??? CPU Scheduling

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??? CPU Scheduling

• ????
• ????(Scheduling Criteria )
• ?????(Scheduling Algorithms)
• ???????(Multiple-Processor Scheduling)
• ????(Real-Time Scheduling)
• ?????(Algorithm Evaluation)

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????

• CPU??????CPU????(CPU utilization)
  ????????(multiprogramming)???multiprogramming
• CPU Burst Cycle?I/O Burst Cycle
• ???????????CPU??????????????.
• CPU burst???(distribution)

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CPU And I/O Bursts?????
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CPU-burst ??????????(histogram)
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CPU ???

• ????????????????????????,??CPU???.
• CPU???????????????????
• ?????(running)?????(wait)???.
• ?????(running)?????????(ready)???.
• ?????(wait)?????????(ready)???Switches from
  waiting to ready.
• ??(terminate).
• ??????????1?4????, ?????????(non-preemptive,??????
  ).
• ???????CPU???????????(preemptive).

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???(Dispatcher)

• ??????????,??CPU?????????????????,?????
• switching context
• ????????
• ??????????????????????
• ????(Dispatch latency)
• ??????????????????????????.

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????

• CPU????(utilization)
• ????CPU??
• ??(Throughput)
• ??????????????
• ????(Turnaround time)
• ????????????
• ????
• ????????ready queue???
• ????(Response time)
• ??????(time-sharing),?????????,
  ?????????????????(????????????)

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?????

• ??CPU???
• ????(throughput)
• ??????(turnaround time)
• ??????(waiting time)
• ??????(response time)

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????(FCFS)???

• Process Burst Time
• P1 24
• P2 3
• P3 3
• ???????????? P1 , P2 , P3
• ????????(Gantt Charts)
• ?????????P1 0 P2 24 P3 27
• ??????? (0 24 27)/3 17

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????(FCFS)???(Cont.)

• ?????????
• P2 , P3 , P1 .
• ?????Gantt chart ?
• ?????????P1 6 P2 0 P3 3
• ???????(6 0 3)/3 3
• ?????????
• ?????(convoy effect), ???????????????CPU??????????
  , ??????????????.

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?????? (SJF) ???

• ?????????????CPU burst?????. ??????????????CPU
  burst???????CPU??.
• ?????
• ?????(nonpreemptive)
• ??CPU????????, ???????????????, ?????CPU burst??.
• ????(preemptive)
• ???????CPU burst???????????????CPU burst?????,
  ????????????????????(preempt). ???????????????????
  ?(Shortest-Remaining-Time-First, SRTF).
• SJF????????
• ???????????????????????.

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????????? SJF??

• Process Arrival Time Burst Time
• P1 0.0 7
• P2 2.0 4
• P3 4.0 1
• P4 5.0 4
• SJF (non-preemptive)
• Average waiting time (0 6 3 7)/4 4

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???????? SJF??

• Process Arrival Time Burst Time
• P1 0.0 7
• P2 2.0 4
• P3 4.0 1
• P4 5.0 4
• SJF (preemptive)
• Average waiting time (9 1 0 2)/4 3

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???CPU burst ?????

• ??????? estimate the length.
• ???????CPU burst??????,??????????(exponential
  averaging).

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???CPU burst?????
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????????

• ? 0
• ?n1 ?n
• ??????CPU burst??.
• ? 1
• ?n1 tn
• ???????????CPU burst??.
• ????????,????
• ?n1 ? tn(1 - ?) ? tn -1
• (1 - ? )j ? tn -1
• (1 - ? )n1 tn ?0
• ????(1 - ?)?????? 1,???????????????????????.

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?????????

• ?????????????????
• CPU???????????? (??????????????).
• Preemptive
• nonpreemptive
• SJF??????????????CPU burst?????????????.
• ????????Starvation
• ???????????????.
• ?????Aging
• ????????????????????.

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?????????(Round Robin,RR)

• ?????????????CPU??(?????time quantum), ???10-100
  milliseconds. ???????, ???????????????ready
  queue???.
• ???n????ready queue?????????? q,?????????????q????
  ???1/n?CPU time.????????? (n-1)q??.
• ??
• q??? ? FIFO
• q??? ? ???context switch??????,q?????,??context
  switch????????????.

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?????20????????????

• Process Burst Time
• P1 53
• P2 17
• P3 68
• P4 24
• The Gantt chart is
• ????, ?SJF??????????(turnaround than SJF),
  ??????????.

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??????Context Switch ??
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??????????????(Turnaround Time)???
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?????(multi-level queue)??

• Ready queue????????queues
• ??, foreground (interactive)
• ??,background (batch)
• ???queue?????????
• foreground RR
• background FCFS
• ????queue?????.
• ???????(Fixed priority scheduling)
• ???????, ???????
• ??????starvation.
• ????(time slice)
• ???queue????????CPU??, ??????queue???????????
• ??80CPU?????queue?RR???, 20 ???queue?FCFS??

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???????
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???????(multi-level feedback queue)

• ?????????queue???
• aging ??????????.
• ???????????????????
• queues???
• ??queue??????
• ??????????????queue???
• ??????????????queue???
• ??????????, ?????????queue???

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???????????

• ??queue
• Q0 time quantum 8 milliseconds
• Q1 time quantum 16 milliseconds
• Q2 FCFS
• ????
• ???????????????Q0,????CPU???, ??8
  milliseconds????.????? 8 milliseconds??,?????Q1.
• ????????????Q1???????16 milliseconds?CPU????.?????
  ???????,??????????Q2.

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Multi-level Feedback Queues
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??????(Multiple-Processor Scheduling)

• ????????.
• ???????????????????(Homogeneous)????.
• ????(Load sharing)
• ????????
• ?????????????????, ???????????????????.

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?????(Real-Time Scheduling)

• ?????(Hard real-time systems)
• ???????????????????.
• ???(Soft real-time computing)
• ?????????????????????????

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????(Dispatch Latency)
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????????

• ???????(Deterministic modeling)
• ?????????????????(workload) ??????????????????????
  .
• ??????(Queueing models)
• ??(Implementation)

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????????CPU???????
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Solaris 2 ???
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Windows 2000 ????
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???????