Problems associated with Deadlock Prevention

1
Problems associated with Deadlock Prevention

• Deadlock prevention , attempts to prevent at
  least one of the three necessary conditions.
• This may lead to inefficient use of resources as
  well as inefficient execution of processes.

2
Deadlock Avoidance

• Basis There exists a priori knowledge about the
  following
• a) The Total Amount of resources at hand
  hence applicable to reusable resources only.
• b) For any process , its maximum demand for
  all types of resources .
• Basic Approaches
• Do not start a process if its demands might
  lead to a deadlock.
• Do not grant incremental resource request to a
  process if this allocation might lead to a
  deadlock.

3
Deadlock Avoidance Principle - 1

• Data Structures used
• N processes P N P1,,Pi,.Pn
• M resources R M R1,..,Rj, , Rm

Where Rj No. of Instances of the jth
resource. Each of the N processes has got a
maximum demand for instances of each of the M
existing resources
4
Deadlock Avoidance Principle - 2

• Data Structures used Contd.
• Claim Matrix C N,M C11,,C1j,.C1m
• 
  Ci1,,Cij,., Cim
• 
  Cn1,,Cnj,.,Cnm

Here C ij ? Max. Requirement of Process Pi for
the instances of the jth resource.
5
Deadlock Avoidance Principle - 3

• Data Structures used Contd.
• Allocation Matrix A N,M A11,,A1j,.A1m
• 
  Ai1,,Aij,., Aim
• 
  An1,,Anj,.,Anm

Here Aij ? Number of Instances of the jth
Resource currently allocated to the Process Pi .
6
Deadlock Avoidance Principle - 4

• Data Structures used Contd.
• Available Matrix V M V1,..,Vj, , Vm

Where Vj No. of Instances of the jth resource
left / existing after allocating some as
specified by the allocation matrix A N,M.
Hence Rj Vj ? A kj ( for k 1 .. N )
7
Deadlock Avoidance Principle - 5

• Constraints

• For all Resources Rj Vj ? A ki ( for j 1
  .. N )
• i.e. all resources are either allocated
  or available.
• 2. Ckj lt Rj for all k, j i.e. No processes
  can claim for more instances of the jth resource
  than that exists.
• 3. Akj lt Ckj , for all k, j i.e. No processes
  is allocated more instances of the jth resource
  than its max. claim.

8
Deadlock Avoidance Policy 1

• Process Initiation Denial No new process is
  started if its resource requirements might lead
  to a deadlock.
• Start a new process P (n1) only if the maximum
  claim of all current processes plus that of the
  new process can be met.
• Rj gt C (n1)j ? C ki ( for k 1 .. N )

9
Deadlock Avoidance Policy 2

• Resource Allocation Denial
• Bankers Algorithm Allocate resources to
  processes so that one always arrives at a safe
  state The current state of the system is
  described by the two vectors Resource RN
  Available VN as well as the two matrices Claim
  CM,N Allocation AM,N .
• A safe state is one in which there exists at
  least one sequence of allocation of resource
  instances to processes that does not result in
  deadlock. Refer to worked out example .

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Bankers Algorithm - 1

• struct State
• int Resource M // The Resource Vector
• int Available M // The Available Vector
• int Claim N M // The Claim Matrix
• int Allocation N M // The Allocation
  Matrix

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Resource Allocation Algorithm

• if ( Allocation I, Request ) gt
  Claim I, )
• / Total Request gt Claim /
• lt Flash Error gt
• else if ( Request gt Available )
• lt Suspend Process gt
• else / Simulate Allocation /
• lt Define New-state by
• Allocation I, ? Allocation I,
  Request
• Available ? Available -
  Request gt
• if ( Safe (New-state))
• lt Carry Out Allocation gt
• else
• lt Restore Original State gt
• lt Suspend Process gt

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Test for Safety / Bankers Algorithm

• boolean Safe (State S)
• int Current_Avail M
• WAIT_LIST lt Number of Processes gt
• Current_Avail M ? Available M
• WAITI_LIST ? All Processes
• Possible ? TRUE
• while (Possible)
• // begin while 1
• ltFind a Process Pk in WAIT_LIST such
  that
• Current_Avail gt Claim K, -
  Allocation K, gt
• if (found)
  // Simulate
  Execution of Pk
• Current_Avail ? Current_Avail
  Allocation K,
• WAIT_LIST ? WAIT_LIST Pk
• else
• Possible ? FALSE
• // end while

13
Deadlock Avoidance Basic Features

• It allows the three necessary conditions for
  deadlock but makes judicious choices to assure
  that the deadlock point is never reached.
• It allows more concurrency than prevention.
• With deadlock avoidance, a decision is made
  dynamically whether the current resource
  allocation request will, if granted, potentially
  lead to a deadlock.
• NOT applicable to consumable resources.