Computer Science Chapter 7: Deadlocks

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Questions and Answers

What does a claim edge indicate in a resource-allocation graph?

A process is currently using a resource.

A process may request a resource in the future. (correct)

A process has received a resource allocation.

A resource has been released by a process.

What does a request edge in a resource-allocation graph represent?

A process is requesting an instance of a resource type. (correct)

A resource type is being released by a process.

A resource type is assigned to a process.

A process is currently using a resource type.

Under what condition is a resource request granted in the resource-allocation graph?

If it results in no cycles when converted to an assignment edge. (correct)

If the process releases its currently held resources.

If all processes can continue executing without blocking.

If a claim edge already exists for that process.

What can be concluded if a resource-allocation graph contains no cycles?

Deadlock cannot occur. Signup and view all the answers

Which algorithm is used for multiple instances of a resource type?

Banker’s Algorithm Signup and view all the answers

What must each process do a priori in the Banker’s Algorithm?

Specify the maximum resources it may use. Signup and view all the answers

In the context of deadlock, what does hold and wait prevention imply?

A process must release all resources before requesting new ones. Signup and view all the answers

When a process gets its resources in the resource-allocation graph, what must happen next?

The process must return the resources after a finite amount of time. Signup and view all the answers

Which method allows a system to enter a deadlock state before recovery strategies are initiated?

Deadlock detection Signup and view all the answers

What does the existence of a cycle in a resource-allocation graph imply when multiple instances of a resource type are present?

Deadlock is a possibility. Signup and view all the answers

What is one of the four necessary conditions for a deadlock to occur?

A process must hold at least one resource and be waiting for additional resources. Signup and view all the answers

What does the 'no preemption' condition in a deadlock mean?

A resource can only be released voluntarily by the process holding it. Signup and view all the answers

Which of the following best describes the mutual exclusion condition in deadlocks?

Only one process can use a resource at a time. Signup and view all the answers

How does 'hold and wait' contribute to the possibility of a deadlock?

A process holding at least one resource is waiting for additional resources. Signup and view all the answers

What is required for a circular wait condition to exist in a deadlock scenario?

There should exist a closed loop of processes, each waiting for a resource held by the next. Signup and view all the answers

What happens to a process that is holding resources when it requests a new resource that cannot be immediately allocated?

All held resources are released. Signup and view all the answers

What is required for deadlock avoidance in a system?

The maximum number of resources that each process may need must be declared. Signup and view all the answers

What defines a system in a 'safe state'?

Every process can complete its execution without leading to a deadlock. Signup and view all the answers

What condition must be true for a system to be considered in an unsafe state?

There is no guarantee that all processes can complete successfully. Signup and view all the answers

Which strategy helps to prevent the circular wait condition?

Implement a total ordering of all resource types. Signup and view all the answers

Study Notes

Chapter 7: Deadlocks

Deadlocks occur when sets of concurrent processes are prevented from completing their tasks.
Deadlocks arise when four conditions hold simultaneously:
- Mutual exclusion: Only one process can use a resource at a time.
- Hold and wait: A process holding at least one resource is waiting to acquire additional resources held by other processes.
- No preemption: A resource can only be released voluntarily, after the process holding it has completed its task.
- Circular wait: A set of waiting processes exists where each process is waiting for a resource held by the next process in the set, and the last process is waiting for a resource held by the first process.
System model:
- System consists of resources (CPU cycles, memory space, I/O devices).
- Each resource type has a number of instances.
- Each process utilizes resources via request and release.

Chapter Objectives

Describe deadlocks and methods for preventing or avoiding them.
Deadlock Prevention: Methods to ensure that the system will never enter a deadlock state.
- Mutual Exclusion: Not required for sharable resources.
- Hold and wait: Guarantee that a process requests all its resources before beginning execution; or the process requests resources only when it has none allocated.
- No preemption: If a process holding some resources requests another resource that cannot be immediately allocated, then all resources currently held are released. Preempted resources are added to the list of resources the process is waiting for.
- Circular wait: Impose a total ordering on resource types and require that each process requests them in increasing order of enumeration.

Deadlock Avoidance

Requires prior information about the maximum resource needs of each process.
Resource allocation state is defined by: Available resources, allocated resources, and maximum demands of processes.
System in safe state if there exists a sequence of all processes such that system can allocate requested resources to each process in the sequence.

Safe State

If a process requests an available resource, the system must decide if immediate allocation leaves the system in a safe state.
System is in a safe state if there exists a sequence of all processes where the resources that a process can still request can be satisfied using available and held resources.
- If resources are not immediately available, the process can wait.
- When a process is finished can obtain resources.
- Return allocated resources and terminate.

Basic Facts

If a system is in a safe state, there are no deadlocks.
If a system is in an unsafe state, there is a possibility of a deadlock.
Avoidance ensures a system never enters an unsafe state.

Resource-Allocation Graph Scheme

Claim edge: process may request a resource.
Claim edge converts to request edge when a process requests a resource.
Request edge converts to an assignment edge when the resource is allocated to the process.
When a resource is released, the assignment edge reconverts to a claim edge.
Resources must be claimed a priori in the system.

Banker's Algorithm

Handles multiple instances of resource types.
Each process must claim its maximum resource use a priori.
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.

Data Structures for Banker's Algorithm

Available: Vector of length m (number of available resources of each type)
Max: n x m matrix (Maximum resources each process may need)
Allocation: n x m matrix (Resources currently allocated to each process)
Need: n x m matrix (Resources each process needs to complete)(Max - Allocation)

Resource-Request Algorithm for Process Pi

If Request ≤ Need, and Request ≤ Available, grant the request.
If either condition fails, the process must wait.

Deadlock Detection

Allow the system to enter a deadlock state.
Detection algorithm.
Recovery scheme.

Single Instance of each Resource Type

Maintain wait-for graph (Nodes are processes).
Periodically invoke an algorithm that searches for a cycle in the graph.
If a cycle exists, a deadlock.
Algorithm to detect cycles in a graph takes O(n²) operations (n = number of processes).

Several Instances of a Resource Type

Available: Vector of length m
Allocation: n x m matrix
Request: n x m matrix

Detection Algorithm (Steps)

Initialize Work and Finish vectors
Set Work = Available, and Finish[i] = true for all i
Find an index i such that Finish[i] = false and Request ≤ Work
Work = Work + Allocationi, and Finish[i] = true.
If all Finish[i] = true, then there are no deadlocks, else there is deadlock.
Algorithm requires O(m x n²) operations.

Recovery from Deadlock

Abort all deadlocked processes.
Abort one process at a time until the deadlock cycle is eliminated.
Consider priority of process, computation time, resources used, resources needed, if interactive or batch.

Recovery from Deadlock: Resource Preemption

Selecting a victim (minimize cost)
Rollback (return to safe state, restart process)
Starvation (same process repeatedly selected as victim).

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Explore the concept of deadlocks in concurrent processes through this quiz. Learn about the four conditions that lead to deadlocks, their impact on system resources, and methods to prevent or avoid them. Test your understanding of key concepts and enhance your knowledge of resource management.