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Deadlock Detection
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Deadlock Detection
The usual method of detecting a deadlock is to construct a diagram showing processes and resources. Each process and each resource unit is depicted as a node. Resource nodes are annotated with the number of available units. Whenever a process holds units of resource, we draw an arrow from the resource to the process, labeled with the number of units held. Whenever a process requests units of a resources (and is waiting), we draw an arrow from the process to the resource, labeled with the number of units requested. This diagram represents a current system state. Whenever a process makes a new request or releases a resource unit, the state diagram is updated accordingly.
A circular wait corresponds to a closed path in this diagram. For example, the kitchen example in the deadlock state looks like:
When there is just one unit of every resource, a circular wait in the diagram is equivalent to a deadlock in the system.
In the banking example, which has multiple units of each resource (its cash reserves in each currency), a loop in the diagram represents only a potential deadlock. Whether it is a real deadlock depends on whether any of the resources has fewer units available than any of the incoming request arrows:
These examples show the principle of a deadlock detector: find all the loops in the current allocation diagram and see if any of them contains a resource that has too few units left to grant any of the incoming requests.
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