Chapter 7 – Deadlock

Resources

  • Examples of computer resources

Printers

Tape drives

Tables

  • Preemptable resources

Can be taken away from a process with no ill effects

  • Nonpreemptable resources

Will cause the process to fail if taken away

  • Reusable resources

Used by one process at a time and not depleted by that use

Examples: Processors, I/O channels, main and secondary memory, files, databases, and semaphores

  • Shared and exclusive resources

Example of shared resource: FILE

Example of exclusive resource: PRINTER

  • Consumable resources

Created (produced) and destroyed (consumed) by a process

Examples: Interrupts, signals, messages, and information in I/O buffers

System Model

  • A system consists of a number of resources to be distributed among a number of competing processes.
  • There are different types of resources R1, R2,..., Rm.

CPU cycles, memory space, I/O devices

  • Each resource type Ri has Wi instances. For example, if two CPUs then resource type CPU has two instances.

Sequence of Events Required to Use a Resource

  • Each process utilizes a resource as follows:

Request a resource:

  • Request is made through a system call
  • Process must wait if request is denied
  • Requesting process may be blocked
  • may fail with error code

Use the resource:

  • The process can operate on the resource.

Release the resource:

  • The process releases the resource. A resource is released through a system call.

Deadlock

  • Formal Definition

A set of processes is deadlocked if each process in the set is waiting for an event that only another process in the set can cause

  • Usually the event is release of a currently held resource
  • None of the processes can …

Run

Release resources

Be awakened

  • Involve conflicting needs for resources by two or more processes

Examples of Deadlock

  • Example 1

System has 2 tape drives.P1 and P2 each hold one tape drive and each needs another one.

  • Example 2

Semaphores A and B, initialized to 1

P0 P1

wait (A);wait(B)

wait (B);wait(A)

  • Example 3

Space is available for allocation of 200K bytes, and the following sequence of events occur

P0
….
Request 80KB;
…
Request 60KB; / P1
…
Request 70KB;
…
Request 80KB;

Deadlock occurs if both processes progress to their second request

Four Conditions for Deadlock

  • Deadlock can arise if four conditions hold simultaneously

Mutual exclusion condition:

  • Only one process at a time can use a resource (non-shareable resource).
  • Each resource is assigned to a process or is available

Hold and wait condition:

  • A process holding at least one resource can request for additional resources

No preemption condition:

  • A resource can be released only voluntarily by the process holding it. That is previously granted resources cannot be forcibly taken away.

Circular wait condition:

  • 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.

Resource-Allocation Graph

  • Deadlocks can be described more precisely 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:

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.

  • E is partitioned into two types as well:

Request edge – directed edge P1 Rj

Assignment edge–directed edge RjPi

  • Different symbols are used to represent processes and resources as given below:

Process:

Resource type of 4 instances:

Pirequests instance of Rj:

Pi is holding an instance of Rj

:

Method of Handling Deadlocks
  • Just ignore the problem altogether
  • Prevention

Ensure that the system will never enter a deadlock state

Requires negating one of the four necessary conditions

  • Dynamic avoidance

Require careful resource allocation

  • Detection and recovery

Allow the system to enter a deadlock state and then recover

We need some methods to determine whether or not the system has entered into deadlock.

We also need algorithms to recover from the deadlock.

The Ostrich Algorithm

  • Pretend there is no problem
  • The system will eventually stop functioning
  • Reasonable if

Deadlocks occur very rarely

Cost of prevention is high

  • UNIX and Windows takes this approach
  • It is a trade off between

Convenience

Correctness

Deadlock Prevention

  • Prevent/denyMutual Exclusioncondition

Use shareable resource.

  • Impossible for practical system.
  • Prevent/Deny
  • Hold and Waitcondition

(a)Pre-allocation - Require processes to request resources before starting

  • A process never has to wait for what it needs

(b)Process must give up all resources and then request all immediately needed

Problems

  • May not know required resources at start of run
  • Low resource utilization – many resources may be allocated but not used for long time
  • Starvation possible – a process may have to wait indefinitely for popular resources.
  • Prevent/deny No Preemptioncondition

(a)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.

Process will be restarted only when it can regain its old resources, as well as the new ones that it is requesting.

(b)The required resource(s) is/are taken back from the process(s) holding it/them and given to the requesting process

Problems

  • Some resources (e.g. printer, tap drives) cannot be preempted without detrimental implications.
  • May require the job to restart
  • Prevent/Deny
  • Circular Wait

Order resources (each resource type is assigned a unique integer) and allow process to request for them only in increasing order

If a process needs several instances of the same resource, it should issue a single request for all of them.

Alternatively, we can require that whenever a process requests an instance of a resource type it has released all the resources which are assigned a smaller inter value.

Problem:

  • Adding a new resource that upsets ordering requires all code ever written to be modified
  • Resource numbering affects efficiency
  • A process may have to request a resource well before it needs it, just because of the requirement that it must request resources in ascending order

An example:

Deadlock Avoidance
  • OS never allocates resources in a way that could lead to a deadlock
  • Processes must tell OS in advance how many resources they will request

Some Definitions

  • State of a system

An enumeration of which processes hold, are waiting for or might request which resource

  • Safe state
  1. No process is deadlocked, and there exits no possible sequence of future request in which deadlock could occur
  2. No process is deadlocked and the current state will not lead to a dead lock state
  3. Safe state is where there is at least one sequence that does not result in deadlock
  • Unsafe state

Is a state that is not safe

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

Deadlock Avoidance with Resource-Allocation Graph

  • This algorithm can be used if we have only one instance of each resource type.
  • In addition to the request and assignment edges, a claim edge is also introduced.
  • Claim edgePiRj indicated that process Pj may request resource Rj in future; represented by a dashed line.
  • Claim edge converts to request edge when a process requests a resource.
  • When a resource is released by a process, assignment edge reconverts to a claim edge.
  • Resources must be claimed a priori in the system. That is, before a process starts executing, all of its claim edges must already appear in the resource-allocation graph.
  • Suppose that process Pi requests resource Rj. The request can be granted only if converting the request edge if converting the request edge PiRj to an assignment edge does not result in a cycle in the resource-allocation graph. That is we use a cycle detection algorithm is used. If no cycle exits, the process Pi will have to wait.


Resource-allocation graph for deadlock avoidance

An unsafe state in the resource-allocation graph

Banker’s Algorithm

  • Applicable to system with multiple instances of resource types.
  • 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 runs each time:

A process requests resource – Is it sage?

A process terminates – Can I allocate released resources to a suspended process waiting for them?

A new state is safe if and only if every process can complete after allocation is made

  • Make allocation and then check system state and deallocate if unsafe

Data Structures for Banker’s algorithm

  • Let n = number of processes, and m = number of resources types.
  • Available: Vector of length m. If available [j] = k, there are k instances of resource type Rjavailable.
  • Max: n x m matrix. Max [i,j] = k mean that process Pimay request at most k instances of 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 Rjto complete its task.

Need [i,j] = Max[i,j] – Allocation [i,j].

Safety Algorithm

  1. Let Work and Finish be vectors of length m and n, respectively.

Initialize: Work = Available

Finish [i]=false for i=1,3, …, n.

  1. Find and i such that both:

(a) Finish [i] = false

(a)

(b) NeediWork

If no such i exists, go to step 4.

  1. Work = Work + Allocationi

Finish[i] = true
go to step 2.

  1. If Finish [i] == true for all i, then the system is in a safe state.

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Needigo 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 = Requesti;

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

5 processes P0 through P4; 3 resource types

A (10 instances),
B (5 instances), and

C (7 instances).

Snapshot at time T0:

Allocation / Max / Available
ABC / ABC / ABC
P0 / 0 1 0 / 7 5 3 / 3 3 2
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

The content of the matrix. Need is defined to be Max – Allocation.

Process / Need
A B C
P0 / 7 4 3
P1 / 1 2 2
P2 / 6 0 0
P3 / 0 1 1
P4 / 4 3 1

The system is in a safe state since the sequence <P1,P3,P4,P2,P0> satisfies safety criteria.

Example P1 Request (1,0,2)

  • Check that Request  Available

that is, (1,0,2)  (3,3,2) true.

Process / Allocation / Need / Available
A B C / A B C / A B C
P0 / 0 1 0 / 7 4 3 / 2 3 0
P1 / 3 0 2 / 0 2 0
P2 / 3 0 1 / 6 0 0
P3 / 2 1 1 / 0 1 1
P4 / 0 0 2 / 4 3 1
  • Executing safety algorithm shows that sequence <P1, P3, P4, P0, P2> satisfies safety requirement.
  • Can request for (3,3,0) by P4 be granted?
  • Can request for (0,2,0) by P0 be granted?

Deadlock Detection Recovery

Allow system to enter deadlock state

  • Need a detection algorithm
  • Need a recovery algorithm

How to Detect a Deadlock Using a Resource-Graph?

  • If each resource type has exactly one instance and the graph has a cycle then a deadlock has occurred. Or if the cycle involves only a set of resource types, each of which has only a single instance, then the deadlock has occurred.

Therefore, a cycle in the graph is both a necessary and sufficient condition for the existence of a deadlock.

Examples:

Resource-allocation graph with a deadlock

Recovery from Deadlocks – 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?

Priority of the process.

How long process has computed, and how much longer to completion.

Resources the process has used.

Resources process needs to complete.

How many processes will need to be terminated?

Is process interactive or batch?

Recovery from Deadlocks – Resource Preemption
  • Selecting a victim – minimize cost.
  • Rollback – return to some safe state, restart process for that state.
  • Starvation – same process may always be picked as victim, include number of rollback in cost factor.

Combined Approach to Deadlock Handling

  • Combine the three basic approaches

prevention

avoidance

detection

allowing the use of the optimal approach for each of resources in the system.

  • Partition resources into hierarchically ordered classes.
  • Use most appropriate technique for handling deadlocks within each class.

7.1

Prepared by Dr. Amjad Mahmood