Operating System Concepts

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Operating System Concepts

• Ku-Yaw Chang
• canseco_at_mail.dyu.edu.tw
• Assistant Professor, Department of Computer
  Science and Information Engineering
• Da-Yeh University

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Chapter 6 CPU Scheduling

• CPU scheduling
• The basis of multiprogrammed OSs
• Make the computer more production
• Switch the CPU among processes
• We introduce
• The basic scheduling concepts
• Several different CPU-scheduling algorithms
• Selecting an algorithm for a particular system

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Chapter 6 CPU Scheduling

1. Basic Concepts
2. Scheduling Criteria
3. Scheduling Algorithms
4. Multiple-Processor Scheduling
5. Real-Time Scheduling

1. Algorithm Evaluation
2. Process Scheduling Models
3. Summary
4. Exercises

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6.1 Basic Concepts

• Several processes are kept in memory at one time
• When a process has to wait, the OS takes the CPU
  away from that process, and gives the CPU to
  another process.
• Almost all computer resources are scheduled
  before use.

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6.1.1 CPU-I/O Burst Cycle

• Observed property of processes
• Process execution consists of a cycle of
• CPU execution
• I/O wait
• Process execution begins with a CPU burst.
• That is followed by an I/O burst, then another
  CPU burst, then another I/O burst, and so on.

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Alternating sequence ofCPU and I/O bursts
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Histogram of CPU-burst times
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6.1.1 CPU-I/O Burst Cycle

• Measure CPU bursts
• An I/O bound program
• Many short CPU bursts
• A CPU bound program
• A few very long CPU bursts
• Help select an appropriate CPU-scheduling
  algorithm

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6.1.2 CPU Scheduler

• Whenever the CPU becomes idle
• Select one of the processes in the ready queue to
  be executed
• Carried out by the short-term scheduler, also
  called CPU scheduler
• A ready queue may be implemented as
• A FIFO queue
• A priority queue
• A tree
• An unordered linked list

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6.1.3 Preemptive Scheduling

• CPU scheduling decisions take place when a
  process
• Switches from running to waiting state
• Switches from running to ready state
• Switches from waiting to ready
• Terminates
• Scheduling only under 1 and 4 is nonpreemptive
• Otherwise is preemptive
• Incur a cost

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6.1.4 Dispatcher

• Dispatcher module gives control of the CPU to the
  process selected by the short-term scheduler
• switching context
• switching to user mode
• jumping to the proper location in the user
  program to restart that program
• Dispatch latency
• time it takes for the dispatcher to stop one
  process and start another running.

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Chapter 6 CPU Scheduling

1. Basic Concepts
2. Scheduling Criteria
3. Scheduling Algorithms
4. Multiple-Processor Scheduling
5. Real-Time Scheduling

1. Algorithm Evaluation
2. Process Scheduling Models
3. Summary
4. Exercises

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6.2 Scheduling Criteria

• CPU utilization (max)
• keep the CPU as busy as possible
• Throughput (max)
• number of processes that complete their execution
  per time unit
• Turnaround time (min)
• amount of time to execute a particular process
• Waiting time (min)
• amount of time a process has been waiting in the
  ready queue
• Response time (min)
• amount of time it takes from when a request was
  submitted until the first response is produced,
  not output (for time-sharing environment)

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Chapter 6 CPU Scheduling

1. Basic Concepts
2. Scheduling Criteria
3. Scheduling Algorithms
4. Multiple-Processor Scheduling
5. Real-Time Scheduling

1. Algorithm Evaluation
2. Process Scheduling Models
3. Summary
4. Exercises

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6.3 Scheduling Algorithms

• Dealing the problem of deciding which of the
  processes in the ready queue to be allocated the
  CPU
• First-Come, First-Served Scheduling
• Shortest-Job-First Scheduling
• Priority Scheduling
• Round-Robin Scheduling
• Multilevel Queue Scheduling
• Multilevel Feedback Queue Scheduling

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6.3.1 First-Come, First-Served Scheduling

• The process that requests the CPU first is
  allocated the CPU first.
• The simplest CPU-scheduling algorithm
• Implementation with a FIFO queue
• Add to the tail of the queue
• Remove from the head to the queue
• The code is simple to write and understand
• Average waiting time is often quite long.

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6.3.1 First-Come, First-Served Scheduling

• Process Burst Time
• P1 24
• P2 3
• P3 3
• Suppose that the processes arrive in the order
  P1 , P2 , P3 The Gantt Chart for the schedule
  is
• Waiting time for P1 0 P2 24 P3 27
• Average waiting time (0 24 27)/3 17

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6.3.1 First-Come, First-Served Scheduling

• Suppose that the processes arrive in the
  order
• P2 , P3 , P1
• The Gantt chart for the schedule is
• Waiting time for P1 6 P2 0 P3 3
• Average waiting time (6 0 3)/3 3
• Much better than previous case.

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6.3.1 First-Come, First-Served Scheduling

• Convoy effect
• All the other processes wait for one big process
  to get off the CPU
• Results in lower CPU and device utilization
• If the shorter processes were allowed to go first
• FCFS is non-preemptive
• Once the CPU has been allocated to a process,
  that process keeps the CPU until it releases the
  CPU.
• Particular troublesome in time-sharing system
• Each user needs to get a share of the CPU at
  regular intervals

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6.3.2 Shortest-Job-First Scheduling

• When the CPU is available, it is assigned to the
  process that has the smallest next CPU burst.
• Associate each process with the length of the
  latters next CPU burst
• Not its total length
• Another term shortest next CPU burst
• FCFS scheduling is used to break the tie
• Provably optimal
• Minimum average waiting time for a given set of
  processes
• Real difficulty
• Knowing the length of the next CPU burst
• Used frequently in long-term scheduling

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6.3.2 Shortest-Job-First Scheduling

• Process Burst Time
• P1 6
• P2 8
• P3 7
• P4 3
• The Gantt Chart for the schedule is
• Waiting time for P1 3 P2 16 P3 9 P4
  0
• Average waiting time (3 16 9 0) / 4 7
• Using FCFS scheme ( 0 6 14 21) / 4 10.25

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6.3.2 Shortest-Job-First Scheduling

• To approximate SJF scheduling
• To predict its value
• Use the length of previous CPU bursts
• exponential average

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Prediction of the length of the next CPU burst
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Examples ofExponential Averaging

• ? 0
• ?n1 ?n
• Recent history does not count.
• ? 1
• ?n1 tn
• Only the actual last CPU burst counts.
• If we expand the formula, we get
• ?n1 ? tn(1 - ?) ? tn -1
• (1 - ? )j ? tn -1
• (1 - ? )n1 tn ?0
• Since both ? and (1 - ?) are less than or equal
  to 1, each successive term has less weight than
  its predecessor.

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6.3.2 Shortest-Job-First Scheduling

• Two schemes
• Nonpreemptive
• Allow the current running process to finish its
  CPU burst
• Preemptive
• If a new process arrives with CPU burst length
  less than remaining time of current executing
  process, preempt.
• Called as Shortest-Remaining-Time-First (SRTF).

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Example of Preemptive SJF

• Process Arrival Time Burst Time
• P1 0 8
• P2 1 4
• P3 2 9
• P4 3 5
• Preemptive SJF
• Average waiting time ( (10-1) (1-1) (17-2)
  (5-3)) / 4 6.5

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6.3.3 Priority Scheduling

• The CPU is allocated to the process with the
  highest priority.
• A priority is associated with each process
• Fixed range of number, such as 0 to 7
• We use low numbers to represent high priority
• Equal-priority processes are scheduled in FCFS
  order
• SJF is a special case of the general
  priority-scheduling algorithm
• The priority is the inverse of the predicted next
  CPU burst

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Example of priority scheduling

• Process Burst Time Priority
• P1 10 3
• P2 1 1
• P3 2 4
• P4 1 5
• P5 5 2
• Priority scheduling
• Average waiting time ( 6 0 16 18 1 ) /
  5 8.2

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6.3.3 Priority Scheduling

• Priorities can be defined
• Internally
• Use some measurable quantity
• Time limits, memory requirements
• Externally
• Set by criteria external to OS
• importance, political factors
• Priority scheduling can be
• Preemptive
• Nonpreemptive
• Major problem
• Indefinite blocking or starvation
• Solution aging
• Gradually increase the priority of processes that
  wait for a long time

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6.3.4 Round-Robin Scheduling

• A small unit of time, called a time quantum ( or
  time slice) is defined.
• Generally from 10 to 100 milliseconds
• The ready queue is treated as a circular, FIFO
  queue.
• The CPU scheduler goes around the ready queue
• Allocate the CPU to each process for a time
  interval of up to 1 time quantum.
• Designed especially for time-sharing systems
• Two cases
• CPU burst less than 1 time quantum
• The process release the CPU voluntarily
• CPU burst longer than 1 time quantum
• Context switch will be executed

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Example of round-robin scheduling

• Process Burst Time
• P1 24
• P2 3
• P3 3
• Round-robin scheduling
• A time-quantum of 4 milliseconds
• Average waiting time ( 6 4 7 ) / 3 5.66
• Often quite long
• RR scheduling is preemptive.

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6.3.4 Round-Robin Scheduling

• n processes in the ready queue and the time
  quantum is q
• Each process gets 1/n of the CPU time in chunks
  of at most q time units at once
• No process waits more than (n-1)q time units
• Performance
• Depend heavily on the size of the time quantum
• q is very large the same as the FCFS policy
• q is very small called processor sharing
• q must be large with respect to context switch,
  otherwise overhead is too high.

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A smaller time quantumincreases context switches
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6.3.4 Round-Robin Scheduling

• Turnaround time also depends on the size of the
  time quantum.
• Rule of thumb
• 80 percent of the CPU burst shouldbe shorter
  thanthe time quantum

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6.3.5 Multilevel Queue Scheduling

• Processes are classified into different groups
• Foreground (or interactive) processes
• Background (or batch) processes
• Multilevel queue-scheduling algorithm
• Partition the ready queue into several separate
  groups
• Each group has its own scheduling algorithm
• Scheduling among the queues
• Fixed-priority preemptive scheduling
• Possibility of starvation
• Time-slice between the queues
• A certain portion of the CPU time

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Multilevel Queue Scheduling
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6.3.6 Multilevel Feedback Queue

• A process can move between the various queues
• Use too much CPU time move to a lower-priority
  queue
• Wait too long move to a higher-priority queue
• This form of aging prevents starvation
• Multilevel-feedback-queue scheduler defined by
• 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 ofMultilevel Feedback Queue

• Three queues
• Q0 time quantum 8 milliseconds
• Q1 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

• The most general and complex scheme

Q0
Q1
Q2
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Chapter 6 CPU Scheduling

1. Basic Concepts
2. Scheduling Criteria
3. Scheduling Algorithms
4. Multiple-Processor Scheduling
5. Real-Time Scheduling

1. Algorithm Evaluation
2. Process Scheduling Models
3. Summary
4. Exercises

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6.4 Multiple-Processor Scheduling

• CPU scheduling is more complex when multiple CPUs
  are available
• Homogeneous system
• Identical processors
• Load sharing can occur
• Separate queue
• Common queue
• Self-scheduling
• Master-slave structure
• Heterogeneous system
• Different processors

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Chapter 6 CPU Scheduling

1. Basic Concepts
2. Scheduling Criteria
3. Scheduling Algorithms
4. Multiple-Processor Scheduling
5. Real-Time Scheduling

1. Algorithm Evaluation
2. Process Scheduling Models
3. Summary
4. Exercises

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6.5 Real-Time Scheduling

• Hard real-time systems
• complete a critical task within a guaranteed
  amount of time.
• Special-purpose software running on dedicated
  hardware
• Lack the full functionality of modern computers
  and operating systems
• Soft real-time computing
• Critical processes receive priority over less
  fortunate ones
• May cause unfair allocation of resources
• A general-purpose system can also support special
  tasks

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6.5 Real-Time Scheduling

• Soft real-time computing
• Priority scheduling
• Real-time process must have the highest priority
• The dispatch latency must be small
• OSs are forced to wait for a system call to
  complete or an I/O block to take place before
  doing a context switch
• Solution system calls must be preemptible
• Insert preemption points
• Make the entire kernel preemptible
• Effective and complex method
• Solaris 2
• Over 100 ms v.s. 2 ms

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Dispatch Latency
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Chapter 6 CPU Scheduling

1. Basic Concepts
2. Scheduling Criteria
3. Scheduling Algorithms
4. Multiple-Processor Scheduling
5. Real-Time Scheduling

1. Algorithm Evaluation
2. Process Scheduling Models
3. Summary
4. Exercises

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6.6 Algorithm Evaluation

• How to select a CPU-scheduling algorithm for a
  particular system?
• Define the criteria
• CPU utilization
• Response time
• Throughput
• A criteria may include several measures
• Maximize CPU utilization under the constraint
  that the maximum response time is 1 second.
• Maximize throughput such that turnaround time is
  linearly proportional to total execution time.

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6.6.1 Deterministic Modeling

• Take a particular predetermined workload and
  defines the performance of each algorithm for
  that workload
• Example
• Assume five processes arrive at time 0, in the
  order given, with the length of the CPU-burst
  time given in milliseconds
• Process Burst Time Priority
• P1 10 3
• P2 1 1
• P3 2 4
• P4 1 5
• P5 5 2
• Consider the FCFS, SJF, and RR(quantum10
  milliseconds) scheduling algorithms for this set
  of processes.
• Which gives the minimum average waiting time?

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6.6.1 Deterministic Modeling

• Advantage
• Simple and fast
• Exact numbers
• Disadvantage
• Too specific, and require too much exact
  knowledge, to be useful
• Main use
• Describing scheduling algorithms and providing
  examples

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6.6.2 Queueing Models

• No static set of processes (and times)
• What can be determined
• The distribution of CPU and I/O bursts
• Measured and approximated by mathematical
  formulas
• Queueing-network analysis
• CPU is a server with its ready queue
• I/O system is a server with its device queue
• Knowing arrival rates and service rates
• Utilization, average queue length, average wait
  time

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6.6.2 Queueing Models

• Littles formula n l W
• n average queue length
• W average waiting time
• l average arrival rate for new processes
• Advantage
• Useful in comparing scheduling algorithms
• Disadvantage
• Approximation only
• Fairly limited applicability

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6.6.3 Simulations

• Programming a model of the computer system
• Simulator
• A variable representing a clock
• Modify the system state to reflect the activities
  of the devices, the processes, and the scheduler
• Data generation
• Random-number
• Probability distribution
• Empirically
• Trace tapes
• Monitor the real system
• Expensive
• More detailed simulation provides more accurate
  results

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Evaluation of CPU schedulersby simulation
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6.6.4 Implementation

• A simulation is of limited accuracy.
• The only completely accurate way
• code it
• put it in the OS
• see how it work
• The cost is high
• In coding
• In the reaction of users

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Chapter 6 CPU Scheduling

1. Basic Concepts
2. Scheduling Criteria
3. Scheduling Algorithms
4. Multiple-Processor Scheduling
5. Real-Time Scheduling

1. Algorithm Evaluation
2. Process Scheduling Models
3. Summary
4. Exercises

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6.7 Process Scheduling Models

• User-level threads
• Managed by a thread library
• Kernel is unaware of them
• Process local scheduling
• Thread scheduling is done local to the
  application
• System global scheduling
• Decide which kernel thread to schedule
• Case study
• Solaris 2
• Windows 2000
• Linux

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Chapter 6 CPU Scheduling

1. Basic Concepts
2. Scheduling Criteria
3. Scheduling Algorithms
4. Multiple-Processor Scheduling
5. Real-Time Scheduling

1. Algorithm Evaluation
2. Process Scheduling Models
3. Summary
4. Exercises

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Summary

• P.184 to 185

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Chapter 6 CPU Scheduling

1. Basic Concepts
2. Scheduling Criteria
3. Scheduling Algorithms
4. Multiple-Processor Scheduling
5. Real-Time Scheduling

1. Algorithm Evaluation
2. Process Scheduling Models
3. Summary
4. Exercises

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Exercises

• 6.2
• 6.3
• 6.4
• 6.7
• 6.10

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The End