Deadlock Detection | Deadlock Prevention

Deadlock Handling-

 

Before you go through this article, make sure that you have gone through the previous article on Deadlock in OS.

 

The various strategies for handling deadlock are-

 

 

1. Deadlock Prevention
2. Deadlock Avoidance
3. Deadlock Detection and Recovery
4. Deadlock Ignorance

 

Deadlock Prevention-

 

• This strategy involves designing a system that violates one of the four necessary conditions required for the occurrence of deadlock.
• This ensures that the system remains free from the deadlock.

 

The various conditions of deadlock occurrence may be violated as-

 

1. Mutual Exclusion-

 

• To violate this condition, all the system resources must be such that they can be used in a shareable mode.
• In a system, there are always some resources which are mutually exclusive by nature.
• So, this condition can not be violated.

 

2. Hold and Wait-

 

This condition can be violated in the following ways-

 

Approach-01:

 

In this approach,

• A process has to first request for all the resources it requires for execution.
• Once it has acquired all the resources, only then it can start its execution.
• This approach ensures that the process does not hold some resources and wait for other resources.

 

Drawbacks-

 

The drawbacks of this approach are-

• It is less efficient.
• It is not implementable since it is not possible to predict in advance which resources will be required during execution.

 

Approach-02:

 

In this approach,

• A process is allowed to acquire the resources it desires at the current moment.
• After acquiring the resources, it start its execution.
• Now before making any new request, it has to compulsorily release all the resources that it holds currently.
• This approach is efficient and implementable.

 

Approach-03:

 

In this approach,

• A timer is set after the process acquires any resource.
• After the timer expires, a process has to compulsorily release the resource.

 

3. No Preemption-

 

• This condition can by violated by forceful preemption.
• Consider a process is holding some resources and request other resources that can not be immediately allocated to it.
• Then, by forcefully preempting the currently held resources, the condition can be violated.

 

A process is allowed to forcefully preempt the resources possessed by some other process only if- It is a high priority process or a system process. The victim process is in the waiting state.

 

4. Circular Wait-

 

• This condition can be violated by not allowing the processes to wait for resources in a cyclic manner.
• To violate this condition, the following approach is followed-

 

Approach-

 

• A natural number is assigned to every resource.
• Each process is allowed to request for the resources either in only increasing or only decreasing order of the resource number.
• In case increasing order is followed, if a process requires a lesser number resource, then it must release all the resources having larger number and vice versa.
• This approach is the most practical approach and implementable.
• However, this approach may cause starvation but will never lead to deadlock.

 

Deadlock Avoidance-

 

• This strategy involves maintaining a set of data using which a decision is made whether to entertain the new request or not.
• If entertaining the new request causes the system to move in an unsafe state, then it is discarded.
• This strategy requires that every process declares its maximum requirement of each resource type in the beginning.
• The main challenge with this approach is predicting the requirement of the processes before execution.
• Banker’s Algorithm is an example of a deadlock avoidance strategy.

 

Deadlock Detection and Recovery-

 

• This strategy involves waiting until a deadlock occurs.
• After deadlock occurs, the system state is recovered.
• The main challenge with this approach is detecting the deadlock.

 

Deadlock Ignorance-

 

• This strategy involves ignoring the concept of deadlock and assuming as if it does not exist.
• This strategy helps to avoid the extra overhead of handling deadlock.
• Windows and Linux use this strategy and it is the most widely used method.
• It is also called as Ostrich approach.

 

To gain better understanding about Deadlock Handling Strategies,

Watch this Video Lecture

 

Next Article- Practice Problems On Deadlock

 

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