What is a Deadlock?
Deadlocks are a set of blocked processes each holding a resource and waiting to acquire a resource held by another process.
How to avoid Deadlocks
Deadlocks can be avoided by avoiding at least one of the four conditions, because all this four conditions are required simultaneously to cause deadlock.
- Mutual Exclusion
Resources shared such as read-only files do not lead to deadlocks but resources, such as printers and tape drives, requires exclusive access by a single process.
- Hold and Wait
In this condition processes must be prevented from holding one or more resources while simultaneously waiting for one or more others.
- No Preemption
Preemption of process resource allocations can avoid the condition of deadlocks, where ever possible.
- Circular Wait
Circular wait can be avoided if we number all resources, and require that processes request resources only in strictly increasing(or decreasing) order.
The above points focus on preventing deadlocks. But what to do once a deadlock has occured. Following three strategies can be used to remove deadlock after its occurrence.
We can take a resource from one process and give it to other. This will resolve the deadlock situation, but sometimes it does causes problems.
In situations where deadlock is a real possibility, the system can periodically make a record of the state of each process and when deadlock occurs, roll everything back to the last checkpoint, and restart, but allocating resources differently so that deadlock does not occur.
- Kill one or more processes
This is the simplest way, but it works.
What is a Livelock?
There is a variant of deadlock called livelock. This is a situation in which two or more processes continuously change their state in response to changes in the other process(es) without doing any useful work. This is similar to deadlock in that no progress is made but differs in that neither process is blocked or waiting for anything.
A human example of livelock would be two people who meet face-to-face in a corridor and each moves aside to let the other pass, but they end up swaying from side to side without making any progress because they always move the same way at the same time.
A deadlock is a situation in which two computer programs sharing the same resource are effectively preventing each other from accessing the resource, resulting in both programs ceasing to function.
The earliest computer operating systems ran only one program at a time. All of the resources of the system were available to this one program. Later, operating systems ran multiple programs at once, interleaving them. Programs were required to specify in advance what resources they needed so that they could avoid conflicts with other programs running at the same time. Eventually some operating systems offered dynamic allocation of resources. Programs could request further allocations of resources after they had begun running. This led to the problem of the deadlock. Here is the simplest example:Program 1 requests resource A and receives it. Program 2 requests resource B and receives it. Program 1 requests resource B and is queued up, pending the release of B. Program 2 requests resource A and is queued up, pending the release of A.
Now neither program can proceed until the other program releases a resource. The operating system cannot know what action to take. At this point the only alternative is to abort (stop) one of the programs.
Learning to deal with deadlocks had a major impact on the development of operating systems and the structure of databases. Data was structured and the order of requests was constrained in order to avoid creating deadlocks.