CPU Scheduling

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CPU Scheduling

• Chapter 6

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Chapter 6 CPU Scheduling

• Basic Concepts
• Scheduling Criteria
• Scheduling Algorithms
• Multiple-Processor Scheduling
• Real-Time Scheduling
• Algorithm Evaluation

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Basic Concepts

• Maximum CPU utilization obtained with
  multiprogramming
• CPUI/O Burst Cycle Process execution consists
  of a cycle of CPU execution and I/O wait.
• CPU burst distribution

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Ready Queue I/O Device Queues
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Process Scheduling Queues

• Job queue set of all processes in the system.
• Ready queue set of all processes residing in
  main memory,ready and waiting to execute.
• Device queues set of processes waiting for an
  I/O device.
• Process migration between the various queues.

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Schedulers

• Long-term scheduler (or job scheduler) selects
  which processes should be brought into the ready
  queue.
• Short-term scheduler (or CPU scheduler) selects
  which process should be executed next and
  allocates CPU.

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Addition of Medium Term Scheduling
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Scheduler Duration Types

• Short-term scheduler is invoked very frequently
  (milliseconds)? (must be fast).
• Long-term scheduler is invoked very infrequently
  (seconds, minutes) ? (may be slow).
• The long-term scheduler controls the degree of
  multiprogramming.
• Processes can be described as either
• I/O-bound process spends more time doing I/O
  than computations, many short CPU bursts.
• CPU-bound process spends more time doing
  computations few very long CPU bursts.

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

• Selects from among the processes in memory that
  are ready to execute, and allocates the CPU to
  one of them.
• CPU scheduling decisions may take place when a
  process
• 1. Switches from running to waiting state.
• 2. Switches from running to ready state.
• 3. Switches from waiting to ready.
• 4. Terminates.
• Scheduling under 1 and 4 is nonpreemptive.
• All other scheduling is preemptive.

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Diagram of Process States
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Representation of Process Scheduling
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Scheduler Types

• Nonpreemptive a process retains control of the
  CPU until blocked or terminated.
• Preemptive the scheduler may preempt a process
  before it blocks or terminates, in order to
  allocate the CPU to another process.
• Preemptive scheduling is necessary on
  user-interactive systems

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Dispatcher

• Dispatcher module 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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CPU Process Switching
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Scheduling Criteria

• CPU utilization the percentage of time the CPU
  is executing a process
• Throughput number of processes that complete
  their execution per time unit
• Turnaround time amount of time to execute a
  particular process
• Waiting time amount of time a process has been
  waiting in the ready queue
• Response time the time it takes the system to
  start responding to user inputs

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Scheduling Criteria

• Balanced Utilization the percentage of time
  that all resources memory, I/O, CPU are
  utilized
• Predictability lack of variability, e.g. 2 sec
  regular response better than 10 sec on occasion
• Fairness the degree to which all processes are
  given equal opportunity
• Priority give preferential treatment to
  processes with higher priorities
• Proportionality response time matches user
  expectations, e.g. expect short time to stop job
• Optimize minimum, maximum, average or variance

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Scheduling Criteria for Different Systems

• All systems
• Fairness equal opportunity for all processes
• Balance keeping all parts of the system busy
• Policy enforcement seeing that stated policy is
  carried out
• Batch Systems
• Throughput maximize jobs per hour
• Turnaround time minimize time between
  submission
• and termination
• Waiting time minimize time in the waiting queue
• CPU utilization keep the CPU busy all the time
• e.g. varies 40 90 utilization

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Scheduling Criteria for Different Systems

• Interactive Systems
• Response time respond to requests quickly
• Proportionality meet users expectations
• Real-time Systems
• Meeting deadlines respond to requests quickly
• Predictability avoid quality degradation
• e.g. in multimedia systems

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Scheduling Algorithms

• First-come, first-served FCFS
• Shortest job first Nonpreemptive SJF
• Shortest remaining time SRTF Preemptive SJF
• Priority scheduling
• Round robin scheduling RR
• Multilevel queue scheduling
• Multilevel feedback queue MRQ

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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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First-Come, First-Served (FCFS) Scheduling

• Process Burst Time
• P1 24
• P2 3
• P3 3
• Suppose that the processes arrive in the order P2
  , P3 , P1 .
• Waiting time for P1 6 P2 0 P3 3
• Average waiting time (6 0 3)/3 3

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Problems with FCFS Scheduling

• Big variations in average waiting time.
• Convoy effect as short processes follow a long
  process from CPU to I/O and back

CPU
I/O
P1
P2
P3
0
30
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Shortest-Job-First (SJR) Scheduling

• Associate with each process the length of its
  next CPU burst. Use these lengths to schedule
  the process with the shortest estimated time.
• Two schemes
• Nonpreemptive 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.
• Depends on good prediction of the burst length

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Example of Nonpreemptive SJF

• Process Arrival Time Burst Time
• P1 0 7
• P2 2 4
• P3 4 1
• P4 5 4
• Average waiting time (0 6 3 7)/4 4

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Example of Preemptive SJF

• Process Arrival Time Burst Time
• P1 0 7
• P2 2 4
• P3 4 1
• P4 5 4
• Average waiting time (9 1 0 2)/4 3

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Prediction of the Length of the Next CPU Burst
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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.

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Examples of Exponential Averaging

• If ? 0
• ?n1 ?n
• Prediction based on stored values, ignores recent
  history.
• If ? 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 - ? )n1 ?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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Problems in SJF Scheduling

• SJF favours short jobs over long jobs.
• In extreme cases, the constant arrival of short
  jobs can lead to starvation of a long job
• Nonpreemptive SJF works best when jobs arrive
  simultaneously
• Short jobs arriving later can be delayed by an
  earlier long job
• Preemptive SJF depends on correct prediction of
  burst times

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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 (say, smallest integer ? highest
  priority).
• Preemptive
• Nonpreemptive
• SJF is a priority scheduling scheme in which
  priority is the predicted next CPU burst time.
• Problem ? Starvation, or indefinite blocking
• low priority processes may never execute.
• Solution ? Aging as time progresses increase
  the priority of the process.

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Priority Dispatch Queues
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Priority Dispatch Queues

• The kernel maintains a priority dispatch queue
  for each priority value in the system.
• Dispatch queues are placed in array dispq
• Each element in dispq is a dispq structure
• Each dispq structure consists of a list of
  runnable proc structures, each linked via their
  p_link member.
• The scheduling algorithms select the process with
  the highest priority to use the CPU next.

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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 (tail) 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 in chunks of at most q time units
  at once. No process waits more than (n-1)q time
  units.
• Performance
• q large ? FIFO
• q small ? q must be large with respect to context
  switch, otherwise overhead is too high.

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Example of RR with Time Quantum 4

• Process Burst Time
• P1 24
• P2 3
• P3 3
• Average waiting time (0 0 17)/3 5.66
• (Average waiting under RR is quite long)

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Example of RR with Time Quantum 20

• Process Burst Time
• P1 53
• P2 17
• P3 68
• P4 24
• Typically, higher average turnaround than SJF,
  but better response time.

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Time Quantum and Context Switch Time
Smaller time quantum means more time spent in
context switching.
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Turnaround Time Varies With The Time Quantum
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Turnaround Time Varies With The Time Quantum

• Generally, larger time quantum
• gt shorter average turnaround time
• If time quantum too large, get FCFS scheduling
• Rule of thumb
• 80 of CPU bursts should be shorter
• than the time quantum

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Scheduling Algorithms

• First-come, first-served Variable waiting convoy
  effect
• Shortest job first Shortest waiting for same
  start time
• Shortest remaining time Shortest waiting, with
  good predict
• Priority scheduling Enforces policy beware
  starvation
• Round robin scheduling Best response for time
  share
• Multilevel queue scheduling
• Multilevel feedback queue