Deadlock

1
Deadlock

• Chapter 3
• Thursday, February 22, 2007

2
Todays Schedule

• Assignment 4 from Chapter 3 posted
• Deadlock - Chapter 3
• Skip multiple resources (3.4.2 3.5.4)

3
Todays Objectives

• You will be able to describe
• Several causes of system deadlock
• The difference between preventing and avoiding
  deadlocks
• How to detect and recover from deadlocks
• How to prevent deadlock

4
Overview

• A lack of process synchronization results in
  deadlock or starvation
• Deadlock
• A system-wide tangle of resource requests that
  begins when two or more jobs are put on hold
• Each job waiting for a vital resource to become
  available
• The jobs come to a standstill
• Resolved via external intervention
• Starvation Infinite postponement of a job

5
Starvation

• Which of the following scheduling algorithms
  could result in starvation? If so, how?
• First-come, First-served
• Shortest job first
• Round robin
• Priority

6
Deadlock

• Affects more than one job, hence more serious
  than starvation
• System (not just a few programs) is affected as
  resources are being tied up
• e.g., Traffic jam

7
Deadlock in Spooling

• Virtual device Sharable devicee.g., a printer
  transformed by installing a high-speed device, a
  disk, between it and the CPU
• Spooling Disk accepts output from several users
  and acts as a temporary storage area for all
  output until printer is ready to accept it
• Deadlock in spooling If printer needs all of a
  job's output before it will begin printing, but
  spooling system fills available disk space with
  only partially completed output

8
Deadlock Defined

• From our more playful days
• Ive got the ball and want the bat
• Youve got the bat and want the ball
• A set of processes is deadlocked if each process
  in the set is waiting for an event that only
  another process in the set can cause.

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Modeling DeadlocksResource Allocation Graphs
Process P1 requests Resource R1
Process P1 holds Resource R1
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How deadlock occurs
A B
C
Cycle formed!!
11
How deadlock can be avoided
(o) (p)
(q)
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Conditions Required for Deadlock

• Mutual exclusion conditioneach resource assigned
  to 1 process or is available
• Hold and wait conditionprocess holding resources
  can request additional
• No preemption conditionpreviously granted
  resources cannot forcibly taken away
• Circular wait conditionmust be a circular chain
  of 2 or more processeseach is waiting for
  resource held by next member of the chain

13
Dealing with Deadlock

• Ostrich Algorithm Ignore deadlock possibility!
• Detection Recovery
• Dynamic Avoidance
• careful resource allocation
• Prevention
• negating one of the four necessary conditions

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Ostrich Algorithm

• Stick your head in the sand and pretend there is
  no problem at all
• Reasonable if
• deadlocks occur very rarely
• cost of prevention is high
• UNIX and Windows takes this approach
• Trade off between
• convenience
• correctness
• What is the typical use of the system?
• What is the probability of deadlock?
• Do the costs associated with dealing w/ deadlock
  outweigh the benefits?

15
Detecting Deadlock Find Cycle

• Note the resource ownership and requests
• A cycle can be found within the graph, denoting
  deadlock

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How to Recover from Deadlock?

• Recovery through preemption
• take a resource from some other process
• depends on nature of the resource
• Recovery through rollback
• checkpoint a process periodically
• use this saved state
• restart the process if it is found deadlocked
• Recovery through killing processes
• crudest but simplest way to break a deadlock
• kill one of the processes in the deadlock cycle

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Avoid Deadlock Resource Trajectories Show
B
Impossible
Printer
I9
I8
I7
I6
Plotter
t
u
Unsafe
r
s
A
p
q
I1
I2
I3
I5
I4
Printer
Plotter
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Safe/Unsafe States

• Safe State
• Not in deadlock
• There is some scheduling order with which all
  processes can finish
• Unsafe State
• Not necessarily deadlock
• Doesnt guarantee deadlock
• But cant guarantee deadlock will be avoided

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Avoid Deadlock
Unsafe State
deadlock
Safe State
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Bankers Algorithm Avoid Deadlock

• Regulate resource allocation
• No loan exceeding banks total capital
• Customers have a pre-set maximum credit
• May not borrow over the limit
• Total of all loans may not exceed banks capital

21
Avoidance (continued)
Safe state Bank still has enough money left
after loans to satisfy the maximum requests of
C1, C2, or C3
The bank started with 10,000 and has remaining
capital of 4,000 after these loans
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Avoidance (continued)
Unsafe state Bank does not have enough money
left after loans to satisfy the maximum requests
of C1, C2, or C3
Table 5.5 The bank has remaining capital of only
1,000 after these loans and therefore is
in an unsafe state
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Avoidance (continued)
Same banking principles can be applied to an
operating system
Table 5.6 A safe state six devices are
allocated and four units are still
available
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Avoidance (continued)
An unsafe state only one unit is available but
every job requires at least two to complete its
execution
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Deadlock Avoidance

• To avoid deadlock, OS must make sure
• Never satisfy a request that moves it from a safe
  state to an unsafe one
• Must identify the job with the smallest number of
  remaining resources
• Number of available resources is always equal to,
  or greater than, the number needed for the
  selected job to run to completion

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Attack Deadlock Conditions

• Attack Mutual Exclusion
• Attack Hold and Wait
• Require all process to request all resources
  before starting execution
• Resource must temporarily release all the
  resources it currently holds
• Attack No preemption
• Just take away resource, does not work
• Attack Circular Wait condition
• Process is entitled to single resource at any
  moment
• Use of global numbering
• All requests made in numerical order

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Summary

• Resources can be preemptable nonpreemptable
• Deadlock can cause processes to halt (stop making
  progress)
• We can detect deadlock
• RAGs are quite useful to detect deadlock
• Ostrich algorithm quite popular

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Summary (cont)

• Trajectories can help in process scheduling to
  avoid deadlock
• Avoid unsafe states
• Prevent Deadlock by attack one of four necessary
  conditions
• Ordering resources avoids circular wait
• Keep processes from starving all processes must
  make some progress

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Tuesday, Feb 27, Memory

• Complete Assignment 4
• Begin reading Chapter 4 Memory Management