Dealing With Deadlock
Deadlock Avoidance
Deadlock avoidance
State matrices example
A computer system with 5 processes and 4 resources
Examine each resource request and determine whether or not
granting the request can lead to deadlock
ALLOCATION
R1 R2 R 3 R4
Define a set of vectors and matrices that characterize the
current state of all resources and processes
ÿ resource allocation state matrix
Alloc ij = the number of units of
resource j held by process i
...
resource j that are unallocated
...
ÿ available vector
Availj = the number of units of
Pp np,1
...
np,r
00
11
11
00
00
00
00
33
66
00
11
00
55
33
11
R1
...
of resource j that the process i will
ever require simultaneously
R1 R2 R 3 ... Rr
P1 n1,1 n1,2 n1,3 ... n1,r
P2 n2,1 n2,2
P3 n3,1
...
ÿ maximum claim matrix
Maxij = the maximum number of units
P1
P2
P3
P4
P5
P1
<n1, n2, n3, ..., nr>
22
00
33
22
44
MAX_CLAIM
R1 R2 R 3 R4
00
11
22
00
00
R2
P2
00
77
33
66
66
11
55
55
55
55
R3
P3
22
00
66
22
66
AVAILABLE
R1 R2 R 3 R4
11 55 22 00
R4
P4
P5
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Deadlock Avoidance
13
Deadlock Avoidance Algorithm
Concept
State definitions
A resource allocation state is safe if the system can allocate
resources to each process up to its maximum claim such that
the system can not deadlock
The OS will examine each resource request and determine
whether or not granting the request can lead to deadlock
ÿ If, after we grant this request, all processes simultaneously make
their maximum claim, will the system deadlock?
There must be an ordering of the processes P1, P2 , ..., Pn, such
that for all processes Pi ,
ÿ Can we satisfy the maximum claims of processes in some order?
MAX_CLAIMPi – ALLOCATIONPi ≤ AVAIL + SALLOCATIONPj
Term. j
ALLOCATION
R1 R2 R 3 R4
P1
P2
P3
P4
P5
0
1
1
0
0
0
0
3
6
0
1
0
5
3
1
2
0
3
2
4
MAX_CLAIM
R1 R2 R 3 R4
0
1
2
0
0
0
7
3
6
6
1
5
5
5
5
2
0
6
2
6
AVAILABLE
R1 R2 R 3 R4
The largest request for resources
that process Pi can make
1 5 2 0
The resources available to
process Pi after processes
P1 through P i–1 terminate
This ordering of processes is called a safe sequence
14
If a safe sequence exists then there exists a process ( P1) that can
execute to completion in the current state, and
Pj can execute to completion at worst after processes P1 - Pj–1
complete
15
Deadlock Avoidance Algorithm
Deadlock Avoidance Example
Example
Safe sequence computation
1. Compute the largest possible resource request a process can
make
MAX_CLAIM
ALLOCATION
MAX_REQUEST
R1 R2 R 3 R4
R1 R2 R 3 R4
R1 R2 R 3 R4
P1 00 00 11 22
00 00 11 22
00 00 00 00
P2 11 77 55 00
11 00 00 00
00 77 55 00
–
=
P3 22 33 55 66
11 33 55 33
11 00 00 33
P4 00 66 55 22
00 66 33 22
00 00 22 00
P5 00 66 55 66
00 00 11 44
00 66 44 22
A computer system with 5 processes and 4 resources
ALLOCATION
R1 R2 R 3 R4
P1 00 00 11 22
P2 11 00 00 00
P3 11 33 55 33
P4 00 66 33 22
P5 00 00 11 44
MAX_CLAIM
R1 R2 R 3 R4
00 00 11 22
11 77 55 00
22 33 55 66
00 66 55 22
00 66 55 66
AVAILABLE
R1 R2 R 3 R4
11 55 22 00
R1
R2
R3
R4
P1
P2
P3
P4
P5
Is this system in a safe state?
Does there exist a safe sequence?
i–1
MAX_CLAIMPi – ALLOCATION Pi ≤ AVAIL + SALLOCATION Pj
R1
R2
R3
R4
P1
P2
P3
P4
P5
2. Attempt to build a safe sequence
j=1
16
17
Deadlock Avoidance Example
Deadlock Avoidance Example
Safe sequence computation
Safe sequence computation
ALLOCATION
P1
P2
P3
P4
P5
R1 R2 R 3 R4
00 00 11 22
11 00 00 00
11 33 55 33
00 66 33 22
00 00 11 44
MAX_CLAIM
AVAILABLE
R1 R2 R 3 R4
00 00 11 22
11 77 55 00
22 33 55 66
00 66 55 22
00 66 55 66
R1 R2 R 3 R4
11 55 22 00
MAX_REQUEST
R1 R2 R 3 R4
00 00 00 00
00 77 55 00
11 00 00 33
00 00 22 00
00 66 44 22
MAX_CLAIM
AVAILABLE
MAX_REQUEST
R1 R2 R 3 R4
R1 R2 R 3 R4
R1 R2 R 3 R4
R1 R2 R 3 R4
11 55 22 00
00
00
11
00
00
P1
P2
P3
P4
P5
ÿ Does there exist a process Pi such that
MAX_REQUEST P ≤ AVAILABLE ?
00
11
11
00
00
00
00
33
66
00
11
00
55
33
11
22
00
00
11
33
22
22 0 4 2 0 00
44
00
00
77
33
66
66
11
55
55
55
55
22
00
66
22
66
2 1 0 0
00
77
00
00
66
00
55
00
22
44
00
00
33
001 3 3 0
22
What if P2 wants to change its allocation to <0, 4, 2, 0>?
i
ÿ If no, then there is no safe sequence, the state is unsafe
ÿ Is the resulting allocation state safe?
ÿ If yes, add Pi to the sequence
ÿ Set AVAILABLE = AVAILABLE + ALLOCATION P
ALLOCATION
i
18
19
Deadlock Avoidance
Dealing With Deadlock
Banker’s algorithm (Dijkstra and Habermann)
Deadlock detection & recovery
When a process makes a request for resources...
Deadlock prevention and avoidance:
ÿ Simulate the effect of granting the process’s request
ÿ Develop and use resource allocation mechanisms and protocols that
prohibit deadlock
ÿ Then check to see if a safe sequence exists
ALLOCATION
MAX_CLAIM
AVAILABLE
R1 R2 R 3 R4
R1 R2 R 3 R4
R1 R2 R 3 R4
R1 R2 R 3 R4
0
1
1
0
0
0
1
2
0
0
1 5 2 0
0
0
1
0
0
P1
P2
P3
P4
P5
0
0
3
6
0
1
0
5
3
1
2
0
3
2
4
0
7
3
6
6
1
5
5
5
5
2
0
6
2
6
Deadlock detection and recovery:
MAX_REQUEST
0
7
0
0
6
0
5
0
2
4
ÿ Let the system deadlock and then deal with it
Detect that a set of processes are deadlocked
Recover from the deadlock
0
0
3
0
2
20
Deadlock Detection & Recovery
21
Deadlock Detection & Recovery
Detecting deadlock
Detecting deadlock
Run Banker’s algorithm and see if a safe sequence exists
How often should the OS check for deadlock?
ÿ Replace MAX_REQUEST with a “ REQUEST ” matrix
ÿ After every resource request?
ÿ If a safe sequence does not exist then the system is deadlocked
ÿ Only when we suspect deadlock has occurred?
ALLOCATION
MAX_CLAIM
AVAILABLE
REQUEST
ALLOCATION
MAX_CLAIM
AVAILABLE
REQUEST
R1 R2 R 3 R4
R1 R2 R 3 R4
R1 R2 R 3 R4
R1 R2 R 3 R4
R1 R2 R 3 R4
R1 R2 R 3 R4
R1 R2 R 3 R4
R1 R2 R 3 R4
0
1
1
0
0
0
1
2
0
0
1 5 2 0
0
0
1
0
0
0
1
1
0
0
0
1
2
0
0
1 5 2 0
0
0
1
0
0
P1
P2
P3
P4
P5
0
0
3
6
0
1
0
5
3
1
2
0
3
2
4
0
7
3
6
6
1
5
5
5
5
2
0
6
2
6
0
2
0
0
2
0
1
0
2
2
0
0
0
0
2
P1
P2
P3
P4
P5
22
0
0
3
6
0
1
0
5
3
1
2
0
3
2
4
0
7
3
6
6
1
5
5
5
5
2
0
6
2
6
0
2
0
0
2
0
1
0
2
2
0
0
0
0
2
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Deadlock Detection & Recovery
Recovering from deadlock
R1
P1
R2
P2
R3
P3
R4
P4
P5
Abort all deadlocked processes & reclaim their resources
Abort one process at a time until all cycles in the RAG
are eliminated
Where to start?
ÿ Select low priority process
ÿ Processes with most allocation of resources
Caveat: ensure that system is in consistent state (e.g., transactions)
Optimization:
ÿ Checkpoint processes periodically; rollback processes to checkpointed
state
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