Threads, CPU Scheduling and Deadlocks

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Chapter 3

• Threads, CPU Scheduling and Deadlocks

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Objectives

• After finish this chapter, you will understand
• CPU Scheduling Concept
• CPU Scheduling Criteria
• Scheduling Algorithms
• What is Deadlock Problem?
• Deadlock Characterization
• Methods for Handling Deadlocks
• Deadlock Prevention
• Deadlock Avoidance

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I-Basic Concepts of CPU Scheduling

• CPU scheduling is the basis of multiprogrammed
  operating systems (Maximum CPU utilization)
• By switching the CPU among processes, the,
  operating system can make the computer more
  productive
• The success of CPU scheduling depends on the
  following observed property of processes Process
  execution consists of a cycle of CPU execution
  and I/O wait. CPUI/O Burst Cycle or Process
  execution consists of a cycle of CPU execution
  and I/O wait.

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Alternating Sequence of CPU And I/O Bursts
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CPU Scheduler

• Whenever the CPU becomes idle, the operating
  system must select one of the processes in the
  ready queue to be executed.
• The selection process is carried out by the
  short-term scheduler (or CPU scheduler).
• The scheduler selects from among the processes in
  memory that are ready to execute, and allocates
  the CPU to one of them.

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• 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.

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Scheduling Criteria

• CPU utilization keep the CPU as busy as
  possible
• 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 amount of time it takes from when
  a request was submitted until the first response
  is produced, not output.

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Optimization Criteria

• Maximized CPU utilization
• Maximized throughput
• Minimized turnaround time
• Minimized waiting time
• Minimized response time

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Scheduling Algorithms

• First Come First Serve (FCFS)
• Shortest Job First (SJF)
• Round Robin (RR)
• ...

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Scheduling AlgorithmsFirst-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. (Shortest Job
  First)

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Priority Scheduling (Shortest Job First)

• Associate with each process the length of its
  next CPU burst. Use these lengths to schedule
  the process with the shortest time.
• SJF is optimal gives minimum average waiting
  time for a given set of processes.
• SJF is a priority scheduling.

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Shortest Job First (SJF)

• 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. (Shortest Job
  First)

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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.

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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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II-The Deadlock Problem

• A set of blocked processes each holding a
  resource and waiting to acquire a resource held
  by another process in the set.
• Example
• System has 2 tape drives.
• P1 and P2 each hold one tape drive and each needs
  another one.
• Example
• semaphores A and B, initialized to 1
• P0 P1
• wait (A) wait(B)
• wait (B) wait(A)

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Bridge Crossing Example

• Traffic only in one direction.
• Each section of a bridge can be viewed as a
  resource.
• If a deadlock occurs, it can be resolved if one
  car backs up (preempt resources and rollback).
• Several cars may have to be backed up if a
  deadlock occurs.

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Deadlock Characterization

• Deadlock can arise if four conditions hold
  simultaneously.
• Mutual exclusion only one process at a time can
  use a resource.
• Hold and wait a process holding at least one
  resource is waiting to acquire additional
  resources held by other processes.
• No preemption a resource can be released only
  voluntarily by the process holding it, after that
  process has completed its task.
• Circular wait there exists a set P0, P1, ,
  P0 of waiting processes such that P0 is waiting
  for a resource that is held by P1, P1 is waiting
  for a resource that is held by
• P2, , Pn1 is waiting for a resource that is
  held by Pn, and P0 is waiting for a resource
  that is held by P0.

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Deadlock Prevention
Restrain the ways request can be made.

• Mutual Exclusion not required for sharable
  resources must hold for nonsharable resources.
• Hold and Wait must guarantee that whenever a
  process requests a resource, it does not hold any
  other resources.
• Require process to request and be allocated all
  its resources before it begins execution, or
  allow process to request resources only when the
  process has none.
• Low resource utilization starvation possible.

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Deadlock Prevention (Cont.)

• No Preemption
• If a process that is holding some resources
  requests another resource that cannot be
  immediately allocated to it, then all resources
  currently being held are released.
• Preempted resources are added to the list of
  resources for which the process is waiting.
• Process will be restarted only when it can regain
  its old resources, as well as the new ones that
  it is requesting.

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Deadlock Avoidance
Requires that the system has some additional a
priori information available.

• Simplest and most useful model requires that each
  process declare the maximum number of resources
  of each type that it may need.
• The deadlock-avoidance algorithm dynamically
  examines the resource-allocation state to ensure
  that there can never be a circular-wait
  condition.
• Resource-allocation state is defined by the
  number of available and allocated resources, and
  the maximum demands of the processes.

22
Review Questions

• Do exercise in CPU Process Scheduling.
• snµt Process enAkñúg Queue List dUcxageRkam³
• Process Burst Time
• P1 3
• P2 5
• P3 3
• P4 7
• P5 8
• k-cUlKUs Gantt Chat tMNageGay Process nImYy²tam
  FCFS, SJF, RR (time quantum3)?
• x-cUlKNna Average Waiting Time rbs Process
  nImYy²tam FCFS, SJF, RR (Time quantum3)?