Re: [Boost-users] [BGL] parallel BGL does not scale

14 Dec 2015

      I have found out the following in the meantime:

I have been running the program with mpirun -np 2. This seems to MASSIVELY slow down the parallel BGL. Why is this the case? Timing connected_components_ps with mpirun -np 1 is in fact competitive with the non-parallel connected_components function. Then I am losing time in the synchronize() calls and end up being slightly slower for the parallel case. How can I use mpirun -np 2 with the parallel BGL?

Thanks,
Alessio

On 14 Dec 2015, at 11:29, Quaglino Alessio <quagla@usi.ch<mailto:quagla@usi.ch>> wrote:

Good Morning,

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