Processor Management

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????? 4

• Processor Management
• (single processor with single-threaded process)

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???????????????? Single processing

• ?? processor ????? (single core)
• ?????????????????? (????????????????)
• ??????????????????????????? (multitasking)
• ?????? OS ???????????????? CPU ?????????
• ???????????????????? ??????????????????
  ????????????????????????? ???????????????????
  ?????????????????????? ????????????????????????
  ??????????????????? ????????????????
• ??????????????????
• ????????????????????? CPU
• ????????????????????????????????? memory

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Process state(???????????? state
??????????????????????????????????
?.?.???????????????????????? ?????????????????????
??????????)

• ?????????????????? process ???? process
  ??????????????????????? ??????????????????????????
  ??????? (???????????????????????????)
• ????? process ???????????? ???????????????????????
  ???? ????? CPU ??????? (?????????????
  ??????????????????????? ?????? ... ???)
• ????? process ????????????? (?????? CPU ????)
  ???????????????????????? (????? ... ???)
• ??? process ???????? I/O ????????????????
  keyboard ???????????????????????
  ????????????????????? process ????????????????????
  ? CPU ?????????? ?????????????????? (????? ...
  ???)

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Job and Process Status
Admitted
Finished
Interrupt
Exit
Scheduler dispatch
I/O or event completion
I/O or event wait
Handled by Process Scheduler Handled by Job
Scheduler
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????? CPU ???? process ?????????????

• ??????????????????????? ready ????????????????
  running
• Process ?????????????? ready ?????????????????????
  ?????
• Process ??????? ready ?????? process
  ?????????????? ?
• ?????????????????? ready ?? 3 ????
  ????????????????????? 3 ???
• ?????????????? ?????????????????? process
  ????????? CPU ???????
• ??????????? process ??????????? ready ???????
  process ???????
• ????????????????????????????????

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????????????????? process ?????

• ??? process ???????????????? CPU ??????????????
  CPU ??????????? process ??????????????????????????
  ????
• ?????? process ??????????? CPU ?????????????
  ??????????????????? CPU ???? ?????? process
  ?????????????????????????????????????????
  ???????????????????????? ?????????????????????????
  ???????????????????? ?????????????????????????????
  ?? CPU ??????????????????
• ??????????????????????????????????????????????????
  ??????????????
• ??????????????????????????????? program counter,
  register content ???????? ??????????????? PCB
  ?????????????????????????????????? process
  ???????????????????
• ???????????????????? Context Switching

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Process Control Block (PCB)

• Process Control Block (PCB) -- data structure
  that contains basic info about the job
• Process identification
• Process status (HOLD, READY, RUNNING, WAITING)
• Process state (process status word, register
  contents, main memory info, resources, process
  priority)
• Accounting (CPU time, total amount of time, I/O
  operations, number input records read, etc.)

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PCBs and Queuing

• PCB of job created when Job Scheduler accepts it
• updated as job goes from beginning to
  termination.
• Queues use PCBs to track jobs.
• PCBs, not jobs, are linked to form queues.
• E.g., PCBs for every ready job are linked on
  READY queue all PCBs for jobs just entering
  system are linked on HOLD queue.
• Queues must be managed by process scheduling
  policies and algorithms.

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Process Scheduling Algorithms

• First Come First Served (FCFS)
• Shortest Job Next (SJN)
• Priority Scheduling
• Shortest Remaining Time (SRT)
• Round Robin
• Multiple Level Queues

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First Come First Served (FCFS)

• Non-preemptive.
• Handles jobs according to their arrival time --
  the earlier they arrive, the sooner theyre
  served.
• Simple algorithm to implement -- uses a FIFO
  queue.
• Good for batch systems not so good for
  interactive ones.
• Turnaround time is unpredictable.

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FCFS Example

• Process CPU Burst (Turnaround Time)
• A 15 milliseconds
• B 2 milliseconds
• C 1 millisecond
• If they arrive in order of A, B, and C.
• What does the time line look like?
• Whats the average turnaround time?

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Shortest Job Next (SJN)

• Non-preemptive.
• Handles jobs based on length of their CPU cycle
  time.
• Use lengths to schedule process with shortest
  time.
• Optimal gives minimum average waiting time for
  a given set of processes.
• optimal only when all of jobs are available at
  same time and the CPU estimates are available and
  accurate.
• Doesnt work in interactive systems because users
  dont estimate in advance CPU time required to
  run their jobs.

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Priority Scheduling

• Non-preemptive.
• Gives preferential treatment to important jobs.
• Programs with highest priority are processed
  first.
• Arent interrupted until CPU cycles are completed
  or a natural wait occurs.
• If 2 jobs with equal priority are in READY
  queue, processor is allocated to one that arrived
  first (first come first served within priority).
• Many different methods of assigning priorities by
  system administrator or by Processor Manager.

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Shortest Remaining Time (SRT)

• Preemptive version of the SJN algorithm.
• Processor allocated to job closest to completion.
• This job can be preempted if a newer job in READY
  queue has a time to completion that's shorter.
• Cant be implemented in interactive system --
  requires advance knowledge of CPU time required
  to finish each job.
• SRT involves more overhead than SJN.
• OS monitors CPU time for all jobs in READY queue
  and performs context switching.

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Round Robin

• Preemptive.
• Used extensively in interactive systems because
  its easy to implement.
• Isnt based on job characteristics but on a
  predetermined slice of time thats given to each
  job.
• Ensures CPU is equally shared among all active
  processes and isnt monopolized by any one job.
• Time slice is called a time quantum
• size crucial to system performance (100 ms to 1-2
  secs)

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If Jobs CPU Cycle lt Time Quantum

• If jobs last CPU cycle job is finished, then
  all resources allocated to it are released
  completed job is returned to user.
• If CPU cycle was interrupted by I/O request, then
  info about the job is saved in its PCB it is
  linked at end of the appropriate I/O queue.
• Later, when I/O request has been satisfied, it is
  returned to end of READY queue to await
  allocation of CPU.

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Time Slices Should Be ...

• Long enough to allow 80 of CPU cycles to run to
  completion.
• At least 100 times longer than time required to
  perform one context switch.
• Flexible -- depends on the system.

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Multiple Level Queues

• Not a separate scheduling algorithm.
• Works in conjunction with several other schemes
  where jobs can be grouped according to a common
  characteristic.
• Examples
• Priority-based system with different queues for
  each priority level.
• Put all CPU-bound jobs in 1 queue and all
  I/O-bound jobs in another. Alternately select
  jobs from each queue to keep system balanced.
• Put batch jobs background queue interactive
  jobs in a foreground queue treat foreground
  queue more favorably than background queue.

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Policies To Service Multi-level Queues

• No movement between queues.
• Move jobs from queue to queue.
• Move jobs from queue to queue and increasing time
  quantums for lower queues.
• Give special treatment to jobs that have been in
  system for a long time (aging).

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?????????? algorithm

• ?????????????? algorithm ?????????????????????????
  ??? ?????????? process ????????????????????????
• ?????????????????? algorithm ?? ?????? preemptive
  ???? non-preemptive ????????????????? time
  quantum ???????????????????????????????? OS
  ???????????????? OS ????????????
  ???????????????????????????????
• CPU-bound jobs
• I/O-bound jobs
• Interactive jobs
• Background jobs
• Real time jobs
• Transactions Processing jobs

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Process Scheduling Policy

• Maximize throughput by running as many jobs as
  possible in a given amount of time.
• Maximize CPU efficiency by keeping CPU busy 100
  of time.
• Ensure fairness for all jobs by giving everyone
  an equal amount of CPU and I/O time.

• Minimize response time by quickly turning around
  interactive requests.
• Minimize turnaround time by moving entire jobs
  in/out of system quickly.
• Minimize waiting time by moving jobs out of READY
  queue as quickly as possible.

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Interrupts

• There are instances when a job claims CPU for a
  very long time before issuing an I/O request.
• builds up READY queue empties I/O queues.
• Creates an unacceptable imbalance in the system.
• Process Scheduler uses a timing mechanism to
  periodically interrupts running processes when a
  predetermined slice of time has expired.
• suspends all activity on the currently running
  job and reschedules it into the READY queue.

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Cache Memory

• Cache memory -- quickly accessible memory thats
  designed to alleviate speed differences between a
  very fast CPU and slower main memory.
• Stores copy of frequently used data in an easily
  accessible memory area instead of main memory.

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Types of Interrupts

• Page interrupts to accommodate job requests.
• Time quantum expiration.
• I/O interrupts when issue READ or WRITE command.
• Internal interrupts (synchronous interrupts)
  result from arithmetic operation or job
  instruction currently being processed.
• Illegal arithmetic operations (e.g., divide by
  0).
• Illegal job instructions (e.g., attempts to
  access protected storage locations).

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Interrupt Handler

• Describe store type of interrupt (passed to
  user as error message).
• Save state of interrupted process (value of
  program counter, mode specification, and
  contents of all registers).
• Process the interrupt
• Send error message state of interrupted process
  to user.
• Halt program execution
• Release any resources allocated to job are
  released
• Job exits the system.
• Processor resumes normal operation.