Process Scheduling

1
Chapter 5

• Process Scheduling

2
Outline

• Basic Concepts
• CPU-I/O Burst Cycle
• CPU Scheduler
• Preemptive Scheduling
• Dispatcher
• Scheduling Criteria
• Scheduling Algorithms
• First-Come, First-Serve Scheduling
• Shortest-Job-First Scheduling
• Priority Scheduling
• Round-Robin Scheduling
• Multilevel Queue Scheduling
• Multilevel Feedback-Queue Scheduling

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Basic Concepts

• CPU One of the primary computer resource
• Maximum CPU utilization obtained with
  multiprogramming
• Several processes are kept in memory
• CPUI/O Burst Cycle Process execution consists
  of a cycle of CPU execution and I/O wait.
• CPU burst distribution determines how to schedule
  the processes.

4
Alternating Sequence of CPU And I/O Bursts
5
Histogram of CPU-burst Times
Normally a process starts with frequent short
CPU bursts and then it stabilizes and shifts to
long but infrequent CPU bursts

• I/O bound processes usually have many small CPU
  bursts
• CPU bound programs usually have few but long CPU
  bursts

CPU-bound program
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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.
• Is it the same as Short-term scheduler? Yes
• 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.(Own will)
• Scheduling Scheme given at 1 and 4 is
  nonpreemptive (Cooperative).
• All other scheduling is preemptive.

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Preemptive vs Non PreemptiveScheduling

• Non Preemptive scheduling
• If a process is being executed then it keeps the
  CPU until one or four conditions of previous
  slide occur.
• MS Windows 3.x which was sort of multitasking
  system used this method
• Not at all efficient.
• Preemptive
• A process in running state can be brought back to
  ready state without any of the conditions of
  previous slide being met.
• Can you guess how it is done? ?

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Problems with preemptive

• Although better than non-preemptive still suffers
  from synchronization problems
• E.g.
• Process P and Q share some data (can be memory
  location or a file)
• P does some calculations changes some of the data
  but before it finishes its time expires
• Q reads the data but is the data valid?

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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.
• Should be minimum.

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Scheduling CriteriaFollowing criteria is kept
under consideration while designing a scheduler

• CPU utilization Keep the CPU as busy as
  possible
• Throughput of processes that complete their
  execution per unit time (Given Time).
• Turnaround time Amount of time to execute a
  particular process (Sum of ready, waiting,
  running, loading times,I/O)
• Waiting time Amount of time a process has been
  waiting in the ready queue
• Response time Amount of time it takes from when
  a request was submitted until the first response
  is produced, not output (for interactive
  environment)
• Time it takes to start responding
• Not the time it takes to output the response

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Optimization Criteria

• So the best CPU Scheduling algorithm is which
• Maximizes CPU utilization
• Maximizes throughput
• Minimizes turnaround time
• Minimizes waiting time
• Minimizes response time

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Outline

• Basic Concepts
• CPU-I/O Burst Cycle
• CPU Scheduler
• Preemptive Scheduling
• Dispatcher
• Scheduling Criteria
• Scheduling Algorithms
• First-Come, First-Serve Scheduling
• Shortest-Job-First Scheduling
• Priority Scheduling
• Round-Robin Scheduling
• Multilevel Queue Scheduling
• Multilevel Feedback-Queue Scheduling

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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 (Cont.)

• 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.
• CPU bound processes will get and hold the CPU
• Convoy effect short process behind long process
  (Bad CPU utilization)

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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 time.
• Two schemes
• nonpreemptive once CPU given to the process it
  cannot be preempted until it 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 know
  as the Shortest-Remaining-Time-First (SRTF).
• SJF is optimal gives minimum average waiting
  time for a given set of processes.
• Shortest-next-cpu-burst algorithm

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Example of Non-Preemptive SJF

• Process Burst Time
• P1 6
• P2 8
• P3 7
• P4 3
• SJF
• Average waiting time (31690)/47
  FCFS10.25

8
P1
P3
P2
P4
3
24
0
16
9
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Shortest-Job-First (SJR) Scheduling

• Good for long-term Scheduler in batch systems
• User specifies process time during job submission
• Lower value may mean faster response
• Too low a value will cause a time-limit-exceeded
  error
• Difficult to implement at the level of short-term
  CPU scheduler
• Determining Length of Next CPU Burst
• Can only predict/ estimate the length of next CPU
  burst.
• Can be done by using the length of previous CPU
  bursts, using exponential averaging.

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Examples of Exponential Averaging

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

?n1 atn (1- a) ?n tn length of the nth CPU
burst ?n1 Our predicted Value a weight of
recent past history 0lt alt1
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Prediction of the Length of the Next CPU Burst
Initial guess for T(1) 10 ms Actual
6 Determination of 2 Cycle based on actual (6)
and last guess (10) T(2) 0.5 x 6 0.5 x 10 8
ms Determination of 3 cycle based on actual(4)
and last guess (8) T(3) 0.5 x 4 0.5 x 8 6
ms Now you can do it ?
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• For example Suppose a process p is given a
  default expected burst length of 5 time units.
  When it is run, the actually burst lengths are
  10,10,10,1,1,1 (although this information is not
  known in advance to any algorithm). The
  prediction of burst times for this process works
  as follows.
• Let e(1) 5, as a default value.
• When process p runs, its first burst actually
  runs 10 time units, so,
• a(1) 10.
• e(2) 0.5 e(1) 0.5 a(1) 0.5 5
  0.5 10 7.5
• This is the prediction for the second cpu burst
• The actual second cpu burst is 10. So the
  prediction for the third cpu burst is
• e(3) 0.5 e(2) 0.5 a(2) 0.5 7.5
  0.5 10 8.75
• e(4) 0.5 e(3) 0.5 a(3) 0.5 8.75
  0.5 10 9.38,
• So, we predict that the next burst will be close
  to 10 (9.38) because the recent bursts have
  been of length 10. At this point, it happens
  that the process starts having shorter bursts,
  with a(4) 1, so the algorithm gradually adjusts
  its estimated cpu burst (prediction)
• e(5) 0.5 e(4) 0.5 a(4) 0.5 9.38
  0.5 1 5.19
• e(6) 0.5 e(5) 0.5 a(5) 0.5 5.19
  0.5 1 3.10
• e(7) 0.5 e(6) 0.5 a(6) 0.5 3.10
  0.5 1 2.05

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Example of Preemptive SJF(Shortest Remaining
Time first)

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

P1
P2
P1
P3
P4
17
1
0
10
5
26
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Outline

• Basic Concepts
• CPU-I/O Burst Cycle
• CPU Scheduler
• Preemptive Scheduling
• Dispatcher
• Scheduling Criteria
• Scheduling Algorithms
• First-Come, First-Serve Scheduling
• Shortest-Job-First Scheduling
• Priority Scheduling
• Round-Robin Scheduling
• Multilevel Queue Scheduling
• Multilevel Feedback-Queue Scheduling

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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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Round Robin (RR)

• 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 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 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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Time Quantum and Context Switch Time

• If context switching time is approx 10 percent of
  the time quantum, then about 10 percent of the
  CPU time will be spent in context switching
• Most modern systems have time quanta ranging from
  10 to 100 milliseconds
• Context switch typically takes less than 10
  microseconds

Remember Context switch takes time ?
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Turnaround Time Varies With The Time Quantum
80 of CPU bursts should be shorter than the time
quantum

• Average Turnaround time doesnt necessarily
  improve as the time-quantum size increases)

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Outline

• Basic Concepts
• CPU-I/O Burst Cycle
• CPU Scheduler
• Preemptive Scheduling
• Dispatcher
• Scheduling Criteria
• Scheduling Algorithms
• First-Come, First-Serve Scheduling
• Shortest-Job-First Scheduling
• Priority Scheduling
• Round-Robin Scheduling
• Multilevel Queue Scheduling
• Multilevel Feedback-Queue Scheduling

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Multilevel Queue

• Ready queue is partitioned into separate
  queuesforeground (interactive)background
  (batch)
• Each queue has its own scheduling algorithm,
  foreground RRbackground FCFS
• Scheduling must be done between the queues.
• Fixed priority scheduling (i.e., serve all from
  foreground then from background). Possibility of
  starvation.
• Time slice 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
31
Multilevel Feedback Queue

• 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

• Consider a case if we have 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 tail of 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 tail of
  queue Q2.

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Multilevel Feedback Queues
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Outline

• Basic Concepts
• CPU-I/O Burst Cycle
• CPU Scheduler
• Preemptive Scheduling
• Dispatcher
• Scheduling Criteria
• Scheduling Algorithms
• First-Come, First-Serve Scheduling
• Shortest-Job-First Scheduling
• Priority Scheduling
• Round-Robin Scheduling
• Multilevel Queue Scheduling
• Multilevel Feedback-Queue Scheduling
• Multi-Processor Scheduling

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Multiple-Processor Scheduling

• CPU scheduling more complex when multiple CPUs
  are available.
• Consider Homogeneous processors within a
  multiprocessor system.
• Load sharing
• Asymmetric multiprocessing only one processor
  accesses the system data structures, alleviating
  the need for data sharing.
• SMP Each processor is self scheduling
• May have separate ready queues

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Multiple-Processor Scheduling Process Affinity

• Cache memory?
• The Data most recently accessed populates it
• Process migration to another process
• Invalidation Repopulation
• Some OS attempt to keep a process running on the
  same processor Affinity
• Soft Affinity Attempt to keep a process running
  on the same processor but not guaranteeing
• Hard Affinity Process cannot migrate to another
  processor

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Multiple-Processor Scheduling Load Balancing

• Utilize the full benefits of multi-processors
• Load balancing Distribute the workload evenly
  across all processors.
• Necessary where each processors has its own
  private queue of eligible processes
• Not necessary where processors have common run
  queue
• Modern OS --private queue or common?
• Push Migration move processes from overloaded to
  idle processors
• Pull Migration Idle processor pulls a waiting
  process from a busy processor
• Any link with Process Affinity?

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Multiple-Processor Scheduling Symmetric
Multithreading

• Multiple logical rather than physical processors
  to run several threads concurrently (HPT)
• Create logical processors on the same physical
  processora view of many Processors to OS
• Each has its own machine-state registers
• Own interrupt handling

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Multiple-Processor Scheduling Symmetric
Multithreading

• SMT a feature provided by hardware
• Os neednt be designed differently
• Performance gains If OS is aware.
• But why?
• Scheduler

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Outline

• Basic Concepts
• CPU-I/O Burst Cycle
• CPU Scheduler
• Preemptive Scheduling
• Dispatcher
• Scheduling Criteria
• Scheduling Algorithms
• First-Come, First-Serve Scheduling
• Shortest-Job-First Scheduling
• Priority Scheduling
• Round-Robin Scheduling
• Multilevel Queue Scheduling
• Multilevel Feedback-Queue Scheduling
• Multi-Processor Scheduling
• OS Examples Self Study
• Algorithm Evaluation

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Algorithm Evaluation

• How do we select the best algorithm?
• Defining a criteria
• CPU Utilization, response time, throughput
• Criteria may include several measures
• e.g. Maximize CPU utilization and Response time
  is 1
• e.g. Maximize Throughput and turnaround time
• Analytic Evaluation Method
• The algorithm and some system workload are used
  to produce a formula or number which gives the
  performance of the algorithm for that workload.
• Deterministic modeling
• Queuing models
• Simulations
• Implementation

42
Deterministic Modeling

• Predetermined workload defines performance of
  each algorithm for that workload. Use of Gantt
  charts.
• Simple and fast
• Exact numbers for comparison
• Answers apply only for the cases considered.
• Performance figures may not be true in general
• Suitable If we are running the same program over
  and over again. We can easily select an
  algorithm.
• Over a set of examples certain trends can be
  analyzed, e.g. If all the processes arrive at
  time 0, the SJF policy will always result in min
  waiting time
• If the Arriving time is different the SJF may not
  always result in min average waiting time.

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Deterministic Modeling

• Process Arrival Time Burst Time
• P1 0.0 7
• P2 2.0 4
• P3 4.0 1
• P4 5.0 4
• Gantt chart
• Average waiting time (9 1 0 2)/4 3

44
Queuing Modeling

• What if the processes that are run, vary from day
  to day? --Is deterministic method useful?
• The distributions of CPU and I/O bursts can be
  determined/ approximated/ estimated.
• Mathematical formula probability of a particular
  CPU burst
• Arrival times distribution can also be estimated.
• Computer system viewed as a network of queues and
  servers ready queue, I/O queue, event queues,
  CPUs, I/O device controllers, etc. e.g. CPU
  ready queue, I/O system device queue
• Input Arrival and service rates
• Output CPU utilization, average queue length,
  average waiting time,

45
Queuing Modeling

• Littles Formula
• n ? W
• where
• n average queue length
• ? average arrival rate
• W average waiting time in a
• queue

46
Queuing Modeling

• Let the average job arrival rate be 0.5

Algorithm Average Wait Time Wtw Average Queue Length(n)
FCFS 4.6 2.3
SJF 3.6 1.8
SRTF 3.2 1.6
RR (q1) 7.0 3.5
RR (q4) 6.0 3.0
47
Queuing Modeling

• Complicated mathematics
• Distributions (uniform, exponential, etc) for the
  arrival and departure rates can be difficult to
  work with
• Assumptions may not be accurate
• Approximation of the real system

48
Simulation

• To get more accurate evaluation
• Workload generated by assuming some distribution
  and a random number generator, or by collecting
  data from the actual system.
• The distributions can be defined mathematically
  or empirically.

49
Simulation
50
Simulation

• Characteristics
• Expensive hours of programming and execution
  time
• May be erroneous because of the assumptions about
  distributions

51
Implementation

• Even simulation is of limited accuracy
• Best way Code and implement the scheduling
  algorithm and see
• High cost coding the algorithm
• Modify the OS to support it
• Reaction of the users to changing OS
• Environment changes if short processes are given
  high priority then user may break their larger
  processes into many short ones
• For example Design of an automatic Interactive/
  non-interactive process classifier Algorithm