1. Understand Process Management
• Deadlock Detection
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3. Recap
In the last class, you have learnt:
• Deadlock Avoidance
• Safe State and Safe sequence
• Banker’s Algorithm
• Data Structures needed for implementing Banker’s
Algorithm
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4. Objectives
On completion of this class, you would be able to
know :
• Deadlock Detection Techniques
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5. Deadlock Detection
• Deadlock may occur if a system does not employ
– Deadlock prevention
– Deadlock avoidance
• In this environment the system must provide
– An algorithm that examines the state of the system to
determine whether a deadlock has occurred
– An algorithm to recover from the deadlock
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6. Deadlock Detection
The Deadlock detection algorithms in the previous slide
are explained with the following conditions
• Single Instance of Each Resource Type
• Several Instances of Resource Type
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7. Single Instance of each Resource Type
• If all resources have only a single instance, then
we can define a Deadlock detection algorithm i.e.,
– A variant of resource allocation graph
– This graph is known as wait-for graph
• A deadlock exists in the system if and only if the
wait-for graph contains a cycle
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8. Resource-Allocation Graph
P5
R1 R3 R4
P1 P2 P3
R2 P4 R5
Fig – 1(a)
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9. Corresponding Wait-for
graph
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P1 P2 P3
P4
Fig – 1(b)
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10. Several Instances of a Resource type
• The wait-for graph scheme is not applicable to a
resource-allocation system with multiple instances of
each resource
• The Deadlock-detection algorithm contains several
time-varying data structures. Namely
– Available
– Allocation
– Request
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11. Several Instances of a Resource Type
• Available: A vector of length m indicates the
number of available resources of each type
• Allocation: An nxm matrix defines the number of
resources of each type currently allocated
• Request: An nxm matrix indicates the current
request of each process
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12. Detection-Algorithm Usage
• When should we invoke the detection algorithm?
The answer depends on
– How often is a deadlock likely to occur?
– How many process will be affected by deadlock when it
happens?
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13. Detection-Algorithm Usage
• Deadlocks occur only when some process makes a
request that cannot be granted immediately
• If the deadlock occurs frequently,
– The detection algorithm should be invoked frequently or
– Invoked for every resource request
• The above methods incur overhead in computation time
• So an alternative method is
– To invoke the algorithm at less frequent intervals for Ex. Once
per hour
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14. Summary
In this class, you have learnt
• Deadlock detection under different condition
namely
• Systems with single instance of each resource type
• Several instance of a resource type
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15. Frequently Asked Questions
1. Explain Deadlock Detection
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16. Quiz
1) --------------------An nxm matrix defines the number
of resources of each type currently allocated
a) Allocation
b) Available
c )None
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17. Other subject materials
• Web designing
• Micro processors
• C++ tutorials
• java
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18. Quiz
2) A --------------- exists in the system if and only if
the wait-for graph contains a cycle
a) Deadlock
b) Request
c) None
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