Operating Systems
CST 352
Deadlock
4/9/2015
CST 352 - Operating Systems
1
Topics
Definitions
Deadlock Conditions
Ostrich Algorithm
Deadlock Detection
Deadlock Recovery
Deadlock Avoidance
Deadlock Prevention
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Definitions
Resources – Parts of a system that are
required for a process to run (e.g.
memory, disk, CPU, etc.)
Preemptable – A resource that can be
taken from a process without adverse
affects (e.g. seized memory)
Non Preemtable– A resource that cannot
be taken away (e.g. a device driver
connected to printer)
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Definitions
Deadlock – In a set of processes, each
process is waiting for a resource that
one of the other process has seized.
Remember the dining philosophers.
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Definitions
A Traffic Deadlock
All Stop…..
Ready…..
Go!
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Definitions
A Traffic Deadlock
Oops….
Deadlock
How is this
deadlock
avoided under
normal
circumstances?
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Definitions
Question:
1.
2.
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Which of the two types of
resource can lead to a deadlock
condition?
Why?
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Definition
Question:
Can there be deadlock in a “firstcome, first-serve” scheduling
scheme?
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Required Conditions
Four conditions necessary for Deadlock:
1. Mutual Exclusion – Only one process can access
a resource at any given time.
2. Hold and Wait – A process may hold a resource
while waiting for others to become available.
3. No preemption – No resource may be forcibly
taken from a holding process.
4. Circular wait – There must be at least two
processes, each waiting for a resource held by the
other.
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Deadlock Concepts
Modeling Deadlock (grid method)
X – axis: progress of process A.
Y – axis: progress of process B.
This gives us a good feel for deadlock overlap,
however, it becomes unmanageable for
more than two processes.
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Modeling
Deadlock
(Resource
Trajectories)
Example: Proc A
and Proc B
competing
for Resource
1 and
Resource 2.
Process B
Deadlock Concepts
Rel 1
Proc A and
Proc B want
Resource 1
Rel 2
Get 1
Deadlock
Pending
Get 2
Get 1
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Proc A and
Proc B want
Resource 2
Get 2
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Rel 1
Rel 2
Process A
11
1.
2.
3.
4.
5.
6.
7.
8.
Process B
Deadlock Concepts
A Executes
B Executes
Rel 1
A Executes
B acquires 2
Rel 2
B acquires 1
B releases 2
Get 1
B releases 1
A can acquire both 1Get 2
and 2
Proc A and
Proc B want
Resource 1
Deadlock
Pending
Get 1
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Get 2
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Proc A and
Proc B want
Resource 2
Rel 1
Rel 2
Process A
12
1.
2.
3.
4.
5.
6.
7.
8.
9.
Process B
Deadlock Concepts
A Executes
B Executes
Rel 1
A Executes
B acquires 2
Rel 2
B acquires 1
A blocks on 1
Get 1
B releases 2
B releases 1
Get 2
A can acquire both 1
and 2
Proc A and
Proc B want
Resource 1
Deadlock
Pending
Get 1
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Get 2
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Proc A and
Proc B want
Resource 2
Rel 1
Rel 2
Process A
13
1.
2.
3.
4.
5.
6.
7.
8.
A Executes
B Executes
A Executes
B acquires 2
A acquires 1
B blocks on 1
A blocks on 2
Deadlock
Process B
Deadlock Concepts
Rel 1
Proc A and
Proc B want
Resource 1
Rel 2
X
Get 1
Deadlock
Pending
Proc A and
Proc B want
Resource 2
Get 2
Get 1
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Get 2
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Rel 1
Rel 2
Process A
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To reduce resource
contention, construct
Process A such that it
does not need both
resource 1 and
resource 2 at the
same time.
This increases the
number of valid
paths through the
“gray” areas.
Process B
Deadlock Concepts
Rel 1
Rel 2
Proc A and
Proc B
want
Resource 1
Proc A and
Proc B
want
Resource 2
Get 1
Get 2
Get 1
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Rel 1
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Get 2
Rel 2
Process A
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Deadlock Concepts
Modeling Deadlock (grid method)
The resource trajectory model has
disadvantages
Unmanageable for complex systems
Dependent on resource acquisition sequencing
This model does suggest solutions to
deadlock (e.g. avoid getting into the
“Deadlock Pending” region.
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Deadlock Concepts
Modeling Deadlock (graph method)
A circle represents a process
A square represents a resource
A directed arch from a circle to a square
represents a process requesting a resource.
P1 requesting R1
P1
R1
A directed arch from square to circle represents a
process holding a resource
P1 holding R1
P1
R1
A Deadlock is condition is represented by a
“cycle” in the graph.
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Deadlock Concepts
Modeling Deadlock (graph method)
Example:
Processes P1, P2, and P3
Resources R1, R2, and R3
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P1
P2
P3
R1
R2
R3
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Deadlock Concepts
Modeling Deadlock (graph method)
1.
P1 requests and holds R1
2.
P2 requests and holds R2
3.
P3 requests and holds R3
4.
P1 requests R2
5.
P2 requests R3
6.
P3 requests R1
The cyclic condition
of the graph indicates
this sequence of event
will cause deadlock.
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P1
P2
P3
R1
R2
R3
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Ostrich Algorithm
Stick your head in the sand and
pretend there is no problem at
all.
Mathematically speaking – this is
unacceptable.
From an engineering point of
view, this may be ok.
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Ostrich Algorithm
Considerations:
What is the half-life of your OS.
What resource contentions could
cause a deadlock.
How often statistically does your
OS experience a visible deadlock.
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Ostrich Algorithm
The Ostrich Algorithm is the easiest
solution to the deadlock problem.
In standard “user” based systems (e.g.
Windows, Unix Workstations, etc.),
this is perfectly acceptable.
Would you consider the “Ostrich
Algorithm” an acceptable solution for
an enterprise management system?
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Deadlock Detection
Graph based approach
For each resource access
Create a node representing the
process accessing the resource.
Create a node representing the
resource.
Make the appropriate directed
association.
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Deadlock Detection
Graph based approach (example)
1.
2.
3.
4.
5.
6.
PA requests R1 and gets it.
PB requests R2 and gets it.
PC requests R2.
PD requests R3 and gets it.
PD requests R1.
PA requests R3.
Every time a resource
request is made:
1. Add the appropriate
node to the graph
2. Check the graph for
cycles.
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PA
R1
PB
R2
PC
R3
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PD
24
Deadlock Detection
Graph based approach
To implement the graph based
approach, the OS needs to:
Build a graph on the fly based on
resource requests.
Provide some form of cycle
detection to detect deadlock
conditions in the constructed graph.
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Deadlock Detection
Q:
1.
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List the four conditions for
deadlock.
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Deadlock Detection
Matrix Approach
Let “n” = number of processes == (P1,
…, Pn).
Let “m” = number of resource classes.
E1 – resource of type 1
E2 – resource of type 2
…
Em – resource of type m
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Deadlock Detection
Matrix Approach
At any point in time, the allocation
of resources can be represented by
a two vectors.
(E1, E2, …, Em) – Existing resource
vector.
(A1, A2, …, Am) – Available resource
vector.
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Deadlock Detection
Matrix Approach
Represent allocation with a matrix
Columns represent resources
Rows represent processes
Represent requests with a matrix
Columns represent resources
Rows represent processes
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Deadlock Detection
Matrix Approach
Current Allocation and Request
c11
c22
…
cn1
c12 … c1m
c22 … c2m
…
…
cn2 … cnm
r11
r22
…
rn1
Current Allocation
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r12 … r1m
r22 … r2m
…
…
rn2 … rnm
Current Request
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Deadlock Detection
Matrix Approach
At any point in time, the resource
state must conform to the equation:
For any j (j -> resource “column”):
n
cij+Aj = Ej
i=1
e.g. allocated instances of resource j + available
instances of resource j = number of existing instances
of resource j.
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Deadlock Detection
Matrix Approach
Process allocation is now done by
comparing vectors.
Vector A <= B iff
For all i < m, Ai <= Bi
Initialize the processes to “r” for “run”.
As the algorithm proceeds, set the process
flags to either “r” for processes capable of
running, or “b” for processes blocked and
waiting for a resource.
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Deadlock Detection
Matrix Approach
Every time there is a resource allocation:
Look for a process (Pi) for which the request row (Ri)
<= available resources (Ai).
Looking for resource demands that can be met.
If such a process is found, add the ith row of the c matrix
to the A vector.
Loop over next request.
Else
Terminate the detection algorithm and set Pi to “b”
All “b” processes are blocked.
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Deadlock Detection
Matrix Approach
In practical application, keep the matrices as
a running total.
Each time there is a resource allocation,
update the matrices.
Each time there is a resource de-allocation,
update the matrices.
Deadlock occurs when two (or more)
processes have cross referenced resources.
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Deadlock Detection
Matrix Approach (Example)
Current
0
C= 0
0
Allocation
0 0
0 0
0 0
DVD
Printer
DVD
CD Rom
Printer
E = [3 2 2]
CD Rom
Available Vector
Resource Vector
A = [3 2 2]
P1 r
P2 r
P3 r
Request
2 0 0
R= 0 0 0
0 0 0
1) P1 requests 2 Printers
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Deadlock Detection
Matrix Approach (Example)
Current
2
C= 0
0
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Allocation
0 0
0 0
0 0
DVD
Printer
DVD
CD Rom
Printer
E = [3 2 2]
CD Rom
Available Vector
Resource Vector
A = [1 2 2]
P1 r
P2 r
P3 r
Request
0 0 0
R= 0 0 0
0 0 0
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Deadlock Detection
Matrix Approach (Example)
Current
2
C= 0
0
Allocation
0 0
0 0
0 0
DVD
Printer
DVD
CD Rom
Printer
E = [3 2 2]
CD Rom
Available Vector
Resource Vector
A = [1 2 2]
P1 r
P2 r
P3 r
Request
0 0 0
R= 1 0 2
0 0 0
2) P2 requests 1 Printer, 2 DVDs
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Deadlock Detection
Matrix Approach (Example)
Current
2
C= 1
0
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Allocation
0 0
0 2
0 0
DVD
Printer
DVD
CD Rom
Printer
E = [3 2 2]
CD Rom
Available Vector
Resource Vector
A = [0 2 0]
P1 r
P2 r
P3 r
Request
0 0 0
R= 0 0 0
0 0 0
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Deadlock Detection
Matrix Approach (Example)
Current
2
C= 1
0
Allocation
0 0
0 2
0 0
DVD
Printer
DVD
CD Rom
Printer
E = [3 2 2]
CD Rom
Available Vector
Resource Vector
A = [0 2 0]
P1 r
P2 r
P3 r
Request
0 0 0
R= 0 0 0
1 1 0
3) P3 requests 1 Printer, 1 CD Rom
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Deadlock Detection
Matrix Approach (Example)
Current
2
C= 1
0
Allocation
0 0
0 2
0 0
DVD
Printer
DVD
CD Rom
Printer
E = [3 2 2]
CD Rom
Available Vector
Resource Vector
A = [0 2 0]
P1 r
P2 r
P3 b
Request
0 0 0
R= 0 0 0
1 1 0
4) P3 blocked
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Deadlock Detection
Matrix Approach (Example)
Current
2
C= 1
0
Allocation
0 0
0 2
0 0
DVD
Printer
DVD
CD Rom
Printer
E = [3 2 2]
CD Rom
Available Vector
Resource Vector
A = [0 2 0]
P1 r
P2 r
P3 b
Request
0 0 0
R= 0 0 0
1 1 0
5) P2 frees up printer
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Deadlock Detection
Matrix Approach (Example)
Current
2
C= 0
0
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Allocation
0 0
0 2
0 0
DVD
Printer
DVD
CD Rom
Printer
E = [3 2 2]
CD Rom
Available Vector
Resource Vector
A = [1 2 0]
P1 r
P2 r
P3 b
Request
0 0 0
R= 0 0 0
1 1 0
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Deadlock Detection
Matrix Approach (Example)
Current
2
C= 0
1
Allocation
0 0
0 2
1 0
DVD
Printer
DVD
CD Rom
Printer
E = [3 2 2]
CD Rom
Available Vector
Resource Vector
A = [0 1 0]
P1 r
P2 r
P3 r
Request
0 0 0
R= 0 0 0
0 0 0
6) P3 can now run
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Deadlock Detection
Matrix Approach (Example)
Current
2
C= 0
1
Allocation
0 0
0 2
1 0
DVD
Printer
DVD
CD Rom
Printer
E = [3 2 2]
CD Rom
Available Vector
Resource Vector
A = [0 1 0]
P1 r
P2 r
P3 r
Request
0 0 1
R= 0 0 0
0 0 0
7) P1 request DVD
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Deadlock Detection
Matrix Approach (Example)
Current
2
C= 0
1
Allocation
0 0
0 2
1 0
DVD
Printer
DVD
CD Rom
Printer
E = [3 2 2]
CD Rom
Available Vector
Resource Vector
A = [0 1 0]
P1 b
P2 r
P3 r
Request
0 0 1
R= 0 0 0
0 0 0
8) P1 blocked
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Deadlock Detection
Matrix Approach (Example)
Current
2
C= 0
1
Allocation
0 0
0 2
1 0
DVD
Printer
DVD
CD Rom
Printer
E = [3 2 2]
CD Rom
Available Vector
Resource Vector
A = [0 1 0]
P1 b
P2 r
P3 r
Request
0 0 1
R= 1 0 0
0 0 0
9) P2 request printer
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Deadlock Detection
Matrix Approach (Example)
Current
2
C= 0
1
Allocation
0 0
0 2
1 0
DVD
Printer
DVD
CD Rom
Printer
E = [3 2 2]
CD Rom
Available Vector
Resource Vector
A = [0 1 0]
P1 b
P2 b
P3 r
Request
0 0 1
R= 1 0 0
0 0 0
10) P2 blocked
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Deadlock Detection
Matrix Approach (Example)
Current
2
C= 0
1
Allocation
0 0
0 2
1 0
DVD
Printer
DVD
CD Rom
Printer
E = [3 2 2]
CD Rom
Available Vector
Resource Vector
A = [0 1 0]
P1 b
P2 b
P3 r
Request
0 0 1
R= 1 0 0
0 0 0
•
This is still not deadlock since P3 holds a resource that could allow P2 to
commence execution.
•
If P1 held all 3 printers, we would have true deadlock.
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Deadlock Detection
Matrix Approach (Example)
Current
2
C= 0
1
Allocation
0 0
0 2
1 0
DVD
Printer
DVD
CD Rom
Printer
E = [3 2 2]
CD Rom
Available Vector
Resource Vector
A = [0 1 0]
P1 b
P2 b
P3 r
Request
0 0 1
R= 1 0 0
0 0 0
• What are the general conditions for true deadlock?
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Deadlock Detection
Hypothesis (extra credit – 50 pts)
Prove or disprove
Deadlock can be detected by rotating the
rows and columns of the C and R matrix
(in conjunction) so that if C and R have
any inverse sub-matrices, the involved
processes are deadlocked.
E and A must be involved.
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Deadlock Recovery
Given the above algorithm, the OS
can detect which process is waiting
for what resource.
Now the OS needs to decide what to
do when a deadlocked process is
detected.
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Deadlock Recovery
Recovery through preemption
Take a resource away from a
process and give it to another.
Only works for preemptable
processes.
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Deadlock Recovery
Recovery through rollback
PCB (TCB) and resource state are periodically
saved at a “checkpoint”
When a deadlock is detected, back the
deadlocked process up to the previous stable
state.
Discard the resource manipulation that occurred
after the checkpoint.
Start the process after it is determined it can run
again.
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Deadlock Recovery
Recovery through process killing
Kill off the offending processes.
The processes that have allocated
contending resources are removed from
the system.
Kill off the lower priority process.
Kill off non-OS (user) processes.
Return their resources to the system.
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Deadlock Recovery
Q:
1.
2.
3.
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How can a system be constructed
so it will never have deadlock?
Give an example.
T or F – The graph model for
deadlock only works well for 2
processes.
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Deadlock Avoidance
Shaded regions with
slanted lines represent
instances where both
processes gain access to a
resource at the same time.
If the resources are
mutually exclusive, it is
impossible for these areas
to be entered by a “run
time trajectory”.
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Process B
Process Trajectories – revisited
Rel 1
Proc A and
Proc B want
Resource 1
Rel 2
Get 1
Deadlock
Pending
Proc A and
Proc B want
Resource 2
Get 2
Get 1
CST 352 - Operating Systems
Get 2
Rel 1
Rel 2
Process A
56
Deadlock Avoidance
Bankers Algorithm
The banker at Klamath First Federal is
granting a group of customers credit.
The banker has limited cash to loan.
Each customer has a credit limit.
The banker assumes no customer will
request their entire credit limit at a
single time.
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Deadlock Avoidance
Bankers Algorithm
The banker grants loans to the customers
based customer credit limit and available
cash reserve.
The bankers job is to avoid the situation
where cash reserves drop so low that the
FDIC shuts the bank down.
Loans that may cause inspection by the
FDIC are called “unsafe” loans.
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Deadlock Avoidance
Bankers Algorithm (Example)
4 customers – Joe, Jay, Jim, and Jill
Initial reserve of 10 dollars (it is a very small
bank).
Each customer has a credit limit.
The goal here is to ensure that two customers
cannot be simultaneously blocked waiting
on a resource.
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Deadlock Avoidance
Bankers Algorithm (Example)
1. Joe requests 1
Name
Has
Limit
Joe
0
6
Jay
0
5
Jim
0
4
Jill
0
7
Reserve: 10
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Deadlock Avoidance
Bankers Algorithm (Example)
1. Joe requests 1 – granted
Name
Has
Limit
Joe
1
6
Jay
0
5
Jim
0
4
Jill
0
7
Reserve: 9
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Deadlock Avoidance
Bankers Algorithm (Example)
1. Joe requests 1 – granted
2. Jay requests 1
Name
Has
Limit
Joe
1
6
Jay
0
5
Jim
0
4
Jill
0
7
Reserve: 9
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Deadlock Avoidance
Bankers Algorithm (Example)
1. Joe requests 1 – granted
2. Jay requests 1 – granted
Name
Has
Limit
Joe
1
6
Jay
1
5
Jim
0
4
Jill
0
7
Reserve: 8
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Deadlock Avoidance
Bankers Algorithm (Example)
1. Joe requests 1 – granted
2. Jay requests 1 – granted
3. Jim requests 2
Name
Has
Limit
Joe
1
6
Jay
1
5
Jim
0
4
Jill
0
7
Reserve: 8
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Deadlock Avoidance
Bankers Algorithm (Example)
1. Joe requests 1 – granted
2. Jay requests 1 – granted
3. Jim requests 2 – granted
Name
Has
Limit
Joe
1
6
Jay
1
5
Jim
2
4
Jill
0
7
Reserve: 6
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Deadlock Avoidance
Bankers Algorithm (Example)
1.
2.
3.
4.
Joe requests 1 – granted
Jay requests 1 – granted
Jim requests 2 – granted
Jill requests 4
Name
Has
Limit
Joe
1
6
Jay
1
5
Jim
2
4
Jill
0
7
Reserve: 6
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Deadlock Avoidance
Bankers Algorithm (Example)
1.
2.
3.
4.
Joe requests 1 – granted
Jay requests 1 – granted
Jim requests 2 – granted
Jill requests 4 – granted
Name
Has
Limit
Joe
1
6
Jay
1
5
Jim
2
4
Jill
4
7
Reserve: 2
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Deadlock Avoidance
Bankers Algorithm (Example)
At any point in time, in this
scenario, the states are safe
because, with at least two
units left, the banker can
delay granting any requests
until Jim pays back his loan.
Jim can ask for the
additional 2 dollars.
Name
Has
Limit
Joe
1
6
Jay
1
5
Jim
2
4
Jill
4
7
Reserve: 2
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Deadlock Avoidance
Bankers Algorithm (Example)
When Jim pays back his loan,
the banker can lend to Jay
or Jill.
Name
Has
Limit
Joe
1
6
Jay
1
5
Jim
0
4
Jill
4
7
Reserve: 4
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Deadlock Avoidance
Bankers Algorithm (Example)
Unsafe State Example
1. Joe requests 1 – granted
2. Jay requests 2 – granted
Name
Has
Limit
Joe
1
6
Jay
2
5
Jim
0
4
Jill
0
7
Reserve: 7
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Deadlock Avoidance
Bankers Algorithm (Example)
Unsafe State Example
1. Joe requests 1 – granted
2. Jay requests 2 – granted
3. Jim requests 2
Name
Has
Limit
Joe
1
6
Jay
2
5
Jim
0
4
Jill
0
7
Reserve: 7
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Deadlock Avoidance
Bankers Algorithm (Example)
Unsafe State Example
1. Joe requests 1 – granted
2. Jay requests 2 – granted
3. Jim requests 2 – granted
Name
Has
Limit
Joe
1
6
Jay
2
5
Jim
2
4
Jill
0
7
Reserve: 5
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Deadlock Avoidance
Bankers Algorithm (Example)
Unsafe State Example
1.
2.
3.
4.
Joe requests 1 – granted
Jay requests 2 – granted
Jim requests 2 – granted
Jill requests 4
Name
Has
Limit
Joe
1
6
Jay
2
5
Jim
2
4
Jill
0
7
Reserve: 5
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Deadlock Avoidance
Bankers Algorithm (Example)
Unsafe State Example
1.
2.
3.
4.
Joe requests 1 – granted
Jay requests 2 – granted
Jim requests 2 – granted
Jill requests 4
With a reserve of 1,
potentially, any other request
could be denied.
If the banker gives the loan
to Jill, the reserve will be down
to 1.
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Name
Has
Limit
Joe
1
6
Jay
2
5
Jim
2
4
Jill
0
7
Reserve: 5
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Deadlock Avoidance
Bankers Algorithm (Example)
1.
2.
3.
4.
Joe requests 1 – granted
Jay requests 2 – granted
Jim requests 2 – granted
Jill requests 4
Granting Jill the loan would
force the system into an unsafe
state.
If all customers demanded their
maximum loan, none of them could
be granted, forcing the FDIC to
come down on the bank (deadlock).
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Name
Has
Limit
Joe
1
6
Jay
2
5
Jim
2
4
Jill
0
7
Reserve: 5
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75
Deadlock Avoidance
Bankers Algorithm (multiple resource)
The bankers algorithm for multiple resource is just
a modification of the matrix approach for deadlock
detection.
The modification made relates to what the
scheduler does during the application of the
algorithm.
Rather than waiting for deadlock to occur, the
scheduler takes proactive action during resource
allocation and de-allocation.
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Deadlock Avoidance
Bankers Algorithm (multiple resource)
The problem with the bankers algorithm for
deadlock avoidance is to operate correctly,
the resources must be pre-determined and
stable.
There are no systems where resources are all
pre-determined and infinitely stable.
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Deadlock Prevention
Attack mutual exclusion
condition
Do not allow any resources to be
mutually exclusive.
Printer spooling was introduced
to do this.
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Deadlock Prevention
Attack the hold and wait condition
Do not allow any process to seize a
resource and hold it.
Require all processes to request their
resources up-front.
This solution introduces
inefficiencies.
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Deadlock Prevention
Attack the No-Preempt
condition
It is close to impossible to get rid
of all non-preemptable resources
in a system.
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Deadlock Prevention
Attack the Circular wait
condition
Only allow a process one
resource at a time.
Require process to adhere to a
resource numbering scheme
sequential allocation
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