I am trying to use the parallel BGL to compute the weakly connected components of a graph. I have managed to obtain the correct results with the code pasted below, however I am getting the following times:

- Sequential BGL 0.01 seconds

- Parallel BGL on 1 core 0.04 seconds

- Parallel BGL on 2 cores 0.64 seconds

That doesn’t look right to me. What am I doing wrong?

using namespace boost;

using boost::graph::distributed::mpi_process_group;

typedef adjacency_list<vecS, distributedS<mpi_process_group, vecS>, undirectedS> Graph;

typedef iterator_property_map<std::vector<int>::iterator, property_map<Graph, vertex_index_t>::type> LocalMap;

int getConnectedComponents(std::vector< std::pair<int,int> > &arcs, const std::vector<int> &isActive) {

// Allocate structures

Graph G(nV+1);

std::vector<int> localComponent(nV+1);

std::vector<int> globalComponent(nV+1);

// Only rank 0 has got the arc data structure

if( rank == 0 ) {

// Add dummy edges to each vertex (required to avoid BGL crash for a single node with no edges)

for( int i=0; i<nV; i++ ) add_edge(vertex(i,G),vertex(i,G),G);

// Add edges to the graph

for(std::vector<std::pair<int,int> >::iterator it = arcs.begin(); it != arcs.end(); ++it)

if( isActive[arcCnt++]==0 )

add_edge(vertex(it->first,G),vertex(it->second,G),G);

}

synchronize(G);

// Compute number of connected components

LocalMap components(localComponent.begin(),get(vertex_index, G));

int num = connected_components_ps(G, components);

// Let rank 0 collect the global component vector

if( rank == 0 ) {

components.set_max_ghost_cells(0);

for( PetscInt i = 0; i < nV+1; i++ ) globalComponent[i] = get(components, vertex(i, G));

}

synchronize(components);

}

Regards,

-------------------------------------------------

Alessio Quaglino

Postdoctoral Researcher

University of Lugano