CS 551: Distributed Deadlock, Wait-For-Graphs

Wait-For-Graphs

A Wait-For Graph (WFG) is a graph where

Each node represents a process;
An edge, P_i --< P_j means that P_i is blocked waiting for P_j to release a resource.

Recall the single-instance SRAG given earlier

A WFG can be derived from that graph.

A Wait-For Graph (WFG) is the same as the SRAG with the resource elements stripped out.

The cycles can be seen more clearly in this form of the graph.

There is deadlock in the system

if and only if there exists a cycle or a knot in the wait-for-graph.

With about half the number of nodes and edges, the WFG requires only about half the storage of a SRAG.

However, WFGs can only be used for SRAGs with single-instance resources.

Resources for SRAGs can have any number of instances;

However, one cannot create WFGs from these.

Note: A similar graph, the Transaction-wait-for graph (TWF), is used in Database systems.

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