Preview Deadlock Condition Resource Allocation Graph Resource

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3/31/2015
Preview
Deadlock Condition
Deadlock Condition
 Resource Allocation Graph
 Four strategies for Dealing Deadlock
 Deadlock Detection and Recovery
A deadlock situation can arise if and only if
the following four conditions hold
simultaneously in a system.
1. Mutual exclusion
2. Hold and wait
3. No preemption
4. Circular wait
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Detection with one resource of each type
Detection with Multiple resource of each type
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Recovery from Deadlock
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
Deadlock Detection Algorithm
COSC450 Operating System Spring 2015
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Resource Allocation Graph
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Resource-Allocation Graph
This graph consists of a set of vertices V and a
set of edges E.
The set of vertices V is partitioned into two types
P = {P1, P2, …, Pn}= set of processes and
R = {R1, R2, …, Rm} = set of resources
Edge from a process to a resource P → R denote
process P request resource R and currently
waiting
Edge from a resource to a process R → P denote
resource R is currently held by process P
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Deadlock Example
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Deadlock Example
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Four Strategies for Dealing Deadlock
Deadlock Detection and Recovery
Just ignore
 Detection and Recover
 Dynamic Avoidance by careful allocation
 Prevention – by negating one of the four
conditions necessary to cause deadlock
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A system let a deadlock occur.
 The system keeps tracking whether there
is any deadlock or not.
 When a deadlock situation is detected by
system, system takes some action to
recover from the deadlock.
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(Detection with one resource of each type)
(Detection with one resource of each type)
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Deadlock Detection with one resource of
each type
 A simple graph algorithm which detect a
cycle in a directed graph can detect a
deadlock
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(Detection with Multiple resource of each type)
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We need matrixes for deadlock detection algorithm.
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Existing resource matrix – Present how many numbers of
resources per each type
Available resource matrix – Present how may number of
resources per each type are available at any moment, since
some of resources are assigned to processes are not
available
Current allocation matrix – present how resources are
currently held by processes
Request matrix – present how many resources are needed
for processes to finish their job.
We can observe following equations from the four matrix structure.
n
C
i 1
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i, j
 Aj  E j
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The deadlock detection algorithm is based on
comparing vectors.
 Let’s define the relation A ≤ B between two
vector A and B means that each element of A is
less than equal to corresponding element of B.
Ex)
A = (1, 2, 0 2), B = (1, 3, 0 2): A ≤ B is true? Yes
A = (1, 2, 0, 2), B = (2, 0, 0, 0): A ≤ B is true? No
A Deadlock detection algorithm
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All process start with unmarked
Look for an unmarked process Pi, for which the
ith row of R is less than or equal to A
If such a process is found, add the ith row of C to
A, mark the process and go back to step 1
If no such a process exists, the algorithm
terminate
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Ex)
E = (4, 2, 3, 1)
A = (2, 1, 0, 0)
Ex)
E = (7, 2, 6)
A = (0, 0, 0)
Ex)
E = (4, 2, 3, 1)
A = (2, 1, 0, 0)
0010
2001
C  2001, R  1010 
0120
2100
- No deadlock
0010
2001
C  2001, R  2111
0120
2100
- Deadlock
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Recovery through preemption
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010 
000 
200
202
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C  303 , R  001 
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211
100 
002 
002 
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010 
000 
200
202
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C  303 , R  000 



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211
100 
002 
002 
Ex)
E = (7, 2, 6)
A = (0, 0, 0)
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Recovery through rollback
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take a resource from some other process
depends on nature of the resource
checkpoint a process periodically
use this saved state
restart the process if it is found deadlocked
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Recovery through killing processes
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Crudest but simplest way to break a deadlock
Kill one of the processes in the deadlock cycle
The other processes get its resources
The killed process need to start from the
beginning
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Preview Deadlock Condition Resource Allocation Graph Resource

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