Topic 7: Concurrency Control, Deadlock and Resources

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Topic 7 Concurrency Control, Deadlock and
Resources

• L E Pages 117-134
• Tanenbaum Pages 75-87, 166-177

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Semaphores A Quick Reminder

• Semaphores are non negative integers which (apart
  from their initialisation) can only be operated
  on by the operations wait (P) and signal (V).
• Signal adds one to the value of a semaphore.
• Wait subtracts one from the value of the
  semaphore unless the semaphore is zero in which
  case the process is blocked until the subtraction
  can be done without making the semaphore
  negative.
• Indivisible (atomic) operations
• cf s s 1

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Using Semaphores To Implement Mutual Exclusion
Semaphore mutex 1
.... non critical code ... wait (mutex) ...
critical code ... signal (mutex) ... non
critical code ...
.... non critical code ... wait (mutex) ...
critical code ... signal (mutex) ... non
critical code ...
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Using Semaphores to Implement Resource Allocation
Begin prepare data wait (printer) print
(data) signal (printer) End.
Semaphore printer 1
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Using Semaphores to Implement Resource Allocation
Begin prepare data wait (printer) print
(data) signal (printer) End.
Semaphore printer 1
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Using Semaphores to Implement Resource Allocation
Begin prepare data wait (printer) print
(data) signal (printer) End.
Semaphore printer 0
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Using Semaphores to Implement Resource Allocation
Begin prepare data wait (printer) print
(data) signal (printer) End.
Semaphore printer 0
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Using Semaphores to Implement Resource Allocation
Begin prepare data wait (printer) print
(data) signal (printer) End.
Semaphore printer 1
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Acquiring Two Resources
Begin prepare data wait (disk) wait
(printer) print (data) signal (disk) signal
(printer) End.
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Two Processes Acquiring Two Resources
Begin prepare data wait (printer) wait
(disk) print (data) signal (disk) signal
(printer) End.
Begin prepare data wait (disk) wait
(printer) print (data) signal
(printer) signal (disk) End.
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Two Processes Acquiring Two Resources
Begin prepare data wait (printer) wait
(disk) print (data) signal (disk) signal
(printer) End.
Begin prepare data wait (disk) wait
(printer) print (data) signal
(printer) signal (disk) End.
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Two Processes Acquiring Two Resources
Begin prepare data wait (printer) wait
(disk) print (data) signal (disk) signal
(printer) End.
Begin prepare data wait (disk) wait
(printer) print (data) signal
(printer) signal (disk) End.
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Two Processes Acquiring Two Resources
Begin prepare data wait (printer) wait
(disk) print (data) signal (disk) signal
(printer) End.
Begin prepare data wait (disk) wait
(printer) print (data) signal
(printer) signal (disk) End.
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Two Processes Acquiring Two Resources
Begin prepare data wait (printer) wait
(disk) print (data) signal (disk) signal
(printer) End.
Begin prepare data wait (disk) wait
(printer) print (data) signal
(printer) signal (disk) End.
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Two Processes Acquiring Two Resources
Begin prepare data wait (printer) wait
(disk) print (data) signal (disk) signal
(printer) End.
Begin prepare data wait (disk) wait
(printer) print (data) signal
(printer) signal (disk) End.
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Two Processes Acquiring Two Resources
Begin prepare data wait (printer) wait
(disk) print (data) signal (disk) signal
(printer) End.
Begin prepare data wait (disk) wait
(printer) print (data) signal
(printer) signal (disk) End.
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Deadlock !
Got Printer, Waiting on Disk
Got Disk, Waiting on Printer
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Dining Philosophers
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Dining Philosophers

• Philosophers do only 2 things
• Think
• Eat
• To eat a philosopher requires two forks
  (chopsticks).

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Dining Philosophers
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Dining Philosophers
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Dining Philosophers
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Dining Philosophers

• The implementation should be simple -

while true do wait (right_fork) wait
(left_fork) eat signal (left_fork) signal
(right_fork) end loop
Acquire resources
Release resources
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But ... Consider the Following Situation
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But ... Consider the Following Situation
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But ... Consider the Following Situation
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But ... Consider the Following Situation
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But ... Consider the Following Situation
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Deadlock !
Got left chopstick, Waiting on right
Got left chopstick, Waiting on right
cf Got Disk, Waiting on Printer
cf Got Printer, Waiting on Disk
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Conditions for Deadlock

• The resources involved are non-sharable.
• Processes hold the resources they have been
  allocated while waiting for new ones.
• Resources cannot be pre-emptied.
• A circular chain of processes exists.

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Solutions to the Deadlock Problem

• Prevent deadlock.
• Detect deadlock and try to recover.
• Avoid deadlock by anticipating events.

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Deadlock Prevention

• Deny one of the four conditions for deadlock-
• The resources involved are unsharable.
• Cf Spooling
• Processes hold the resources they have been
  allocated while waiting for new ones.
• Make processes request all resources at once
• Resources cannot be pre-emptied while in use.
• Force a process to release all held resources
• A circular chain of processes exists.
• Allocate an order on resource types

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Deadlock Detection

• Detection algorithms work by detecting circular
  wait patterns.
• Processes are represented as nodes and resource
  dependencies as arcs.

process holds A and wants B
B
A

• Periodically search for loops.

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Deadlock Detection Continued

• Example of a loop representing deadlock

B
A
C
D
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Recovering from Deadlock

• Abort all deadlocked processes.
• Common approach
• Restart deadlocked processes from a checkpoint.
• Relies on non-determinacy of the system
• Abort deadlocked processes one by one until
  deadlock broken.
• Ordering can be chosen to reduce cost.
• Release resources one by one until deadlock
  broken.
• Need to re-invoke detection algorithm

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Deadlock Avoidance

• Tries to predict/anticipate when deadlock is
  likely to occur.
• cf Deadlock Prevention
• How is deadlock avoided?
• Check each resource request and if it would cause
  deadlock then abort it.
• State Graph
• Problem
• Allocation of a resource does not always produce
  a situation of deadlock immediately

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Deadlock Avoidance
Instructions process B
D
Instructions process A
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Resource Issues and Scheduling

• The issue of resource allocation is intimately
  associated with the issue of scheduling, i.e.
  deciding which process to run next.
• Recall that processes cycle through 3 states.
• Ready
• Running
• Blocked

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The Process State Model cont
Ready
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The Process State Model cont
Process is selected to run
Ready
Running
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EITHER
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The Process State Model .. contd
Process is selected to run
Ready
Running
Process finishes its time slice
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OR
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The Process State Model .. contd
Process is selected to run
Ready
Running
Blocked
Process blocks (e.g. I/O)
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The Process State Model cont
Process is selected to run
Ready
Running
Blocking operation finishes (e.g. I/O)
Blocked
Process blocks (e.g. I/O)
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Resource Issues and Scheduling

• General scheduling model shown below.

Ready Queue
CPU
Blocked Queue
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Scheduling and the Dispatcher

• We designed and built a dispatcher.
• Higher level decisions taken by a process called
  a scheduler.
• Scheduler tries to make best use of resources.
• Most systems use some form of round-robin
  scheduling algorithm.
• Simple scheme which gives each non-blocked
  process a fixed size time-slice.

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Round Robin Scheduling
49
Round Robin Scheduling
50
Round Robin Scheduling
51
Round Robin Scheduling
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Round Robin Scheduling
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Round Robin Scheduling
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Round Robin Scheduling
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Priorities

• Sometimes want different priority processes.
• These can be implemented by either
• Multiple ready queues.
• Giving different processes different length
  time-slices.
• Giving different processes more time slices.

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Minimising Transitions to the Blocked State

• Important transitions to blocked state increase
  overhead because processes not running for their
  full time-slice.
• Can minimise number of transitions by ensuring
  resources are available when required.
• This requires additional scheduling strategies,
  e.g. give resource intensive processes a high
  priority.
• Why ?

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Summary

• Recapped on semaphores and mutual exclusion.
• Highlighted the problem of deadlock.
• Reviewed techniques for overcoming deadlock
  (prevention, detection and avoidance).
• Briefly covered process scheduling and impact of
  resource allocation strategy.

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Coming Next Week

• Memory Management.
• Book Reminder
• Fundamentals of Operating Systems, A. M.
  Lister and R. D. Eager, Fifth Edition, Macmillan
  Computer Science Series, ISBN. 0-333-59848-2,
  13.99.
• Operating Systems Design and Implementation, A.
  S. Tanenbaum, Prentice-Hall International,
  ISBN. 0-13-630195-9, about 29.99.
• www.comp.lancs.ac.uk/computing/staff/kc/keiths_tea
  ching.html