Random access to vertices using Boost :: graph
I am trying to iterate over my vertices of an acceleration graph in parallel using OpenMP. This seems to require an iterator that supports random access to the elements (such as itr[i]
getting the i
th element ). However, the iterator that vertices(g)
returns (a vertex_iterator
) doesn't seem to support this. Is there an efficient, clean way to achieve this? Ideally, I want a standard for the loop, for example:
for (int i = 0; i < num_vertices; i++) {
vertex v = itr[i];
// Compute on vertex
}
which will collaborate with OpenMP. Thank!
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Using adjacency_list<..., vecS, ...>
or adjacency_matrix
would allow this to be done with integral type vertex descriptors.
Thinking a little out of the box, have a look at the Parallel Boost Graph Library (Parallel BGL). It is very likely that it does what you want (and more), but better?
Tiny Demo
Sample output (on my system):
Generated 50000000 vertices in 1879ms
Using 8 threads.
Sum of volumes for 50000000 vertices in 94ms: 2.5603e+10
Full list:
#include <boost/graph/adjacency_list.hpp>
#include <boost/graph/random.hpp>
#include <chrono>
#include <iostream>
#include <omp.h>
#include <random>
static std::mt19937 prng { std::random_device{}() };
struct MyVertex {
uintmax_t volume = [] { static std::uniform_int_distribution<int> pick(0, 1024); return pick(prng); }();
};
using namespace boost;
using G = adjacency_list<vecS, vecS, directedS, MyVertex>;
G generate() {
using namespace std::chrono;
auto start = high_resolution_clock::now();
G g;
generate_random_graph(g, 50000000, 0, prng);
auto end = high_resolution_clock::now();
std::cerr << "Generated " << num_vertices(g) << " vertices " << "in " << duration_cast<milliseconds>(end-start).count() << "ms\n";
return g;
}
int main() {
auto const g = generate();
using namespace std::chrono;
auto start = high_resolution_clock::now();
double sum = 0;
#pragma omp parallel
{
#pragma omp single
std::cerr << "Using " << omp_get_num_threads() << " threads.\n";
#pragma omp for reduction(+:sum)
for (G::vertex_descriptor u = 0; u < num_vertices(g); ++u) {
sum += g[vertex(u, g)].volume;
}
}
auto end = high_resolution_clock::now();
std::cerr << "Sum of volumes for " << num_vertices(g) << " vertices "
<< "in " << duration_cast<milliseconds>(end-start).count() << "ms: " << sum << "\n";
}
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