ITEC 502 ??? ??? ? ??

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ITEC 502 ??? ??? ? ??

• Chapter 5
• CPU Scheduling
• Mi-Jung Choi
• mjchoi_at_postech.ac.kr
• DPNM Lab., Dept. of CSE, POSTECH

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Contents

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

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Basic Concepts

• Process Scheduling is
• the basis of multi-programmed operating system
• Terminology
• CPU scheduling, Process scheduling, Kernel Thread
  scheduling
• used interchangeably, we use process scheduling

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Basic Concepts

• In a single-processor system
• Only one process can run at a time
• Any others must wait until the CPU is free and
  can be rescheduled
• When the running process goes to the waiting
  state,
• the OS may select another process to assign CPU
  to improve CPU utilization
• Every time one process has to wait, another
  process can take over use of the CPU
• Process scheduling is
• a fundamental function of operating-system
• to select a process from the ready queue and
  assign the CPU
• Maximum CPU utilization obtained with
  multiprogramming
• by process scheduling

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Diagram of Process State

• It is important to realize that only one process
  can be running on any processor at any instant
• Many processes may be ready and waiting states

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Process Scheduling

• Two types of queues one ready queue, a set of
  device queues
• Two types of resources CPU, I/O
• Arrow indicates the flow of processes in the
  system

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CPU - I/O Burst Cycle

• Process execution consists of
• a cycle of CPU execution (CPU burst) and I/O
  wait (I/O burst)
• Process alternate between these two states
• Process execution begins with a CPU burst, which
  is followed by an I/O burst, and so on
• Eventually, the final CPU burst ends with a
  system call to terminate execution
• CPU burst distribution of a process
• varies greatly from process to process and from
  computer to computer

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SchedulingIntroduction to Scheduling

• Bursts of CPU usage alternate with periods of I/O
  wait
• a CPU-bound process
• an I/O bound process

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Alternating Sequence of CPU I/O Bursts
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Histogram of CPU-burst Times

• CPU burst distribution is generally characterized
  
• as exponential or hyper-exponential
• with large number of short CPU burst and small
  number of long CPU burst
• I/O bound process has many short CPU bursts
• CPU bound process might have a few long CPU bursts

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Process Scheduler

• is one of OS modules
• selects one of the processes in memory that are
  ready to execute, and allocates the CPU to the
  selected process
• CPU scheduling decisions may take place when a
  process
• 1. switches from running to waiting state I/O
  request, invocation of wait() for the termination
  of other process
• 2. switches from running to ready state when
  interrupt occurs
• 3. switches from waiting to ready at completion
  of I/O
• 4. terminates
• Scheduling under 1 and 4 is non-preemptive
• Scheduling under 2 and 3 is preemptive

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Non-preemptive vs. Preemptive

• Non-preemptive scheduling
• Once the CPU has been allocated to a process, the
  process keeps the CPU until it releases the CPU
• either by terminating or by switching to the
  waiting state
• used by Windows 3.x
• Preemptive scheduling
• Current running process can be switched with
  another at any time
• because interrupt can occur at any time
• Most of modern OS provides this scheme (Windows
  XP, Mac OS X, UNIX)
• incurs a cost associated with access to shared
  data among processes
• affects the design of the OS kernel
• Certain OS (UNIX) waits either for a system call
  to complete or for an I/O block to take place
  before doing a context switch
• protects critical kernel code by disabling and
  enabling the interrupt at the entry and exit of
  the code

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Dispatcher

• Dispatcher module is a part of a Process
  Scheduler
• gives control of the CPU to the process selected
  by the short-term scheduler this involves
• 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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Context Switch
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Scheduling Criteria

• Based on the scheduling criteria, the performance
  of various scheduling algorithm could be
  evaluated
• Different scheduling algorithms have different
  properties
• CPU utilization ratio () of the amount of time
  while the CPU was busy per time unit
• Throughput of processes that complete their
  execution per time unit
• Turnaround time the interval from the time of
  submission of a process to the time of
  completion. Sum of the periods spent waiting to
  get into memory, waiting in the ready queue,
  executing on the CPU, and doing I/O
• Waiting time Amount of time a process has been
  waiting in the ready queue, which is affected by
  scheduling algorithm
• Response time In an interactive system, 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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Optimization Criteria

• It is desirable to maximize
• The CPU utilization
• The throughput
• It is desirable to minimize
• The turnaround time
• The waiting time
• The response time
• However in some circumstances, it is desirable to
  optimize the minimum or maximum values rather
  than the average
• Interactive systems, it is more important to
  minimize the variance in the response time than
  minimize the average response time

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Process Scheduling Algorithms

• First-Come, First-Served Scheduling (FCFS)
• Shortest-Job-First Scheduling (SJF)
• Priority Scheduling
• Round-Robin Scheduling
• Our measure of comparison is the average waiting
  time
• CPU utilization, Throughput, Turnaround time

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First-Come, First-Served (FCFS) 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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FCFS 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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FCFS Scheduling
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FCFS Scheduling

• FCFS scheduling algorithm is non-preemptive
• Once the CPU has been allocated to a process,
  that process keeps the CPU until it releases the
  CPU, either by terminating or by requesting I/O
• is particularly troublesome for time-sharing
  systems
• Convoy effect occurs
• When one CPU-bound process with long CPU burst
  and many I/O-bound process which short CPU burst
• All I/O bound process waits for the CPU-bound
  process to get off the CPU while I/O is idle
• All I/O- and CPU- bound processes executes I/O
  operation while CPU is idle
• results in low CPU and device utilization

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Shortest-Job-First (SJF) Scheduling

• associate with each process the length of its
  next CPU burst
• use these lengths to schedule the process with
  the shortest time
• Two schemes
• non-preemptive once CPU given to the process it
  cannot be preempted until completes its CPU burst
• preemptive if a new process arrives with CPU
  burst length less than remaining time of current
  executing process, preempt. This scheme is
  known as the Shortest-Remaining-Time-First (SRTF)
• SJF is optimal gives minimum average waiting
  time for a given set of processes

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Example of Non-Preemptive 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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Example of Preemptive 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

0
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Preemptive SJF Scheduling
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Non-preemptive SJF Scheduling
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Determining Length of Next CPU Burst

• can only estimate the length
• can be done by using the length of previous CPU
  bursts, using exponential averaging
• The value of tn contains our most recent
  information
• ?n stores the past history
• The parameter ? controls the relative weight of
  recent and past history in our prediction

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Prediction of the Length of the Next CPU Burst

• In this example, ?0 10, ? ½
• ?1 ? x t0 (1- ?) x ?0 ½ x 6 ½ x 10 8
• ?2 ? x t1 (1- ?) x ?1 ½ x 4 ½ x 8 6

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Examples of Exponential Averaging

• ? 0
• ?n1 ?n ?n-1 ?n-2 . ?0
• 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 -j
• (1 - ? )n 1 ?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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Priority Scheduling

• A priority number (integer) is associated with
  each process
• The CPU is allocated to the process with the
  highest priority (smallest integer ? highest
  priority)
• Preemptive
• Non-preemptive
• SJF is a priority scheduling where priority is
  the predicted next CPU burst time
• Problem ? Starvation low priority processes may
  never execute
• Solution ? Aging as time progresses, increase
  the priority of the process

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Example of Non-Preemptive Priority

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

P2
P5
P4
P1
P3
16
6
18
19
0
1
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Preemptive Priority Scheduling
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Non-preemptive Priority Scheduling
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Round Robin (RR) Scheduling

• Each process gets a small unit of CPU time (time
  quantum), usually 10-100 milliseconds
• After this time has elapsed, the process is
  preempted and added to the end of the ready queue
• If there are n processes in the ready queue and
  the time quantum is q, then each process gets 1/n
  of the CPU time ( q time units) in chunks of at
  most n x q time units at once
• No process waits more than (n-1) x q time units
• Performance depends on the size of the time
  quantum
• q large ? RR is same as FIFO
• q small ? provides high concurrency each of n
  processes has its own processor running at 1/n
  the speed of the real processor

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Example of RR with Time Quantum 20

• Process Burst Time
• P1 53
• P2 17
• P3 68
• P4 24
• The Gantt chart is
• Typically, higher average turnaround than SJF,
  but better response

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RR Scheduling (1)

• Time Quantum 2

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RR Scheduling (2)

• Time Quantum 5

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Time Quantum and Context Switch Time

• The effect of context switching on the
  performance of RR scheduling, for example one
  process of 10 time quantum
• quantum 12 time units, finished in less than 1
  time quantum
• quantum 6 time units, requires 2 quanta, 1
  context switch
• quantum 1 time units, requires 10 quanta, 9
  context switch

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Round Robin (RR) Scheduling

• The time quantum q must be large with respect to
  context switch, otherwise overhead is too high
• If the context switching time is 10 of the time
  quantum, then about 10 of the CPU time will be
  spent in context switching
• Most modern OS have time quanta ranging from 10
  to 100 milliseconds,
• The time required for a context switch is
  typically less than 10 microseconds thus the
  context-switch time is a small fraction of the
  time quantum

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Turnaround Time varies with the Time Quantum

• The turnaround time depends on the size ofthe
  time quantum
• The average turnaroundtime can be improvedif
  most processes finish their next CPU burst in a
  single time quantum
• When SJF and RR used
• If quantum 6 and 7, average turnaround time
  10.5
• If quantum 1, average turnaround time 11

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Scheduling Algorithm with multi-Queues

• Multi-level Queue Scheduling
• Multi-level Feedback Queue Scheduling

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Multilevel Queue

• Ready queue is partitioned into separate
  queues foreground (interactive) background
  (batch)
• The processes are permanently assigned to one
  queue, generally based on some property, or
  process type
• Each queue has its own scheduling algorithm
• foreground RR
• background FCFS
• Scheduling must be done between the queues
• Fixed priority scheduling - serve all from
  foreground then from background, Possibility of
  starvation
• Time slice scheduling each queue gets a certain
  amount of CPU time which it can schedule amongst
  its processes i.e., 80 to foreground in RR, 20
  to background in FCFS

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Multilevel Queue Scheduling

• No process in the batch queue could run unless
  the queues with high priority were all empty
• If an interactive editing process entered the
  ready queue while a batch process was running,
  the batch process would be preempted

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Multi-level Queue Scheduling

• Two-level Queues SJF with priority 0, FCFS with
  priority 1
• Fixed Priority Queue
• Preemptive

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Multilevel Feedback Queue

• A process can move between the various queues
  aging can be implemented in 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

• 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 RR.
  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 RR and receives 16
  additional milliseconds. If it still does not
  complete, it is preempted and moved to queue Q2
• The job is serverd based on FCFS in queue Q2

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Multilevel Feedback Queues
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Multi-level Feedback Queue Scheduling

• Three-level Queues
• RR with quantum 3, RR with quantum 6, FCFS with
  priority 1

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Scheduling Algorithms Goals
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Scheduling in Batch Systems (1)

• An example of shortest job first scheduling

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Scheduling in Batch Systems (2)

• Three level scheduling

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Scheduling in Interactive Systems (1)

• Round Robin Scheduling
• list of runnable processes
• list of runnable processes after B uses up its
  quantum

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Scheduling in Interactive Systems (2)

• A scheduling algorithm with four priority classes

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Multiple-Processor Scheduling

• CPU scheduling more complex when multiple CPUs
  are available
• Homogeneous processors within a system or
  heterogeneous processors within a system
• Asymmetric multiprocessing vs. Symmetric
  multiprocessing (SMP)
• Symmetric Multiprocessing (SMP) each processor
  makes its own scheduling decisions
• Asymmetric multiprocessing only one processor
  accesses the system data structures, alleviating
  the need for data sharing
• Load sharing on SMP system
• keeps the workload evenly distributed across all
  processors in an SMP system
• Push migration vs. Pull migration

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Real-Time Scheduling

• Hard real-time systems required to complete a
  critical task within a guaranteed amount of time
• Soft real-time computing requires that critical
  processes receive priority over less fortunate
  ones

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Scheduling in Real-Time Systems

• Schedulable real-time system
• Given
• m periodic events
• event i occurs within period Pi and requires Ci
  seconds
• Then the load can only be handled if

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Policy versus Mechanism

• Separate what is allowed to be done with how it
  is done
• a process knows which of its children threads are
  important and need priority
• Scheduling algorithm parameterized
• mechanism in the kernel
• Parameters filled in by user processes
• policy set by user process

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Thread Scheduling (1)

• On operating systems that support kernel-level
  thread, it is kernel-level threads, not
  processes, that are being scheduled by the
  operating system.
• Local Scheduling How the threads library
  decides which thread to put onto an available LWP
• Global Scheduling How the kernel decides which
  kernel thread to run next

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Thread Scheduling (2)

• Possible scheduling of user-level threads
• 50-msec process quantum
• threads run 5 msec/CPU burst

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Thread Scheduling (3)

• Possible scheduling of kernel-level threads
• 50-msec process quantum
• threads run 5 msec/CPU burst

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Summary

• CPU scheduling is the task of selecting a waiting
  process from the ready queue and allocating the
  CPU to it
• The CPU is allocated to the selected process by
  the dispatcher
• FCFS scheduling is simple, cause short processes
  to wait for long time
• SJF scheduling is provably optimal, providing the
  shortest averaging waiting time. But predicting
  the length of the next CPU bursts is difficult
• Priority scheduling allocates the CPU to the
  heights priority process
• Both priority and SJF may suffer from starvation.
  Aging is a technique to prevent starvation
• RR scheduling is more appropriate for a
  time-shared system
• Major problem of RR scheduling is the selection
  of the time quantum
• FCFS is non-preemptive, RR is preemptive, SJF and
  Priority may be preemptive and non-preemptive
• Multilevel queue allows different scheduling
  algorithm for each queue
• Multilevel feedback queue allow process to move
  from one queue to another

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Review

• Basic Concepts
• Scheduling Criteria
• Scheduling Algorithms
• Multiple-Processor Scheduling
• Real-Time Scheduling
• Thread Scheduling