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Operating Systems (CS 340 D)
Princess Nora UniversityFaculty of Computer & Information
SystemsComputer science Department
(Chapter-7)Deadlocks
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Chapter 7: Deadlocks
1. The Deadlock Problem
2. Deadlock Characterization
3. Methods for Handling
Deadlocks
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OBJECTIVES:
To develop a description of deadlocks, which prevent sets of concurrent processes from completing their tasks
To present methods for Handling Deadlocks
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The Deadlock Problem
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The Deadlock Problem
In a multiprogramming environment, several processes may compete for a finite number of resources.
A process requests resources; if the resources are not available at that time, the process enters a waiting state.
Sometimes, a waiting process is never again able to change state, because the resources it has requested are held by other waiting processes.
This situation is called a DEADLOCK.
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What is a deadlock problem??o A set of blocked processes each holding a resource
and waiting to acquire a resource held by another process in the set
Example o System has 2 disk driveso P1 and P2 each hold one disk drive and each need
the other one Note :– Most OSs do not prevent or deal with deadlocks
The Deadlock Problem (cont..)
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Bridge Crossing Example
Traffic only in one direction Each section of a bridge can be viewed as a
resource If a deadlock occurs, it can be resolved if one
car backs up (preempt resources and rollback) Several cars may have to be backed up if a
deadlock occurs Starvation is possible
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System Model
The resources are partitioned into several types (R1, R2, . . ., Rm )o E.g.: CPU cycles, memory space, I/O devices
Each resource type Ri has Wi instances.
o E.g : If a system has (2) CPUs, then the resource type CPU has (2)
instances. The resource type printer may have (5) instances.
Instances are identical
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Each process utilizes a resource as follows:
1. Request >> If the request cannot be granted immediately ,then the requesting process must wait until it can acquire the resource.
2. Use 3. Release
A set of processes is in a deadlocked state when every process in the set is waiting for an event that can be caused only by another process in the set.
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System Model (cont.)
Resource forms:
Example: consider a system with one printer and one DVD drive.
Suppose that process Pi is holding the DVD and process Pj is holding the printer. If Pi requests the printer and Pj requests the DVD drive, a deadlock occurs.
Physical resources Logical resources
e.g. printers, DVD drives, memory space, and CPU cycles
e.g. files
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The Deadlock Characterization
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Deadlock Characterization
1.Mutual exclusion: the resources are sharable or non sharable mutual exclusion happen when only one process at a time can use a resource…(i.e. No resource sharing)
2.Hold and wait: a process holding at least one resource is waiting to acquire additional resources held by other processes
Deadlock can arise if four conditions hold simultaneously:
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Deadlock Characterization (cont..)
3. No preemption: a resource can be released only voluntarily by the process holding it, after that process has completed its task
4.Circular wait: There exists a set {P0, P1, …, Pn} of waiting
processes such that :oP0 is waiting for a resource that is held by P1,
o P1 is waiting for a resource that is held by P2, …, Pn–1 is waiting for a resource that is held by Pn,
o Pn is waiting for a resource that is held by P0.P0 P1 P2 Pn
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Resource-Allocation Graph
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Resource-Allocation Graph
Deadlocks can be described in terms of a directed graph called a system resource-allocation graph.
This graph consists of a set of vertices V and a set of edges E.
V is partitioned into two types:
E is partitioned into two types:
P = {P1, P2, …, Pn}, the set consisting of all the processes in the system
Request edge – directed edge Pi Rj
R = {R1, R2, …, Rm}, the set consisting of all resource types in the system
Assignment edge – directed edge Rj Pi
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Resource-Allocation Graph (Cont.)
Process
Resource Type with 4 instances
Pi requests instance of Rj
Pi is holding an instance of RjPi
Pi
Rj
Rj
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Example of a Resource Allocation Graph
The sets P, R, and E: P = {P1, P2, P3} R = {R1, R2, R3, R4} E = {P1 → R1, P2 → R3, R1 →
P2, R2 → P2, R2 → P1, R3 → P3}
Resource instances: (1) instance of resource type
R1 (2) instances of resource type
R2 (1) instance of resource type R3
(3) instances of resource type R4
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Example of a Resource Allocation Graph
Process states:
Process P1 is holding an instance of resource type R2 and is waiting for an instance of resource type R1.
Process P2 is holding an instance of R1 and an instance of R2 and is waiting for an instance of R3.
Process P3 is holding an instance of R3.
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Basic Facts
How we recognize deadlock by using a Resource Allocation Graph??
If graph contains no cycles no deadlock
If graph contains a cycle
If only one instance per resource type, then there is a deadlock
If several instances per resource type, possibility of deadlock .
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Resource Allocation Graph With NO Deadlock
Example (1): The graph contains NO cycles NO deadlock is
occurred.
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It ‘s a dead lock situation >>> Processes P1, P2, and P3 are deadlocked…….why?? There is no chance to break the cycle.
Resource Allocation Graph With a Deadlock
Example (2): The graph contains
(2)cycles may be a deadlock is occurred.
Cycle (1) : P1 → R1 → P2 → R3 → P3 → R2 → P1 Cycle (2) : P2 → R3 → P3 → R2 → P2
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Graph With A Cycle But No Deadlock
NO dead lock situation .....WHY???? process P4 may release its instance of resource
type R2…… That resource can then be allocated to P3, breaking the cycle.
Example (3): The graph contains
(1)cycle may be a deadlock
Cycle : P1 → R1 → P3 → R2 → P1
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Methods for Handling Deadlocks
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Methods for Handling Deadlocks
OS can deal with the deadlock problem in one of three ways
1-prevent or avoid deadlocks
2-Allow 3-Ignore
ensuring that the system will never enter a deadlocked state.
Allow the system to enter a deadlocked state, detect it, and recover.
Ignore the problem
Used by most operating systems, including Unix & windows
This method is cheaper than 1 or 2 .
Solution: The system must be restarted manually
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To ensure that deadlocks never occur, the system can use either
1. a deadlock prevention or 2. a deadlock-avoidance scheme
Method (1) for Handling Deadlocks
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Deadlock prevention
Deadlock avoidance
provides a set of methods to ensure that at least one of the necessary 4 conditions (explained before ) cannot hold.
These methods prevent deadlocks by constraining how requests for resources can be made.
• requires that the operating system be given additional information in advance concerning which resources a process will request and use during its lifetime.
• With this additional knowledge, the operating system can decide for each request whether or not the process should wait.
Method(1) for Handling Deadlocks
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1-Deadlock Prevention
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Deadlock Prevention
1- Mutual Exclusion – not required for sharable resources; must hold for non-sharable resources. That is, at least one resource must be nonsharable. Sharable
resources, in contrast, do not require mutually exclusive access and thus cannot be involved in a deadlock
2- Hold and Wait – must guarantee that whenever a process requests a resource, it does not hold any other resources by using one of the following protocols:
Problems-> Low resource utilization; starvation possible
Remove the possibility of deadlock occurring by denying one of the four necessary conditions:
•The process must request all its resources before execution.
• or allow process to request resources only when the process has none
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Deadlock Prevention (Cont.)
3- 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
Process will be restarted only when it can regain its old resources, as well as the new ones that it is requesting
It is applied to resources whose state easily saved and restored later, such as CPU registers and memory space but it not applicable for resources such as printers
The main Advantages is more better resource utilization Problems
The cost of removing a process's resources starvation possible It is not applicable for all resource types such as printers
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One way to ensure that this condition never holds is to impose a total ordering of all resource types and to require that each process requests resources in an increasing order of enumeration.
let R = {R1, R2, ..., Rm} be the set of resource types. We assign to each resource type a unique integer number, which allows us to compare two resources and to determine whether one precedes another in our ordering.
Formally, we define a one-to-one function F: R→N, where N is the set of natural numbers.
4- Circular Wait –
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Deadlock Prevention (Cont.)
Example : if the set of resource types R includes tape drives, disk drives, and printers,
then the function F might be defined as follows:
F(tape drive) = 1
F(disk drive) = 5
F(printer) = 12
We can now consider the following protocol to prevent deadlocks:
Each process can request resources only in an increasing order of enumeration.
That is, a process can initially request any number of instances of a resource type —say, Ri . After that, the process can request instances of resource type Rj if and only if F(Rj ) > F(Ri ). …………..(Protocol#1)
4- Circular Wait – cont..
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Deadlock Prevention (Cont.)
EX: a process that wants to use the tape drive and printer at the same time must first request the tape drive and then request the printer.
Alternatively, we can require that a process requesting an instance of resource type Rj must have released any resources Ri such that F(Ri ) ≥ F(Rj ). …..(Protocol#2)
Problems Resources must be requested in ascending
order of resource number rather than as needed
4- Circular Wait – cont..
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2-Deadlock Avoidance
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Deadlock Avoidance
Simplest and most useful model requires that each process declare the maximum number of resources of each type that it may need
The deadlock-avoidance algorithm dynamically examines the resource-allocation state to ensure that there can never be a circular-wait condition
Resource-allocation state is defined by the number of available and allocated resources, and the maximum demands of the processes
Requires that the system has some additional a priori information available
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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 sequence <P1, P2, …, Pn> of ALL the processes in the systems such that 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
o That is: If Pi resource needs are not immediately available, then Pi can wait until all Pj have finished
o When Pj is finished, Pi can obtain needed resources, execute, return allocated resources, and terminate
o When Pi terminates, Pi +1 can obtain its needed resources, and so on
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Basic Facts
If a system is in safe state ⇒ no deadlocks
If a system is in unsafe state ⇒ possibility of deadlock
Avoidance ⇒ ensure that a system will never enter an unsafe state.
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Safe, Unsafe , Deadlock State
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Avoidance algorithms
The avoidance algorithms ensure that the system will always remain in a safe state.
Single instance of a resource type Use a resource-allocation graph
Multiple instances of a resource type Use the banker’s algorithm
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Resource-Allocation Graph Scheme
Claim edge Pi → Rj indicated that process Pj may request resource Rj; represented by a dashed line
Claim edge converts to request edge when a process requests a resource
Request edge converted to an assignment edge when the resource is allocated to the process
When a resource is released by a process, assignment edge reconverts to a claim edge
Resources must be claimed a priori in the system
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Resource-Allocation Graph
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Unsafe State In Resource-Allocation Graph
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Resource-Allocation Graph Algorithm
Suppose that process Pi requests a resource Rj
The request can be granted only if converting the request edge to an assignment edge does not result in the form of a cycle in the resource allocation graph
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Banker’s Algorithm
Multiple instances
Each process must a priori claim maximum use
When a process requests a resource it may have to wait
When a process gets all its resources it must return them in a finite amount of time
Banker's Algorithm consist of (2) sub-algorithms:
o Safety algorithmo Recourse-Request Algorithm
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Banker’s Algorithm
When a new process enters the system, it must declare the maximum number of instances of each resource type that it may need. This number may not exceed the total number of resources in the system.
When a process requests a set of resources, the system must determine whether the allocation of these resources will leave the system in a safe state.
o If it will, the resources are allocated; o otherwise, the process must wait until some other
process releases enough resources.
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Data Structures for the Banker’s Algorithm
Available: Vector of length m. If available [j] = k, there are k instances of resource type Rj available
Max: n x m matrix. If Max [i,j] = k, then process Pi may request at most k instances of resource type Rj
Allocation: n x m matrix. If Allocation[i,j] = k then Pi is currently allocated k instances of Rj
Need: n x m matrix. If Need[i,j] = k, then Pi may need k more instances of Rj to complete its taskNeed [i,j] = Max[i,j] – Allocation [i,j]
Let n = number of processes, and m = number of resources types.
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Safety Algorithm1. Let Work and Finish be vectors of length m and n, respectively.
Initialize: Work = Available Finish [i] = false for i = 0, 1, …, n- 1
2. Find any i such that both: (a) Finish [i] = false (b) Needi Work If no such i exists, go to step 4
3. Work = Work + Allocationi
Finish[i] = truego to step 2
4. If Finish [i] == true for all i, then the system is in a safe state
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Resource-Request Algorithm for Process Pi Request = request vector for process Pi. If Requesti [j] = k
then process Pi wants k instances of resource type Rj
1. If Requesti Needi go to step 2. Otherwise, raise error condition, since process has exceeded its maximum claim
2. If Requesti Available, go to step 3. Otherwise Pi must wait, since resources are not available
3.Pretend to allocate requested resources to Pi by modifying the state as follows:
Available = Available – Request;Allocationi = Allocationi + Requesti;
Needi = Needi – Requesti; If safe the resources are allocated to Pi If unsafe Pi must wait, and the old resource-allocation
state is restored
Example of Banker’s Algorithm
A system with 5 processes (p0,p1,p2,p3,p4), 3 resource types )A,B,C) with instances (10,5,7)
Snapshot at time T0:Allocation Max need
A B C A B CP0 0 1 0 7 5 3
P1 2 0 0 3 2 2
P2 3 0 2 9 0 2
P3 2 1 1 2 2 2
p4 0 0 2 4 3 3
Example of Banker’s Algorithm (Cont.)
Allocation MAX Need Available
A B C A B C A B C A B C
P0 0 1 0 7 5 3 7 4 3 3 3 2
P1 2 0 0 3 2 2 1 2 2
P2 3 0 2 9 0 2 6 0 0
P3 2 1 1 2 2 2 0 1 1
p4 0 0 2 4 3 3 4 3 1
-Is system now in safe state?
- If a request R1= (1,0,2) arrive, Can this request be granted immediately?
Example of Banker’s Algorithm (Cont.)
Allocation MAX Need Available
A B C A B C A B C A B C
P0 0 1 0 7 5 3 7 4 3 2 3 0
P1 3 0 2 3 2 2 0 2 0
P2 3 0 2 9 0 2 6 0 0
P3 2 1 1 2 2 2 0 1 1
p4 0 0 2 4 3 3 4 3 1
- Calculate new state after granting request R1= (1,0,2)
Example of Banker’s Algorithm (Cont.)
-If a request R4= (3,3, 0) arrive, Can this request be granted immediately?
-If a request R0= (0,2, 0) arrive, Can this request be granted immediately?
-If a request R3= (1, 1, 1) arrive, Can this request be granted immediately?
Allocation MAX Need Available
A B C A B C A B C A B C
P0 0 1 0 7 5 3 7 4 3 2 3 0
P1 3 0 2 3 2 2 0 2 0
P2 3 0 2 9 0 2 6 0 0
P3 2 1 1 2 2 2 0 1 1
p4 0 0 2 4 3 3 4 3 1
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Thank you
End of Chapter 7
