Deadlock

Part II
Process Management
Chapter 7: Deadlocks

System Model
qSystem resources are utilized in the following way:
vRequest: If a process makes a request to use a system
resource which cannot be granted immediately, then the
requesting process blocks until it can acquire the
resource.
vUse:The process can operate on the resource.
vRelease: The process releases the resource.
qDeadlock: A set of process is in a deadlock state
when every process in the set is waiting for an
event that can only be caused by another process in
the set.

Deadlock: Necessary Conditions
qFor a deadlock to occur, each of the following four
conditions must hold.
vMutual Exclusion: At least one resource must
be held in a non-sharable way.
vHold and Wait: A process must be holding a
resource and waiting for another.
vNo Preemption: Resource cannot be preempted.
vCircular Wait: A waits for B, B waits for C, C
waits for A.

Handling Deadlocks
qDeadlock Prevention and Avoidance: Make
sure deadlock can never happen.
vPrevention: Ensure one of the four conditions fails.
vAvoidance: The OS needs more information so that
it can determine if the current request can be
satisfied or delayed.
qDeadlock : Allow a system to enter a deadlock
situation, detect it, and recover.
qIgnore Deadlock: Pretend deadlocks never
occur in the system.

Deadlock Prevention: 1/4
Mutual Exclusion

qBy ensuring that at least one of the four
conditions cannot hold, we can prevent the
occurrence of a deadlock.
qMutual Exclusion: Some sharable resources
must be accessed exclusively (e.g., printer),
which means we cannot deny the mutual
exclusion condition.

Hold and Wait
qNo process can hold some resources and then
request for other resources.
qTwo strategies are possible:
vA process must acquire all resources before it runs.
vWhen a process requests for resources, it must hold
none (i.e., returning resources before requesting for
more).
qResource utilization may be low, since many
resources will be held and unused for a long time.
qStarvation is possible. A process that needs some
popular resources my have to wait indefinitely.

No Preemption
qResources that are being held by the requesting
process are preempted. There are two strategies:
vIf a process is holding some resources and requesting for
some others that are being held by other processes, the
resources of the requesting process are preempted. The
preempted resources become available.
vIf the requested resources are not available:
ØIf they are being held by processes that are waiting for
additional resources, these resources are preempted
and given to the requesting process.
ØOtherwise, the requesting process waits until the
requested resources become available. While it is
waiting, its resources may be preempted.
ØThis works only if the state of the process and resources
can be saved and restored easily (e.g., CPU & memory).

Circular Waiting
qTo break the circular waiting condition, we can
order all resource types (e.g., tapes, printers).
qA process can only request resources higher than the
resource types it holds.
qSuppose the ordering of tapes, disks, and printers
are 1, 4, and 8. If a process holds a disk (4), it can
only ask a printer (8) and cannot request a tape (1).
qA process must release some lower order resources
to request a lower order resource. To get tapes (1), a
process must release its disk (4).
qIn this way, no deadlock is possible. Why?

Deadlock Avoidance: 1/5
qEach process provides the maximum number of
resources of each type it needs.
qWith these information, there are algorithms that
can ensure the system will never enter a deadlock
state. This is deadlock avoidance.
qA sequence of processes <P1, P2, …, Pn> is a safe
sequence if for each process Pi in the sequence, its
resource requests can be satisfied by the remaining
resources and the sum of all resources that are
being held by P1, P2, …, Pi-1. This means we can
suspend Pi and run P1, P2, …, Pi-1 until they
complete. Then, Pi will have all resources to run.

q A state is safe if the system can
allocate resources to each
process (up to its maximum, of
course) in some order and still deadlock
avoid a deadlock.
q In other word, a state is safe if
there is a safe sequence. unsafe
Otherwise, if no safe sequence
exists, the system state is unsafe.
q An unsafe state is not
necessarily a deadlock state.
On the other hand, a deadlock
safe
state is an unsafe state.

qA system has 12 tapes and three processes A, B, C.
At time t0, we have:
Max needs Current holding Will need
A 10 5 5
B 4 2 2
C 9 2 7

qThen, <B, A, C> is a safe sequence (safe state).
qThe system has 12-(5+2+2)=3 free tapes.
qSince B needs 2 tapes, it can take 2, run, and
return 4. Then, the system has (3-2)+4=5 tapes. A
now can take all 5 tapes and run. Finally, A
returns 10 tapes for C to take 7 of them.

qA system has 12 tapes and three processes A, B, C. At
time t1, C has one more tape:
Max needs Current holding Will need
A 10 5 5
B 4 2 2
C 9 3 6
qThe system has 12-(5+2+3)=2 free tapes.
qAt this point, only B can take these 2 and run. It
returns 4, making 4 free tapes available.
qBut, none of A and C can run, and a deadlock occurs.
qThe problem is due to granting C one more tape.

qA deadlock avoidance algorithm ensures that
the system is always in a safe state. Therefore,
no deadlock can occur.
qResource requests are granted only if in doing
so the system is still in a safe state.
qConsequently, resource utilization may be
lower than those systems without using a
deadlock avoidance algorithm.

Banker’s Algorithm: 1/2
qThe system has m resource types and n processes.
qEach process must declare its maximum needs.
qThe following arrays are used:
vAvailable[1..m]: one entry for each resource. Available[i]=k
means resource type i has k units available.
vMax[1..n,1..m]: maximum demand of each process.
Max[i,j]=k means process i needs k units of resource j.
vAllocation[1..n,1..m]: resources allocated to each process.
Allocation[i,j]=k means process i is currently allocated k
units of resource j.
vNeed[1..n,1..m]: the remaining resource need of each
process. Need[i,j]=k means process i needs k more units of
resource j.

Banker’s Algorithm: 2/2
qWe will use A[i,*] to indicate the i-th row of
matrix A.
qGiven two arrays A[1..m] and B[1..m], A ≤ B if A[i]
≤ B[i] for all i. Given two matrices A[1..n,1..m]
and B[1..n,1..m], A[i,*] ≤B[i,*] if A[i,j] ≤B[i,j] for
all j.
qWhen a resource request is made by process i, this
algorithm calls the Resource-Request algorithm to
determine if the request can be granted. The
Resource-Request algorithm calls the Safety
Algorithm to determine if a state is safe.

Safety Algorithm
1. Let Work[1..m] and Finish[1..n] be two working
arrays.
2. Work := Available and Finish[i]=FALSE for all i
3. Find an i such that both
v Finish[i] = FALSE // process i is not yet done
v Need[i,*] ≤ Work // its need can be satisfied
If no such i exists, go to Step 5
4. Work = Work + Allocation[i,*] // run it and reclaim
Finish[i] = TRUE // process i completes
go to Step 3
5. If Finish[i] = TRUE for all i, the system is in a
safe state.

Resource-Request Algorithm
1. Let Request[1..n,1..m] be the request matrix. Request[i,j]=k
means process i requests k units of resource j.
2. If Request[i,*]≤Need[i,*], go to Step 3. Otherwise, it is an
error.
3. If Request[i,*]≤Available, go to Step 4. Otherwise, process i
waits.
4. Do the following:
Available = Available – Request[i,*]
Allocation[i,*] = Allocation[i,*]+Request[i,*]
Need[i,*] = Need[i,*] – Request[i,*]
If the result is a safe state (Safety Algorithm), the request is
granted. Otherwise, process i waits and the
resource-allocation tables are restored back to the original.

Example: 1/4
q Consider a system of 5 processes A, B, C, D and E, and 3
resource types (X=10, Y=5, Z=7). At time t0, we have
Allocation Max Need=Max-Alloc Available
X Y Z X Y Z X Y Z X Y Z
A 0 1 0 7 5 3 7 4 3 3 3 2
B 2 0 0 3 2 2 1 2 2
C 3 0 2 9 0 2 6 0 0
D 2 1 1 2 2 2 0 1 1
E 0 0 2 4 3 3 4 3 1
q A safe sequence is <B,D,E,C,A>. Since B’s [1,2,2]≤ Avail’s
[3,3,2], B runs. Then, Avail=[2,0,0]+[3,3,2]=[5,3,2]. D runs
next. After this, Avail=[5,3,2]+[2,1,1]=[7,4,3]. E runs next.
q Avail=[7,4,3]+[0,0,2]=[7,4,5]. Since C’s [6,0,0]≤Avail=[7,4,5], C
runs. After this, Avail=[7,4,5]+[3,0,2]=[10,4,7] and A runs.
q There are other safe sequences: <D,E,B,A,C>, <D,B,A,E,C>, …

Example: 2/4
q Now suppose process B asks for 1 X and 2 Zs. More
precisely, RequestB = [1,0,2]. Is the system still in a safe state
if this request is granted?
q Since RequestB = [1,0,2] ≤ Available = [3,3,2], this request
may be granted as long as the system is safe.
q If this request is actually granted, we have the following:

A 0 1 0 7 5 3 7 4 3 2 3 0
B 3 0 2 3 2 2 0 2 0
C 3 0 2 9 0 2 6 0 0
D 2 1 1 2 2 2 0 1 1
E 0 0 2 4 3 3 4 3 1

[3,0,2]=[2,0,0]+[1,0,2] [0,2,0]=[1,2,2]-[1,0,2] [2,3,0]=[3,3,2]-[1,0,2]

Example: 3/4
A 0 1 0 7 5 3 7 4 3 2 3 0
B 3 0 2 3 2 2 0 2 0
C 3 0 2 9 0 2 6 0 0
D 2 1 1 2 2 2 0 1 1
E 0 0 2 4 3 3 4 3 1

qIs the system in a safe state after this allocation?
qYes, because the safety algorithm will provide a
safe sequence <B,D,E,A,C>. Verify it by
yourself.
qTherefore, B’s request of [1,0,2] can safely be
made.

Example: 4/4
A 0 1 0 7 5 3 7 4 3 2 3 0
B 3 0 2 3 2 2 0 2 0
C 3 0 2 9 0 2 6 0 0
D 2 1 1 2 2 2 0 1 1
E 0 0 2 4 3 3 4 3 1

q After this allocation, E’s request RequestE=[3,3,0]
cannot be granted since RequestE=[3,3,0] ≤[2,3,0] is
false.
q A’s request RequestA=[0,2,0] cannot be granted because
the system will be unsafe.
q If RequestA=[0,2,0] is granted, Available=[2,1,0].
q None of the five processes can finish and the system is
unsafe.

Deadlock Detection
qIf a system does not use a deadlock prevention
or a deadlock avoidance algorithm, then a
deadlock situation may occur. Thus, we need
vAn algorithm that can examine the system
state to determine if a deadlock has
occurred. This is a deadlock detection
algorithm.
vAn algorithm that can help recover from a
deadlock. This is a recovery algorithm.
qA deadlock detection algorithm does not have
to know the maximum need Max and the
current need Need. It uses only Available,
Allocation and Request.

Deadlock Detection Algorithm
1. Let Work[1..m] and Finish[1..n] be two working arrays.
2. Work := Available and Finish[i]=FALSE for all i
3. Find an i such that both
v Finish[i] = FALSE // process i is not yet done
v Request[i,*] ≤ Work // its request can be satisfied
If no such i exists, go to Step 5
4. Work = Work + Allocation[i,*] // run it and reclaim
Finish[i] = TRUE // process i completes
go to Step 3
5. If Finish[i] = TRUE for all i, the system is in a safe state. If
Finish[i] = FALSE, then process Pi is deadlocked.

Use Request here rather than Need in the safety algorithm

Example: 1/2
Allocation Request Available
X Y Z X Y Z X Y Z
A 0 1 0 0 0 0 0 0 0
B 2 0 0 2 0 2
C 3 0 3 0 0 0
D 2 1 1 1 0 0
E 0 0 2 0 0 2
q Suppose maximum available resource is [7,2,6] and the
current state of resource allocation is shown above.
q Is the system deadlocked? No. We can run A first, making
Available=[0,1,0].
q Then, we run C, making Available=[3,1,3]. This is followed
by D, making Available=[5,2,4], and followed by B and E.

Example: 2/2
Allocation Request Available
X Y Z X Y Z X Y Z
A 0 1 0 0 0 0 0 0 0
B 2 0 0 2 0 2
C 3 0 3 0 0 1
D 2 1 1 1 0 0
E 0 0 2 0 0 2

qSuppose C requests for one more resource Z.
qNow, A can run, making Available=[0,1,0].
qHowever, none of B, C, D and E can run.
Therefore, B, C, D and E are deadlocked!

The Use of a Detection Algorithm
qFrequency
vIf deadlocks occur frequently, then the
detection algorithm should be invoked
frequently.
vOnce per hour or whenever CPU utilization
becomes low (i.e., below 40%). Low CPU
utilization means more processes are
waiting.

How to Recover: 1/3
qWhen a detection algorithm determines a
deadlock has occurred, the algorithm may
inform the system administrator to deal with it.
Of, allow the system to recover from a deadlock.
qThere are two options.
vProcess Termination
vResource Preemption
qThese two options are not mutually exclusive.

Recovery: Process Termination: 2/3
qAbort all deadlocked processes
qAbort one process at a time until the deadlock cycle
is eliminated
qProblems:
vAborting a process may not be easy. What if a
process is updating or printing a large file? The
system must find some way to maintain the state
of the file and printer before they can be reused.
vThe termination may be determined by the
priority/importance of a process.

Recovery: Resource Preemption: 3/3

qSelecting a victim: which resources and which
processes are to be preempted?
qRollback: If we preempt a resource from a
process, what should be done with that process?
vTotal Rollback: abort the process and restart it
vPartial Rollback: rollback the process only as far as
necessary to break the deadlock.
qStarvation: We cannot always pick the same
process as a victim. Some limit must be set.

Deadlock

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Deadlock