#dead lock

THIS CHARACTER IS TOO FUN

Deadlock is so tuff but I uninstalled it because all my friends got off it and now my PC has only gotten worse and probably couldn't run it anyone :(

What is Deadlock?

Deadlock is a situation that occurs in OS when any process enters a waiting state because another waiting process is holding the demanded resource. Deadlock is a common problem in multi-processing where several processes share a specific type of mutually exclusive resource known as a soft lock or software.

Example of Deadlock

A real-world example would be traffic, which is going only in one direction.

Here, a bridge is considered a resource.

So, when Deadlock happens, it can be easily resolved if one car backs up (Preempt resources and rollback).

Several cars may have to be backed up if a deadlock situation occurs.

So starvation is possible.

Read More

What is Circular wait?

One process is waiting for the resource, which is held by the second process, which is also waiting for the resource held by the third process etc. This will continue until the last process is waiting for a resource held by the first process. This creates a circular chain.

For example, Process A is allocated Resource B as it is requesting Resource A. In the same way, Process B is allocated Resource A, and it is requesting Resource B. This creates a circular wait loop.

Example of Circular wait

For example, a computer has three USB drives and three processes. Each of the three processes able to holds one of the USB drives. So, when each process requests another drive, the three processes will have the deadlock situation as each process will be waiting for the USB drive to release, which is currently in use. This will result in a circular chain.

Deadlock Detection in OS

A deadlock occurrence can be detected by the resource scheduler. A resource scheduler helps OS to keep track of all the resources which are allocated to different processes. So, when a deadlock is detected, it can be resolved using the below-given methods:

Deadlock Prevention in OS

It’s important to prevent a deadlock before it can occur. The system checks every transaction before it is executed to make sure it doesn’t lead the deadlock situations. Such that even a small change to occur dead that an operation which can lead to Deadlock in the future it also never allowed process to execute.

It is a set of methods for ensuring that at least one of the conditions cannot hold.

No preemptive action:

No Preemption — A resource can be released only voluntarily by the process holding it after that process has finished its task

If a process which is holding some resources request another resource that can’t be immediately allocated to it, in that situation, all resources will be released.

Preempted resources require the list of resources for a process that is waiting.

The process will be restarted only if it can regain its old resource and a new one that it is requesting.

If the process is requesting some other resource, when it is available, then it was given to the requesting process.

If it is held by another process that is waiting for another resource, we release it and give it to the requesting process.

Mutual Exclusion:

Read More

Mutual Exclusion is a full form of Mutex. It is a special type of binary semaphore which used for controlling access to the shared resource. It includes a priority inheritance mechanism to avoid extended priority inversion problems. It allows current higher priority tasks to be kept in the blocked state for the shortest time possible.

Resources shared such as read-only files never lead to deadlocks, but resources, like printers and tape drives, needs exclusive access by a single process.

Hold and Wait:

In this condition, processes must be stopped from holding single or multiple resources while simultaneously waiting for one or more others.

Circular Wait:

It imposes a total ordering of all resource types. Circular wait also requires that every process request resources in increasing order of enumeration.

Deadlock Avoidance Algorithms

It is better to avoid a deadlock instead of taking action after the Deadlock has occurred. It needs additional information, like how resources should be used. Deadlock avoidance is the simplest and most useful model that each process declares the maximum number of resources of each type that it may need.

Avoidance Algorithms

The deadlock-avoidance algorithm helps you to dynamically assess the resource-allocation state so that there can never be a circular-wait situation.

A single instance of a resource type.

Use a resource-allocation graph

Cycles are necessary which are sufficient for Deadlock

Multiples instances of a resource type.

Cycles are necessary but never sufficient for Deadlock.

Uses the banker’s algorithm

Advantages of Deadlock

Here, are pros/benefits of using Deadlock method

This situation works well for processes which perform a single burst of activity

No preemption needed for Deadlock.

Convenient method when applied to resources whose state can be saved and restored easily

Feasible to enforce via compile-time checks

Needs no run-time computation since the problem is solved in system design

Disadvantages of Deadlock

Here, are cons/ drawback of using deadlock method

Delays process initiation

Processes must know future resource need

Pre-empts more often than necessary

Dis-allows incremental resource requests

Deadlock characteristics

Mutual Exclusion

Hold and wait

No preemption

Circular wait

Deadlocks can be prevented by eliminating the aforementioned conditions.

Eliminating mutual exclusion

It is not possible to do this as some resources, such as the drivers and printers, are inherently non-shareable.

Eliminating hold and wait

Allocate all necessary resources to the process before it begins to run. This eliminates the hold and wait for conditions, but it results in low system utilization. For instance, if a process needs printing at a later time and we have allocated the printer before the start of its execution, the printer will be blocked until that process is completed.

This process will make new requests for a resource after releasing a set of resources, but this may lead to starvation.

Eliminating no preemption

Preempt resources from the process if they are required by another high-priority process.

Eliminate circular wait

A numerical number will be allocated to each resource. A process may request that the number of resources is increased or decreased. For example, if P1 is given R5 resources, the next time P1 requests R4, R3, or any other resource less than R5, the request will be denied. Only requests for resources greater than R5 will be granted.

Deadlock avoidance

A deadlock can be avoided using the Bankers’ Algorithm.

Bankers’ Algorithm

The Bankers’ Algorithm is a resource allocation and deadlock avoidance algorithm that examines all resource requests made by systems. It checks for the safe state and makes the request if the system remains in the safe state after request approval. If there is no safe state, the request is denied.

 Inputs required for the Bankers’ Algorithm:

Maximum need or resources     required by each process

The resources currently     allocated by each process

Free resources available in     the system

 Request for the resource will only be granted:

If the request made by the     process is </= the freely available resource in the system

What is Deadlock?

Deadlock is a situation that occurs in OS when any process enters a waiting state because another waiting process is holding the demanded resource. Deadlock is a common problem in multi-processing where several processes share a specific type of mutually exclusive resource known as a soft lock or software.

Example of Deadlock

A real-world example would be     traffic, which is going only in one direction.

Here, a bridge is considered a     resource.

So, when Deadlock happens, it     can be easily resolved if one car backs up (Preempt resources and     rollback).

Several cars may have to be     backed up if a deadlock situation occurs.

So starvation is possible.

Read More

What is Circular wait?

One process is waiting for the resource, which is held by the second process, which is also waiting for the resource held by the third process etc. This will continue until the last process is waiting for a resource held by the first process. This creates a circular chain.

For example, Process A is allocated Resource B as it is requesting Resource A. In the same way, Process B is allocated Resource A, and it is requesting Resource B. This creates a circular wait loop.

Example of Circular wait

For example, a computer has three USB drives and three processes. Each of the three processes able to holds one of the USB drives. So, when each process requests another drive, the three processes will have the deadlock situation as each process will be waiting for the USB drive to release, which is currently in use. This will result in a circular chain.

Deadlock Detection in OS

A deadlock occurrence can be detected by the resource scheduler. A resource scheduler helps OS to keep track of all the resources which are allocated to different processes. So, when a deadlock is detected, it can be resolved using the below-given methods:

Deadlock Prevention in OS

It’s important to prevent a deadlock before it can occur. The system checks every transaction before it is executed to make sure it doesn’t lead the deadlock situations. Such that even a small change to occur dead that an operation which can lead to Deadlock in the future it also never allowed process to execute.

It is a set of methods for ensuring that at least one of the conditions cannot hold.

No preemptive action:

No Preemption – A resource can be released only voluntarily by the process holding it after that process has finished its task

If     a process which is holding some resources request another resource that     can’t be immediately allocated to it, in that situation, all resources     will be released.

Preempted     resources require the list of resources for a process that is waiting.

The     process will be restarted only if it can regain its old resource and a new     one that it is requesting.

If     the process is requesting some other resource, when it is available, then     it was given to the requesting process.

If     it is held by another process that is waiting for another resource, we     release it and give it to the requesting process.

Mutual Exclusion:

Read More

Mutual Exclusion is a full form of Mutex. It is a special type of binary semaphore which used for controlling access to the shared resource. It includes a priority inheritance mechanism to avoid extended priority inversion problems. It allows current higher priority tasks to be kept in the blocked state for the shortest time possible.

Resources shared such as read-only files never lead to deadlocks, but resources, like printers and tape drives, needs exclusive access by a single process.

Hold and Wait:

In this condition, processes must be stopped from holding single or multiple resources while simultaneously waiting for one or more others.

Circular Wait:

It imposes a total ordering of all resource types. Circular wait also requires that every process request resources in increasing order of enumeration.

Deadlock Avoidance Algorithms

It is better to avoid a deadlock instead of taking action after the Deadlock has occurred. It needs additional information, like how resources should be used. Deadlock avoidance is the simplest and most useful model that each process declares the maximum number of resources of each type that it may need.

Avoidance Algorithms

The deadlock-avoidance algorithm helps you to dynamically assess the resource-allocation state so that there can never be a circular-wait situation.

A single instance of a resource type.

Use     a resource-allocation graph

Cycles     are necessary which are sufficient for Deadlock

Multiples instances of a resource type.

Cycles     are necessary but never sufficient for Deadlock.

Uses     the banker’s algorithm

Advantages of Deadlock

Here, are pros/benefits of using Deadlock method

This     situation works well for processes which perform a single burst of     activity

No     preemption needed for Deadlock.

Convenient     method when applied to resources whose state can be saved and restored     easily

Feasible     to enforce via compile-time checks

Needs     no run-time computation since the problem is solved in system design

Disadvantages of Deadlock

Here, are cons/ drawback of using deadlock method

Delays     process initiation

Processes     must know future resource need

Pre-empts     more often than necessary

Dis-allows     incremental resource requests