Deadlock Avoidance

A decision is made dynamically whether the current resource
allocation request will, if granted, potentially lead to a
deadlock

Requires knowledge of future process requests
Deadlock
Avoidance
Resource
Allocation Denial
Process Initiation
Denial
• do not grant an
incremental resource
request to a process if
this allocation might
lead to deadlock
• do not start a
process if its
demands might lead
to deadlock

Referred to as the banker’s algorithm

State of the system reflects the current allocation of resources to
processes

Safe state is one in which there is at least one sequence of resource
allocations to processes that does not result in a deadlock

Unsafe state is a state that is not safe
Dijkstra's Banker's
Algorithm

Definitions
 Each
process has a LOAN, CLAIM,
MAXIMUM NEED
LOAN: current number of resources held
 MAXIMUM NEED: total number
resources needed to complete
 CLAIM: = (MAXIMUM - LOAN)

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Ceng 334 - Operating Systems
Assumptions

Establish a LOAN ceiling (MAXIMUM NEED) for
each process

MAXIMUM NEED < total number of
resources available (ie., capital)

Total loans for a process must be less than or equal to
MAXIMUM NEED

Loaned resources must be returned back in finite
time
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Ceng 334 - Operating Systems
Algorithm
1.
Search for a process with a claim that can
satisfied using the current number of
remaining resources (ie., tentatively grant the
claim)
2. If such a process is found then assume that it
will return the loaned resources.
3. Update the number of remaining resources
2.4
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4. Repeat steps 1-3 for all processes and mark
them
Ceng 334 - Operating Systems

DO NOT GRANT THE CLAIM if at least
one process can not be marked.

Implementation
A
resource request is only allowed if
it results in a SAFE state
 The system is always maintained in a
SAFE state so eventually all requests
will be filled
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Ceng 334 - Operating Systems

Advantages



It works
Allows jobs to proceed when a prevention
algorithm wouldn't
Problems



Requires there to be a fixed number of resources
What happens if a resource goes down?
Does not allow the process to change its
Maximum need while processing
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Ceng 334 - Operating Systems
Determination of a Safe State

State of a system consisting of four processes and three resources

Allocations have been made to the four processes. Is this a safe
state?
Amount of
existing
resources
Resources
available
after
allocation
P3 Runs to Completion
Thus, the state defined
originally is a safe state
Safe and Unsafe States (1)
(a)
(b)
(c)
(d)
Demonstration that the state in (a) is safe
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Ceng 334 - Operating Systems
(e)
Safe and Unsafe States (2)
(a)
(d)
(b)
Demonstration that the sate in (b) is not safe
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Ceng 334 - Operating Systems
(c)
The Banker's Algorithm for a Single Resource
(a)
(b)

Three resource allocation states



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Ceng 334 - Operating Systems
safe
safe
unsafe
(c)
Banker's Algorithm for Multiple Resources
2.4
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Example of banker's algorithm with multiple resources
Ceng 334 - Operating Systems

It is not necessary to preempt and rollback processes, as in
deadlock detection

It is less restrictive than deadlock prevention

Maximum resource requirement for each process
must be stated in advance

Processes under consideration must be independent
and with no synchronization requirements

There must be a fixed number of resources to
allocate

No process may exit while holding resources
Deadlock Strategies
Deadlock prevention strategies are very
conservative
• limit access to resources by imposing restrictions on
processes
Deadlock detection strategies do the
opposite
• resource requests are granted whenever possible
Deadline Detection
Algorithms
 A check for deadlock can be made as frequently as each resource
request or, less frequently, depending on how likely it is for a
deadlock to occur. Tradeoffs involved in determining frequency
Advantages:
 Frequent checks lead to early detection
 The algorithm is simpler if it is done more often (there have
been fewer changes to the pattern of resource allocation)
 Disadvantage
 frequent checks consume considerable processor time
Recovery Strategies

Abort all deadlocked processes

Back up each deadlocked process to some previously defined
checkpoint and restart all processes

Successively abort deadlocked processes until deadlock no longer
exists

Successively preempt resources until deadlock no longer exists
Thank You