18 Questions
What is the condition among the four that can lead to a deadlock?
Mutual exclusion
Which algorithm is designed to prevent deadlocks by ensuring that the system will never enter an unsafe state?
Banker's Algorithm
What term refers to a situation where a process holds resources while waiting for additional resources that are currently being held by other processes?
Hold and wait
Which of the following conditions can cause a deadlock when resources are not preemptable?
Circular wait
What characterizes a system where processes are allocated resources and may request additional resources while still holding onto existing resources?
Hold and wait
In a deadlock situation, what term refers to the process of determining the sequence of processes to be terminated in order to recover from the deadlock?
Deadlock resolution strategy
What is the primary goal of the Banker's Algorithm?
To guarantee that the system will never enter an unsafe state
In the Banker's Algorithm, what does the 'Need' matrix represent?
Resources required by each process to complete execution
How does the Banker's Algorithm deal with the 'Hold and Wait' condition?
By only allocating resources to a process if it has none
What does the 'Available' vector represent in the context of resource allocation?
Instances of resources that are free and available for allocation
In the context of resource allocation, what does it mean for a system to be in an 'unsafe state'?
A system where available resources can't satisfy resource requests leading to potential deadlocks
What is the significance of each process having to a priori claim its maximum resource use in the Banker's Algorithm?
It helps in determining if allocating requested resources will keep the system in a safe state
What does the matrix Need represent in the context of the Banker's Algorithm?
The instances of resources that a process still needs to complete its task
In the Banker's Algorithm, what does a safe state signify?
A state where deadlock cannot occur
What does the sequence < P1, P3, P4, P0, P2> satisfying safety criteria indicate?
A sequence where each process can finish without getting blocked
In the Banker's Algorithm, why is hold and wait considered a potentially unsafe condition?
It can cause deadlock if resources are not allocated properly
What happens if a process requests more resources than are available in the Banker's Algorithm?
The request will be denied to maintain system safety
Why do some resource allocation methods not allow preemption?
To prevent processes from holding onto resources indefinitely
Study Notes
Deadlocks
- A deadlock is a situation in a multiprogramming environment where several processes are competing for a finite number of resources, and each process is waiting for another process to release a resource.
- In a deadlock, processes never finish executing, and system resources are tied up, preventing other jobs from starting.
System Model
- A process utilizes a resource in three stages:
- Request: The process requests the resource.
- Use: The process operates on the resource.
- Release: The process releases the resource.
Deadlock Characterization
- A deadlock can arise if four conditions hold simultaneously:
- Mutual exclusion
- Hold and wait
- No preemption
- Circular wait
Methods for Handling Deadlocks
- If a system is in a safe state, it means no deadlocks are possible.
- If a system is in an unsafe state, it means there is a possibility of deadlock.
- Avoidance algorithms ensure that a system will never enter an unsafe state.
- Two types of avoidance algorithms:
- Single instance of a resource type: uses a resource-allocation graph.
- Multiple instances of a resource type: uses the banker's algorithm.
Banker's Algorithm
- Used for 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.
- The algorithm uses three matrices:
- Available: a vector of length m, representing the number of available instances of each resource type.
- Max: an n x m matrix, representing the maximum number of instances of each resource type that each process may request.
- Allocation: an n x m matrix, representing the number of instances of each resource type that each process is currently allocated.
- Need: an n x m matrix, representing the number of instances of each resource type that each process may need to complete its task.
Example of Banker's Algorithm
- The algorithm is used to determine if a system is in a safe state.
- A safe state is one where there is a sequence of processes that can execute without leading to a deadlock.
- The algorithm checks if a request can be granted by checking if the request is less than or equal to the available resources.
Learn about the Banker's Algorithm through an example scenario involving 5 processes and 3 resource types. Understand how the Need matrix is calculated based on Allocation and Max matrices.
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