3.Graph_algs_2 - UBI529 3. Distributed Graph Algorithms 2.4...

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Unformatted text preview: UBI529 3. Distributed Graph Algorithms 2.4 Distributed Path Traversals Distributed BFS Algorithms Distributed DFS Algorithms 3 Bellman-Ford BFS Tree Algorithm : Use a variant of the flooding algorithm. Each node and each message store an integer which corresponds to the distance from the root. The root stores 0, every other node initially . The root starts the flooding algorithm by sending a message 1 to all neighbors. A node u with integer x receives a message y from a neighbor v: if y < x then node u stores y (instead of x) and sends y+1 to all neighbors (except v). 4 Distributed Bellman-Ford BFS Algorithm 1. Initially, the root sets L(r0) = 0 and all other vertices set L(v) = 1. 2. The root sends out the message Layer(0) to all its neighbors. 3. A vertex v, which gets a Layer(d) message from a neighbor w does: If d + 1 < L(v) then parent(v) = w; L(v) = d + 1; Send Layer(d + 1) to all neighbors except w . Time complexity: O(D). Message Complexity: O(n|E|). 5 Analysis Analysis of Algorithm : The time complexity of Algorithm 3.10 is O(D), the message complexity is O(n|E|), where D is the diameter of the graph. Proof : We can prove the time complexity by induction. We claim that a node at distance d from the root has received a message d by time d. The root knows by time 0 that it is the root. A node v at distance d has a neighbor u at distance d-1. Node u by induction sends a message d to v at time d-1 or before, which is then received by v at time d or before. Message complexity : A node can reduce its integer at most n-1 times; each of these times it sends a message to all it neighbors. If all nodes do this we have O(n|E|) messages. 6 Remarks There are graphs and executions that produce O(n|E|) messages. How does the algorithm terminate? Algorithm 3.8 has the better message complexity; algorithm 3.10 has the better time complexity. The currently best known algorithm has message complexity O(|E|+n log3 n) and time complexity O(D log3 n). How do we find the root?!? Leader election in an arbitrary graph: FloodMax algorithm. Termination? Idea: Each node that believes to be the max builds a spanning tree (More for example in Chapter 15 of Nancy Lynch Distributed Algorithms) 7 Distributed DFS Distributed DFS algorithm: There is a single message called the token 1. Start exploration (visit) at root r....
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This note was uploaded on 01/13/2012 for the course CEN 303 taught by Professor Kayhaner during the Spring '11 term at Izmir Institute of Technology.

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3.Graph_algs_2 - UBI529 3. Distributed Graph Algorithms 2.4...

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