Topik 4 Deadlock

8/13/2019 Topik 4 Deadlock

1/22

Pn. Nin Hayati Binti Mohd Yusoff


2/22

EXAMPLES:

"It takes money to make money".

You can't get a job without experience; you can't get experience without a

job.

BACKGROUND:

The cause of deadlocks: Each process needing what another process has. This

results from sharing resources such as memory, devices, links.

Under normal operation, a resource allocations proceed like this:

1. Request a resource (suspend until available if necessary ).

2. Use the resource.

3. Release the resource.


3/22

In a multiprogramming system, processes request resources.

If those resources are being used by other processes then the processenters a waiting state.

However, if other processes are also in a waiting state, we have

deadlock. The formal definition of deadlock is as follows:

A set of processes is in a deadlock state if everyprocess in the set is waiting for an event (release)that can only be caused by some other process in

the same set.


4/22

Process-1 requests the printer, gets it

Process-2 requests the tape unit, gets it Process-1 and

Process-1 requests the tape unit, waits Process-2 are

Process-2 requests the printer, waits deadlocked!


5/22

A process must request a resource before using it. It must release the resource after

using it.

(request -> use -> release)

A process cannot request a number more than the total number of resourcesavailable in the system


6/22

A process cannot request a number more than the total number of resources availablein the systemA set of blocked processes each holding a resource and waiting toacquire a resource held by another process in the set.

Example System has 2 tape drives. P1and P2each hold one tape drive and each needs another one.

Example semaphoresAandB, initialized to 1

P0 P1

wait (A); wait(B)wait (B); wait(A)


7/22


8/22

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.

request edge directed edge P1 Rj

assignment edge directed edge Rj Pi

A set of vertices Vand a set of edges E.


9/22

Process

Resource Type with 4 instances

Pirequests instance of Rj

Piis holding an instance of Rj

Pi

Rj

Pi

Rj


10/22

P2 Requests P3

R Assigned to P


11/22


12/22

Methods for handling deadlocks are

Deadlock

prevention

Deadlock

avoidance

Deadlock detection

and recovery


13/22

A deadlock occurs if and only if the following four conditions hold in asystem simultaneously: !

Mutual Exclusion

Hold and Wait

No Preemption

Circular Wait


14/22

There must be at least one process that is holding at least

one resource and waiting for other resources that are being

hold by other processes.

At least one of the resources is non-sharable (that is; only alimited number of processes can use it at a time and if it is

requested by a process while it is being used by another one,

the requesting process has to wait until the resource is

released.).


Mutual Exclusion

Hold and Wait


15/22

There exists a set of processes: {P1, P2, ..., Pn} such that

P1 is waiting for a resource held by P2

P2 is waiting for a resource held by P3

...

Pn-1 is waiting for a resource held by Pn

Pn is waiting for a resource held by P1

No resource can be preempted before the holding processcompletes its task with that resource


No Preemption

Circular Wait


16/22

If we have prior knowledge of how resources will be requested, it'spossible to determine if we are entering an "unsafe" state.

Possible states are:deadlock - no forward progress can be made.Unsafe state - A state that may allow deadlock.Safe state - A state is safe if a sequence of processes exist such

that there are enough resources for the first to finish, andas each finishes and releases its resources there areenough for the next to finish.

The rule is simple: if a request allocation would cause an unsafe state, donot honor that request.

NOTE: all deadlocks are unsafe, but all unsafes are NOT deadlocks.


17/22

NOTE: all deadlocks are unsafe, but all unsafes are NOT deadlocks

UNSAFE SAFE

DEADLOCK

Only with luck willprocesses avoid

deadlock.

O.S. can avoiddeadlock.


18/22

If a system has no deadlock prevention and no deadlock avoidance

scheme, then it needs a deadlock detection scheme withrecovery from deadlock capability. For this, information should be kept on the allocation of resources to

processes, and on outstanding allocation requests.

Then, an algorithm is needed which will determine whether the system has

entered a deadlock state.


19/22

If the system is in a deadlock state, some methods for recovering it

from the deadlock state must be applied. There are various ways for

recovery:

Allocate one resource to several processes, by violating mutual

exclusion.

Preempt some resources from some of the deadlocked

processes.

Abort one or more processes in order to break the deadlock.


20/22


21/22

ANY QUESTIONS?


22/22

END OF CHAPTER

4.0 RESOURCE MANAGEMENT4.1 Understand Processor Management4.1.1

Explain the role of control blocks and interrupts in the dispatching

process.

4.1.2 Describe the various types of scheduling processes:a. Long-term schedulingb. Medium-term schedulingc. Short-term scheduling

Topik 4 Deadlock

Documents

Transcript of Topik 4 Deadlock