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  Deadlocks

Deadlocks:

Definition:

A set of process will be in a deadlock state if each of the process in the set is waiting for an event that can be caused by only another process in the set. In other term, each of the members of the set of deadlock processes is waiting for a resource that can be only released by a deadlock process. None of the processes can run, none of them can release any resources, and none of them can be awakened. It is significant to note down that the number of processes and the number and type of resources possessed and requested are not important. The resources might be either physical or logical. For examples physical resources are Tape Drivers, Printers, Memory Space, and CPU Cycles. For example logical resources are Semaphores, Files and Monitors.

Example:

The simplest instance of deadlock is where process 1 has been allocated non-shareable resources A, say, a tap drive, and process 2 has be allocated non-sharable resource B, say, a printer. Now, if it turns out that process 1 requires resource B (printer) to proceed and process 2 requires resource A (the tape drive) to proceed and these are the two processes only in the system, each is blocked the other and all useful work in the system stops. This situation is termed deadlock. The system is in deadlock state since each process holds a resource being requested by the other process neither process is desire to release the resource it holds.

Necessary and Sufficient Deadlock Conditions:

Coffman conditions that must simultaneously hold for there to be a deadlock.

1. Mutual Exclusion Condition:

The resources involved are non-shareable. At least one resource (thread) ought to be held in a non-shareable mode, that is, just one process at a time claims exclusive control of the resource. If another process requests that resource, the requesting process ought to be delayed till the resource has been released.

2. Hold and Wait Condition:

Requesting process hold already, resources while waiting for requested resources. There must exist a process that is already holding a resource allocated to it while waiting for extra resource that are currently being held by other processes.

3. No-Preemptive Condition:

Resources already allocated to a process can’t be preempted. Resources can’t be eliminated from the processes are utilized to completion or released voluntarily by the process holding it.

4. Circular Wait Condition:

In the system the processes form a circular list or chain where each of the process in the list is waiting for a resource held by the next process in the list.

Strategies for dealing with Deadlock:

  • Avoid the problem altogether that means. Ostrich algorithm it may take place very infrequently, cost of detection/prevention etc may not be worth it.
  • recovery and detection
  • avoidance by careful resource allocation
  • Prevention by structurally negating one of the four essential conditions.

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