Uniprocessor Scheduling

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Lecture 7

• Uniprocessor Scheduling

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CPU Scheduling

• ?? ????? ????? ???? ??? ???? ??? ??? ??? ?????
  ???? CPU? ???? ??
• ? ?
• High processor utilization
• High throughput
• number of processes completed per unit time
• Low response time
• elapsed time from the submission of a request to
  the beginning of the response

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???? ??? ??

• Long-term which process to admit
• Medium-term which process to swap in or out
• Short-term which ready process to execute next

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Queuing Diagram for Scheduling
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Long-Term Scheduling

• ?? ????? ????? ??? ? ??? ??? ???? ???? ??
• ?? ????? ??(the degree of multi-programming)?
  ????
• ?? ????? ???? ??
• ?? ????? ????(blocked)? ? ???? ????
• CPU ???? ?? ? ??
• ? ????? ?? CPU ?? ??? ????
• ? ????? ?? ??? ????
• Processor-bound ????? I/O-bound ????? ???? ??? ?
  ??? ???? ?? ???

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Medium-Term Scheduling

• Swapping decisions based on the need to manage
  multiprogramming
• ?? ????? swapping ? ???? ??
• ??? ?? ?????? ?? ??

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Short-Term Scheduling

• ?? ????? ??? ??? ???? ??
• CPU scheduling
• Dispatcher
• ?? ??? ???? ??? ??? ??? ????? ??
• ??? ?????? CPU? ??
• ??? ??
• ??? ??? ??
• ???? ??? ?? ??? ????? ??? ??? ?? PC(program
  count) ??
• CPU ????? ???? ??(events)
• clock interrupts
• I/O interrupts
• operating system calls and traps
• signals

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CPU-I/O Burst Cycle
? ? ? load store add
store read from file store
increment index write to file
load store add store read from
file ? ? ?

• ??? ????? CPU ??? I/O ??? ??? ???? ??? ????? ??
• ? ???? CPU burst (typically of 5 ms)? I/O
  burst(usually longer)? ??
• ????? CPU burst? ???? CPU burst? ??
• CPU-bound process? I/O-bound process?? ? ? CPU
  bursts? ???

CPU Burst
I/O Burst
CPU Burst
I/O Burst
CPU Burst
I/O Burst
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CPU ???? ??(Criteria)

• ??? ?? ??
• Response Time(?? ??) ????? ??? ??? ??? ????? ???
  ??
• Turnaround Time(?? ?? ??) ????? ??? ??? ??? ??
  ??? ??
• ??? ?? ??
• processor utilization(CPU ???)
• Fairness(???)
• Throughput(???) ?? ??? ?? ???? ???? ?

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CPU ???? ????? ??

• ????(selection function)
• ready queue? ?? ????? ??? ??? ??? ????? ????
  ??(??)
• ????(decision mode)
• ????? ??? ??? ??
• Non-preemptive(???)
• ?? ????? ????? ?? ??? ????? I/O? ?? ????? ? ???
  ??? ???? ??
• Preemptive(??)
• ?? ???? ????? ???? ready queue? ??? ?? ??? ?????
  ???? ??
• ??? ????? ?? ???? CPU? ???? ???? ???? ? ?? ????
  ????

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CPU ???? ????

• ?? ???(First Come First Served) ????
• ??? ??(Round-Robin) ????
• ?? ???? ??(Shortest Process NextSPN) ????
• ?? ?? ??(Shortest Remaining TimeSRT) ????
• ?? ??? ??(Highest Response Ratio NextHRRN) ????
• ????(Priority) ????
• ??? ??? ?(Multi-level Feedback Queue) ????
• Multiprocessor ????
• ??? ????(Real-Time Scheduling)

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CPU ???? ????? ??
- Service time total processor time needed in
one (CPU-I/O) cycle - Jobs with long service time
are CPU-bound jobs and are referred to as long
jobs
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?? ??? ???? First Come First Served (FCFS)

• ????
• ready queue ?? ?? ???? ???? ?? ????? ??
• First Come First Served(FCFS)
• ????
• Non-preemptive(???)
• ????? ????? ????? ? ??? ????

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FCFS ????? ??

• ?? I/O? ???? ?? ????? CPU? ??? ? ??
• CPU-bound process? ????
• I/O-bound process? CPU-bound process? ??? ???
  ???? ??
• I/O-bound process? I/O? ?????? ???? ????? ?? I/O?
  ???? ???(poor device utilization)
• I/O bound process? ? ?? ?? ????? ?????? I/O
  device? ???? ?? ? ??

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???-??(Round-Robin) ????

• ???? FCFS? ??
• ???? preemptive(??)
• ????? ??? time slice (quantum, typically from 10
  to 100 ms) ??? ????
• clock interrupt? ???? ?? ???? ????? ready queue?
  ??? FCFS ???? ??? ????? ????

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Time Quantum for Round Robin (1)

• Clock interrupt? dispatching ??? ?? ???? ? ??
  ????? ??
• should be larger than the typical interaction
• ?? ?? ??? ?? ? ??

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Time Quantum for Round Robin (2)
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Round Robin ???? ??

• ??? CPU-bound process? ????
• I/O bound process? time slice?? ?? ???? CPU? ????
  I/O ??? ?? ????? ??
• CPU-bound process? ?? time slice ?? ???? ready
  queue? ?????. ??, ?? ??? ???? ?? ??? ??
• ??? ?? ??? ??(virtual round robin)
• I/O? ????? ?? ?? ??? ????? ready queue?? ?? ?????
  ??? ?? ?(auxiliary queue)? ???

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Queuing for Virtual Round Robin
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Shortest Process Next (SPN) ????

• ????
• ?? ?? CPU burst time? ?? ????? ????
• ?? ?? ??(Shortest Job First) ????
• ???? non-preemptive(???)
• I/O bound process? ?? ????
• ? ????? CPU burst time? ?? ????? ??

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CPU burst time ??(1)

• Let Ti be the execution time for the i-th
  instance of the process
• the actual duration of the i-th CPU burst of this
  process
• Let Si be the predicted value for the i-th CPU
  burst of this process. The simplest choice is
• Sn1 (1/n) S_i1 to n Ti
• To avoid recalculating the entire sum we can
  rewrite this as
• Sn1 (1/n) Tn ((n-1)/n) Sn
• But this convex combination gives equal weight to
  each instance

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CPU burst time ??(2)

• But recent instances are more likely to reflect
  future behavior
• A common technique for that is to use exponential
  averaging
• Sn1 a Tn (1-a) Sn 0 lt a lt 1
• more weight is put on recent instances whenever a
  gt 1/n

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SPN ???? ??

• ?? ????? ????? ??? ??? ? ????? starvation ? ????
  ??
• ??? ????? ??? ??(??? ??)?? ?????
• CPU bound process? ?? ????? ???? ??? I/O? ???? ??
  ????? CPU? ??? ? ??
• ??? ?? ???? ???? ???
• SPN ????? ????? ???? ??? ????
• shortest jobs are given preferences

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Shortest Remaining Time (SRT) ????

• ????
• ??? ?? ?? ??? ?? ?? ????? ??
• SPN ????? ???
• ???? preemptive(??)
• ??? ????? ?? ?? ??? ?? ???? ????? ?? ?? ???? ??
  ?? ????

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Shortest Remaining Time (SRT) ????

• ? ?
• FCFS ??????? ??? ????? ??
• ?? ??? ? ?????? ?? ??? ?? ???? ??
• ???-??? ?? ????? ??? ???? ??
• ????? ??? SPN?? ???? ??? ?? ??
• ????? ?? ?? ?? ? ???? ??? ?? ??? ?? ????? ??

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Highest Response Ratio Next (HRRN) ????

• ????
• ?? ??(RR Response Ratio)? ?? ? ????? ??
• RR (CPU ?? ?? ??? ??? ??)/??? ??? ??
• ?? ?????? ?? ?? ??? ???? ???
• ???? nonpreemptive(???)

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Highest Response Ratio Next (HRRN) ????

• ? ?
• ????? ?? ??? ???? ??? ???
• ????? ?? ????? ?? ????? ??
• ?? ??? ? ????? ?? ?? ??? ???? ?? ????? ???? ?? ??
  ??? ?????? ?? ??

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???? ???? Priority Scheduling

• ???? ?? ?? ??? ????
• ????? ????? ??
• ?? ????? ????? ??, ?? ??? ????? ?? ????? ????
  FCFS ???? ??
• ???? preemptive(??)
• ? ???? ??? ???? ??? ready queue? ???? ??? ? ??
• Multi-level queue
• ?? ???? ????? starvation ? ? ??
• ?? ????(dynamic priority) ??
• ?? ?? ??? ?? ????? ????? ???

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??? ??? ? ???? Multilevel Feedback Queue
Scheduling

• Preemptive scheduling with dynamic priorities
• Several ready queues with decreasing priorities
• P(RQ0) gt P(RQ1) gt ... gt P(RQn)
• ??? ????? RQ0? ???
• Time slice? ???? ????? RQ1?? ???. ??? ? ?? ?????
  RQ2?, ???? RQn?? ???? ??

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??? ??? ? ???? Multilevel Feedback Queue
Scheduling

• I/O-bound process? ?? ???? ??? ???? ??? CPU-bound
  process? ?? ???? ?? ????
• Dispatcher? RQi-1?? RQ0 ??? ?? ???? ?? RQi ?? ???
  ????? ????
• ? ????? starvation ? ???? ??
• ?? ???? ??? ?? ??? ????? ?? ???? ??
  ????(aging)??? ??

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Multiple Feedback Queues

• ? ???? FCFS ????
• ?? ?? ???? ???? Round-Robin ????

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Time Quantum for MFQ Scheduling

• ?? ??? time slice? ??? ??, ? ????? ?? ?? ??? ???
  ??? ? ??
• ?? ??? ?? time slice? ??????? ?? ?? ?? ??? ?? ?
  ??
• ? time slice of RQi 2i-1

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???? ????? ?? ??
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Fair-Share Scheduling(FSS)

• Users application runs as a collection of
  processes (threads)
• User is concerned about the performance of the
  application
• Need to make scheduling decisions based on
  process sets

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Traditional UNIX Scheduling

• ??? ??? ??? ??? ????? ??
• CPU ???? ????? ?? ????
• ??? ????? ?? ????? ??
• ?? ????? ??(background) ?????? ?? ??? ??? ?? ??
• Multilevel feedback using round robin within each
  of the priority queues
• If a running process does not block or complete
  within 1 second, it is preempted
• Priorities are recomputed once per second
• Base priority divides all processes into fixed
  bands of priority levels

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Bands

• Decreasing order of priority
• Swapper
• Block I/O device control
• File manipulation
• Character I/O device control
• User processes
• ? ???? ?? ???? ??? ???? ??? ??? ??? ????? ??? ?
  ??? ?? ??

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Bands of process priorities

• Kernel and user priorities

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