DEADLOCK PREVENTION & RECOVERY

1. DEADLOCK PREVENTION &
RECOVERY
I am S.RAJAPRIYA
I am here because I love to give a fright.
You can find me at rajapriya@vvvcollege.org

2. DEADLOCK
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✘ The Deadlock Problem
✘ System Model
✘ Deadlock Characterization
✘ Methods for Handling
Deadlocks
✘ Deadlock Prevention
✘ Deadlock Avoidance
✘ Deadlock Detection
✘ Recovery from Deadlock

3. The Deadlock Problem
A set of blocked processes each holding a resource and
waiting to acquire a resource held by another process in
the set
Example
System has 2 disk drives
P1 and P2 each hold one disk drive and each needs
another one
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4. “A Deadlock is a permanent blocking of a
set of threads”
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5. SYSTEM MODEL
✘ Resources typesR1, R2, . . ., Rm
CPU cycles, memory space, I/O devices
✘ Each resource type Ri has Wi instances.
✘ Each process utilizes a resource as follows:
✗ request
✗ use
✗ release
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6. Mutual exclusion :
only one process at a time can
use a resource. If another process request
that resource, the requesting process must
be delayed until the resource has been
released
Hold and wait :
a process holding at least one
resource is waiting to acquire additional
resources held by other processes
Deadlock Characterization
No preemption :
a resource can be released only
voluntarily by the process holding it, after
that process has completed its task.
Circular wait :
there exists a set {P0, P1, …, P0}
of waiting processes such that P0 is
waiting for a resource that is held by P1,
P1 is waiting for a resource that is held by
P2, …, Pn–1 is waiting for a resource that is
held by
Pn, and P0 is waiting for a resource that is
held by P0.
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7. RESOURCE ALLOCATION GRAPH
Deadlock can be described more precisely in terms of a directed
graph called a system resource allocation graph.
A set of vertices V and a set of edges E.
V is partitioned into two types:
• P = {P1, P2, …, Pn}, the set consisting of all the processes in the
system
• R = {R1, R2, …, Rm}, the set consisting of all resource types in the
system
• request edge – directed edge P1  Rj
• assignment edge – directed edge Rj  Pi
Example
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8. Resource-Allocation Graph
✘ Pictorially, we represent
✘ Process
✘ Resource Type with 4 instances
✘ Pi requests instance of Rj
✘ Pi is holding an instance of Rj
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Pi
Pi

9. Example of a Resource Allocation Graph
• The sets P,R,E:
o P={p1,p2,p3}
o R={R1,R2,R3,R4}
o E={P1->R1,P2->R3,R1->P2,
R2->P2,R2->P1,R3->P3}
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10. Resource Allocation Graph with Deadlock
• Suppose that process P3 request an
instance of resource type R3.
• Since no resource is currently
available, a request edge P3->R2 is
added to the graph.
• At this point two cycles exist in the
system:
✘ P1->R1->P2->R3->P3->R2->P1
✘ P2->R3->P3->R2->P2
• Processes P1,P2,P3 are deadlocked.
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11. Graph with a Cycle but No Deadlock
• In this example we also have a
cycle
✘ P1->R1->P3->R2->P1
• However, there is no cycle.
• Observe that process P4 may
release its resource R2.That
resource can then be allocated to
P3,breaking the cycle.
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12. ✘ If graph contains no cycles  no deadlock
✘ If graph contains a cycle 
✗ if only one instance per resource type, then deadlock
✗ if several instances per resource type, possibility of
deadlock
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13. DEADLOCK PREVENTION
✘ By ensuring that at least one of these conditions cannot hold,
We can prevent the occurrence of a deadlock
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14. Eliminate Mutual Exclusion –
not required for sharable
resources, must hold for non-sharable
resources. For example, a printer cannot
be shared by several processes. Read
only files are good example of sharable
resource
Eliminate Hold and Wait –
must guarantee that
whenever a process requests a
resource, it does not hold any other
resources.
Deadlock Prevention
✗ Allocate all required resources to the
process before the start of its
execution, this way hold and wait
condition is eliminated but it will
lead to low device utilization.
✗ For example, if a process requires
printer at a later time and we have
allocated printer before the start of
its execution printer will remain
blocked till it has completed its
execution.
✗ The process will make a new
request for resources after releasing
the current set of resources. This
solution may lead to starvation.
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15. Eliminate No Preemption –
✗ If a process that is holding
some resources requests
another resource that cannot
be immediately allocated to it,
then all resources currently
being held are released.
✗ Preempted resources are
added to the list of resources
for which the process is
waiting.
Deadlock Prevention
✘ Process will be restarted
only when it can regain its
old resources, as well as the
new ones that it is
requesting.
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16. Deadlock Prevention
Eliminate Circular Wait –
• Each resource will be assigned
with a numerical number. A
process can request the
resources increasing/decreasing
order of numbering.
• For Example, if P1 process is
allocated R5 resources, now next
time if P1 ask for R4, R3 lesser than
R5 such request will not be granted,
only request for resources more
than R5 will be granted.
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17. A picture is worth a thousand words
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18. Safe State!When a process requests an available resource, system must decide if
immediate allocation leaves the system in a safe state.
System is in safe state if there exists a safe sequence of all processes.
Sequence <P1, P2, …, Pn> is safe if for each Pi, the resources that Pi
can still request can be satisfied by currently available resources
+ resources held by all the Pj, with j<I.
✗ If Pi resource needs are not immediately available, then Pi can wait
until all Pj have finished.
✗ When Pj is finished, Pi can obtain needed resources, execute,
return allocated resources, and terminate.
✗ When Pi terminates, Pi+1 can obtain its needed resources, and so
on.
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19. 19
Safe, Unsafe and Deadlock State
Place your screenshot here

20. Recovery from Deadlock
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Process Termination
Resource Preemption

21. Process Termination
Abort all deadlocked
processes
Abort one
process at a
time until the
deadlock
cycle is
eliminated
In which
order should
we choose to
abort?
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22. Resource Preemption
Selecting a victim
Starvation –
same process
may always be
picked as victim,
include number of
rollback in cost
factor
Rollback – return
to some safe
state, restart
process for that
state.
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23. Thankyou
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