# Parallel Graph Algorithms

LightGraphs.Parallel is a module for graph algorithms that are parallelized. Their names should be consistent with the serial versions in the main module. In order to use parallel versions of the algorithms you can write:

using LightGraphs
import LightGraphs.Parallel

g = path_graph(10)
bc = Parallel.betweenness_centrality(g)

The arguments to parallel versions of functions match as closely as possible their serial versions with potential addition default or keyword arguments to control parallel execution. One exception is that for algorithms that cannot be meaningfully parallelized for certain types of arguments a MethodError will be raised. For example, dijkstra_shortest_paths works for either a single or multiple source argument, but since the parallel version is slower when given only a single source, it will raise a MethodError.

g = Graph(10)
# these work
LightGraphs.dijkstra_shortest_paths(g,1)
LightGraphs.dijkstra_shortest_paths(g, [1,2])
Parallel.dijkstra_shortest_paths(g, [1,2])
# this doesn't
Parallel.dijkstra_shortest_paths(g,1)

Note that after importing or using LightGraphs.Parallel, you must fully qualify the version of the function you wish to use (using, e.g., LightGraphs.betweenness_centrality(g) for the sequential version and Parallel.betweenness_centrality(g) for the parallel version.)

The following is a current list of parallel algorithms:

• Centrality measures:
• Parallel.betweenness_centrality
• Parallel.closeness_centrality
• Parallel.pagerank
• Parallel.radiality_centrality
• Parallel.stress_centrality
• Distance measures:

• Parallel.center
• Parallel.diameter
• Parallel.eccentricity
• Parallel.radius
• Shortest paths algorithms:

• Parallel.bellman_ford_shortest_paths
• Parallel.dijkstra_shortest_paths
• Parallel.floyd_warshall_shortest_paths
• Paralell.johnson_shortest_paths
• Traversal algorithms:

• Parallel.bfs
• Parallel.greedy_color

Also note that in some cases, the arguments for the parallel versions may differ from the serial (standard) versions. As an example, parallel Dijkstra shortest paths takes advantage of multiple processors to execute centrality from multiple source vertices. It is an error to pass a single source vertex into the parallel version of dijkstrashortestpaths.