processes  with  the  same  priority  are  scheduled  RR  

•  Processes  dynamically  change  priority  –  Increases  over  &me  if  process  blocks  before  end  of  quantum  

– Decreases  over  &me  if  process  uses  en&re  quantum  ECE344  Opera&ng  Systems  Ding  Yuan 35  

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Unix  Scheduler  

priority  value  =  nice  +  base  +  (recent  CPU  usage/2)  •  The  lower  the  value,  the  higher  the  priority  •  nice  -­‐-­‐-­‐  sta&c  priority  [-­‐20,  19],  default  0  •  base  -­‐-­‐-­‐  a  constant  (60  in  Unix)  •  recent  CPU  usage  is  also  called  CPU  decay  =  (last  value  +  CPU  count  used  by  this  process)  /  2  

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ECE344  Opera&ng  Systems  Ding  Yuan   37  

Process  A  priority   recent  CPU  

Process  B  priority   recent  CPU  

Process  C  priority   recent  CPU  

0  1  ..  60  

0  1  ..  60  

0  1  ..  60  

60  

75   30  

&mer  int.   60   0   0  60  

60  Context    switch  

Context    switch  

0  60  

67.5   15   75   30   60  

Context    switch  

63.75   7.5  8  9  ..  67  

67.5   15   75   30  

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Proper&es  

•  How  will  it  treat  processes  that  have  been  wai&ng  for  a  long  &me?  

•  How  about  a  process  that  do  not  finish  quantum  before  wai&ng?  

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Mo&va&on  of  Unix  Scheduler  •  The  idea  behind  the  Unix  scheduler  is  to  reward  interac&ve  processes  over  CPU  hogs  

•  Interac&ve  processes  (shell,  editor,  etc.)  typically  run  using  short  CPU  bursts  –  They  do  not  finish  quantum  before  wai4ng  for  more  input  

•  Want  to  minimize  response  &me  –  Time  from  keystroke  (puvng  process  on  ready  queue)  to  execu&ng  keystroke  handler  (process  running)  

–  Don’t  want  editor  to  wait  un&l  CPU  hog  finishes  quantum  

•  This  policy  delays  execu&on  of  CPU-­‐bound  jobs  –  But  that’s  ok  

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Scheduling  Algorithms  

•  First  Come  First  Serve  (FCFS)  •  Short  Job  First  (SJF)  •  Priority  Scheduling  •  Round  Robin  •  Mul&-­‐Queue  &  Mul&-­‐Level  Feedback  •  Earliest  Deadline  First  Scheduling  

ECE344  Opera&ng  Systems  Ding  Yuan   40  

Batch    Systems  

Interac4ve    Systems  

Real-­‐4me  Systems  

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Earlieas  Deadline  First  (EDF)  •  Each  job  has  an  arrival  &me  and  a  deadline  to  finish  

–  Real  life  analogy?  •  Always  pick  the  job  with  the  earliest  deadline  to  run  

•  Op&mal  algorithm  (provable):  if  the  jobs  can  be  scheduled  (by  any  algorithm)  to  all  meet  the  deadline,  EDF  is  one  of  such  schedules  

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Scheduling  Summary  

•  Scheduler  (dispatcher)  is  the  module  that  gets  invoked  when  a  context  switch  needs  to  happen    

•  Scheduling  algorithm  determines  which  process  runs,  where  processes  are  placed  on  queues  

•  Many  poten&al  goals  of  scheduling  algorithms  –  U&liza&on,  throughput,  wait  &me,  response  &me,  etc.  

•  Various  algorithms  to  meet  these  goals  –  FCFS/FIFO,  SJF,  Priority,  RR  

•  Can  combine  algorithms  – Mul&ple-­‐level  feedback  queues  –  Unix  example  

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Scheduling  problems  in  “big  data  analy&cs”  

•  Hadoop  MapReduce  – Users  run  computa&on  jobs  –  In  nature  it  is  a  batch  system  

•  FCFS,  SJF  –  But  also  needs  to  consider  for  fairness  and  priority  among  mul&ple  users  

–  Further  complicated  by    •  sharing  the  compu&ng  cluster  with  other  jobs  •  some  jobs  may  have  deadlines  

–  Lots  of  interes&ng  problems!  

ECE344  Opera&ng  Systems  Ding  Yuan   43  


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