Module%202.1:%20CPU%20Scheduling

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Module 2.1 CPU Scheduling

• Scheduling Types
• Scheduling Criteria
• Scheduling Algorithms
• Performance Evaluation

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CPU SCHEDULING

• The basic problem is as follows How can OS
  schedule the allocation of CPU cycles to
  processes in system, to achieve good
  performance?
• Components of CPU scheduling subsystem of OS
• Dispatcher gives control of the CPU to the new
  process
• Scheduler - selects next process from those in
  main memory (short-term scheduler)
• Swapper - manages transfer of processes between
  main memory and secondary storage (medium-term
  scheduler)
• Long-Term Scheduler - in a batch system,
  determines which and how many jobs to admit into
  system.

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Types of Scheduling Algorithms

• Preemptive process may have CPU taken away
  before completion of current CPU burst (e.g. end
  of CPU quantum.)
• Non-preemptive processes always run until CPU
  burst completes
• Static Priority
• Dynamic Priority

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Performance Criteria for Scheduling

• Scheduling (as an Optimization task) How to
  best order the ready queue for efficiency
  purposes.
• CPU utilization of time CPU in use
• Throughput of jobs completed per time unit
• Turnaround Time wall clock time required to
  complete a job
• Waiting Time amount of time process is ready but
  waiting to run
• Response Time in interactive systems, time until
  system responds to a command
• Response Ratio (Turnaround Time)/(Execution
  Time) -- long jobs should wait longer
• The overhead of a scheduling algorithm (e.g.,
  data kept about execution activity, queue
  management, context switches) should also be
  taken into account.
• Kleinrock's Conservation Law
• No matter what scheduling algorithm is used, you
  cannot help one class of jobs without hurting the
  other ones.
• Example A minor improvement for short jobs (say,
  on waiting time) causes a disproportionate
  degradation for long jobs.

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Optimization Criteria

• Max CPU utilization
• Max throughput
• Min turnaround time
• Min waiting time
• Min response time

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Basic Scheduling Algorithm

• FCFS - First-Come, First-Served
• Non-preemptive
• Ready queue is a FIFO queue
• Jobs arriving are placed at the end of queue
• first job in queue runs to completion of CPU
  burst
• Advantages simple, low overhead
• Disadvantages long waiting time, inappropriate
  for interactive systems, large fluctuations in
  average turnaround time are possible.

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FCFS Example

• Pid Arr CPU Start Finish Turna Wait
  Ratio----------------------------------
  A 0 3 0 3 3 0 1.0 B 1
  5 3 8 7 2 1.4 C 3 2 8
  10 7 5 3.5 D 9 5 10 15
  6 1 1.2 E 12 5 15 20 8
  3 1.6---------------------------------
  - A 0 1 0 1 1 0 1.00 B
  0 100 1 101 101 1 1.01 C 0 1
  101 102 102 101 102.00 D 0 100 102
  202 202 102 2.02

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RR - Round Robin

• Preemptive version FCFS
• Treat ready queue as circular
• arriving jobs placed at end
• first job in queue runs until completion of CPU
  burst, or until time quantum expires
• if quantum expires, job again placed at end

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Properties of RR

• Advantages simple, low overhead, works for
  interactive systems
• Disadvantages if quantum too small, too much
  time wasted in context switching if too large,
  approaches FCFS.Typical value 10 - 100
  msecRule of thumb choose quantum so that large
  majority (80-90) of jobs finish CPU burst in one
  quantum

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SJF - Shortest Job First

• non-preemptive
• ready queue treated as a priority queue based on
  smallest CPU-time requirement
• arriving jobs inserted at proper position in
  queue
• shortest job (1st in queue) runs to completion
• Advantages provably optimal w.r.t. average
  turnaround time
• Disadvantages in general, unimplementable. Also,
  starvation possible!
• Can do it approximately use exponential
  averaging to predict length of next CPU burst
  gt pick shortest predicted burst next!

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Exponential Averaging

• t n1 a tn (1 - a) t n
• tn1 predicted length of next CPU burst
• tn actual length of last CPU burst
• tn previous prediction
• a 0 implies make no use of recent history
• (t n1 t n)
• a 1 implies tn1 tn (past prediction not
  used).
• a 1/2 implies weighted (older bursts get less
  and less weight).

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Prediction of the Length of the Next CPU Burst
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SRTF - Shortest Remaining Time First

• Preemptive version of SJF
• Ready queue ordered on length of time till
  completion (shortest first)
• Arriving jobs inserted at proper position
• shortest job runs to completion (i.e. CPU burst
  finishes) or until a job with a shorter
  remaining time arrives (i.e. placed in the ready
  queue.)

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Performance Evaluation

• Deterministic Modeling (vs. Probabilistic) Look
  at behavior of algorithm on a particular
  workload, and compute various performance
  criteria
• Example
• workload - Job 1 24 units
  Job 2 3 units Job 3 3 units
• Gantt chart for FCFS
• Job 1 Job 2 Job 3 0 24
  27 30
• Total waiting time 0 24 27 51
• Average waiting time 51/3 17
• Total turnaround time 24 27 30 81
• Average turnaround time 81/3 27

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RR and SJF

• Chart for RR with quantum of 3
• Job 1 Job 2 Job 3 Job 1 0 3
  6 9 30
• Total waiting time 6 6 3 15
• Avg. waiting time 15 / 3 5
• Chart for SJF
• Job 2 Job 3 Job 1 0 3
  6 30
• Total waiting time 6 0 3 9
• Avg. waiting time 9 / 3 3
• Can see that SJF gives minimum waiting time. RR
  is intermediate. (This can be proved in general.)

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HPF - Highest Priority First

• general class of algorithms
• each job assigned a priority which may change
  dynamically
• may be preemptive or non-preemptive
• Problem how to compute priorities?
• SJF is a special case of priority the longer the
  CPU burst, the lower the priority.
• Priority can be internally computed, e.g., CPU
  burst vs. I/O burst. Or it can be externally
  defined depending on the importance of the
  process, (e.g. using nice command in Unix).
• Effective Priority System (Internal) User
  defined
• System type of process age (dynamically
  changes)

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Windows XP Priorities

• 6 priority classes (shown with Task Manager)
• 7 default relative priorities/values (shown with
  Process Explorer)
• 16-31 (time critical)
• 1-16 (others)

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Multilevel Queue Scheduling
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Multilevel Feedback Queue

• A process is admitted to one class of queues
• Schedule top queue processes first
• A process can move between the various queues
  aging can be implemented this way
• Multilevel-feedback-queue scheduler defined by
  the following parameters
• number of queues
• scheduling algorithms for each queue
• method used to determine when to upgrade a
  process
• method used to determine when to demote a process
• method used to determine which queue a process
  will enter when that process needs service

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Example of Multilevel Feedback Queue

• Attractive scheme for I/O bound jobs
• Three queues
• Q0 RR with time quantum 8 milliseconds
• Q1 RR time quantum 16 milliseconds
• Q2 FCFS
• Scheduling
• A new job enters queue Q0 which is served FCFS.
  When it gains CPU, job receives 8 milliseconds.
  If it does not finish in 8 milliseconds, job is
  moved to queue Q1.
• At Q1 job is again served FCFS and receives 16
  additional milliseconds. If it still does not
  complete, it is preempted and moved to queue Q2.

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Multilevel Feedback Queues
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Further Readings

• When is processor affinity used?
• In Windows XP, what is the default base priority
  for a process when it gets created?