lect9 deadlocks

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OPERATING S YSTEMS

(IS ZC362 LECTURE

9)

DEADLOCK



Process states

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The Deadlock Problem

A set of blocked processes each holding aresource and waiting to acquire a resourceheld by another process in the set

Permanent blocking of a set of processes thateither compete for system resources orcommunicate with each other

No efficient solution

Involve conflicting needs for resources by twoor more processes

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Examples Example 1

System has 2 disk drives

P 1 and P 2 each hold one disk drive and eachneeds another one

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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 Note – Most OSes do not prevent or deal with

deadlocks

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System Model

Processes P1, P2, …, Pn

Resource types R 1, R 2, . . ., R mCPU cycles, memory space, I/O devices

Each resource type R i has W i instances.

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Resource Allocation Request

If the request cannot be granted immediately then therequesting process must wait until it can acquire theresource.

Use

Process operates on resource. Release

Process release the resource.

Resources

Physical resource -- Printer,Memory space….. Logical resource --- Semaphore,Files…..



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Reusable Resources

Used by only one process at a time and notdepleted by that use

Processes obtain resources that they laterrelease for reuse by other processes

Processors, I/O channels, main and secondarymemory, devices, and data structures such asfiles, databases, and semaphores

Deadlock occurs if each process holds oneresource and requests the other



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Example of Deadlock Space is available for allocation of

200Kbytes, and the following sequence ofevents occur

Deadlock occurs if both processes progressto their second request

P1

. . .

. . .Request 80 Kbytes;

Request 60 Kbytes;

P2

. . .

. . .Request 70 Kbytes;

Request 80 Kbytes;



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Consumable Resources Created (produced) and destroyed (consumed)

Interrupts, signals, messages, and information inI/O buffers

There is no limit on the number of consumableresources of a particular type.



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Example of Deadlock Deadlock occurs if receive is blocking

P1

. . .

. . .

Receive(P2);

Send(P2, M1);

P2

. . .

. . .

Receive(P1);

Send(P1, M2);



Deadlock Characterization

Mutual exclusion: only one process at a time can use a resource.

At least one resource must be in a non sharablemode; that is only one process at a time can use a

resource. If any other process request the resource, the

requesting process must be delayed until theresource has been released.

Hold and wait: a process holding at least one resource is waiting to

acquire additional resources held by other processes

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Deadlock Characterization

No preemption: a resource can be released only voluntarily by the

process holding it, after that process has completed itstask .(The resource cannot be preempted).

Circular wait: there exists a set {P 0, P 1, …, P 0} of waiting processessuch that

P 0 is waiting for a resource that is held by P 1,

P 1 is waiting for a resource that is held by P 2, …,

P n –1 is waiting for a resource that is held by P n, and

P n is waiting for a resource that is held by P 0.

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Deadlock can arise if all four conditions hold simultaneously.



Traffic Deadlock

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Resource-Allocation Graph

V is partitioned into two types:

P = {P 1, P 2, …, P n }, the set consisting of all theprocesses in the system

R = {R 1, R 2, …, R m }, the set consisting of all

resource types in the system

request edge – directed edge P 1 R j assignment edge – directed edge R j P i

A set of vertices V and a set of edges E .

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Resource-Allocation Graph (Cont.) Process

Resource Type with 4 instances

P i requests instance of R j

P i is holding an instance of R j

P i

R j

P i

R j 17



Example of a Resource Allocation Graph

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Resource Allocation Graph With A Deadlock

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Graph With A Cycle But No Deadlock

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Basic Facts If graph contains no cycles no deadlock

If graph contains a cycle

if only one instance per resource type, then

deadlock if several instances per resource type, possibility of

deadlock

Most of the Operating Systems are nothandling Deadlock problems

It is too costly to prevent / avoid / detect – recoverdeadlock

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Methods for Handling Deadlocks Ensure that the system will never enter a

deadlock state

Allow the system to enter a deadlock state

and then recover

Ignore the problem and pretend that

deadlocks never occur in the system; used bymost operating systems, including UNIX

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Deadlock Prevention

Ensure that at least one of the 4 necessarycondition be prevented.

Mutual Exclusion – not required for sharableresources; must hold for non-sharableresources

Only one process may use a resource at a time

Restrain the ways request can be made

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Hold and Wait – must guarantee thatwhenever a process requests a resource, itdoes not hold any other resources

Require process to request and be allocated all itsresources before it begins execution

Implementation: System calls requestingresources precede all other function calls

Low resource utilization; Starvation possible

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Deadlock Prevention (Cont.)



Alternative method: Allow a process to request resources only when

the process has no resources.

Release all resources before requesting for a new

one This method is inefficient

A process may be held up for a long time waiting forall of its resource request be filled, when it could have

proceeded with only some of the resources. A process may not know in advance all of the

resources that it will require.

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Deadlock Prevention (Cont.) No Preemption

If a process that is holding some resources requests

another resource that cannot be immediately allocated toit, then all resources currently being held are released

Preempted resources are added to the list of resourcesfor which the process is waiting

Process will be restarted only when it can regain its oldresources, as well as the new ones that it is requesting

Alternate Approach:

If a process request some resources,

Allot to the process if the resource is free

Else, check whether they are allocated to some otherprocess that is waiting for additional resource. If so,preempt the resource from the waiting process and allotthem to the requesting process.

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Circular Wait

Impose a total ordering of all resource types,and require that each process requests

resources in an increasing order ofenumeration

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Example

Assign a unique integer number to individual resource which allowsus to compare two resources and determine one proceed other.

Define a one – to – one function F: R

N F(tape drive)=1

F(disk drive)=5

F(printer) =12

Process can request resources only in an increasing order of

enumeration.

So R(j) is acceptable to a process if and only if F(R(i))< F(R(j)). IfF(R(i))> F(R(j)) then release (R(i)) before R(j) entered in.

Example: Process needs tape drive and printer must request thetape driver first then printer.

Disadvantage of Deadlock Prevention

Low device Utilization

Reduced system throughput.



Deadlock Avoidance Requires that the system has some additional a priori

Information available .(Each request requires the

information of the resources currently available, theresources currently allocated to each process,future

request and release of each process etc…)

Simplest and most useful model requires that each process

declare the maximum number of resources of each typethat it may need

The deadlock-avoidance algorithm dynamically examinesthe 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 maximumdemands of the processes

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Safe State

A state is a safe state if the system can

allocate resources to each process in someorder and still avoid a deadlock.

If the system is in safe state only if there exists

a safe sequence. When a process requests an available

resource, system must decide if immediate

allocation leaves the system in a safe state

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Example 3 processes and 12 CD drives

At time t0 the system is in safe state <P1,P0,P2> System will go to unsafe state if process P2 request and allocated one

more resource.

P2 9 3

Avoidance algorithm ensures that the system will never deadlock.

Initially the system is in safe state if a process is requesting a resource,the resource is granted only if the allocation leaves the system in safestate.

In this scheme if a process request a resource that is currentlyavailable, it may still have to wait. So resource utilization can be lower.

Max. Needs Currently holding

P0 10 5

P1 4 2

P2 9 2



System is in safe state if there exists a sequence <P 1, P 2 ,…, P n > of ALL the processes is the systems such that foreach Pi, the resources that Pi can still request can be

satisfied by currently available resources + resources heldby all the P j, with j < i

That is:

If Pi resource needs are not immediately available, then

Pi can wait until all Pj have finished When Pj is finished, Pi can obtain needed resources,

execute, return allocated resources, and terminate

When Pi terminates, Pi +1 can obtain its needed

resources, and so on If no such sequence exists, then the system state is said

to be unsafe.

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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 willnever enter an unsafe state.

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Safe, Unsafe , Deadlock State

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