When I noticed that the preparatory steps took much longer than actually
solving the system, I profiled by hand: commenting and uncommeting the
code, and measuring elapsed times.
The first point where there is significant slow down comes at that method
set to next positive.
I attach a simplified code where I use three methods. The second is the one
I use. The third is a naive implementation, slower, and the first is the
faster, but works for a special case where the descendants are direct
children.
Probably, the problem is not the filtered iterator, but maybe I should
re-think the algorithm, to avoid a costly search for active descendants.
On Sat, May 25, 2024, 12:17 AM Wolfgang Bangerth <bange...@colostate.edu>
wrote:
> On 5/24/24 15:24, Felipe Ponce wrote:
> > *
> > *
> > Is there a better alternative to iterate active descendants? Why is
> > /set_to_next_positive/ so slow here?//
>
> Is it? I would also expect it to be fast, but have also learned that you
> can't
> judge where a code is slow without benchmarking it. So: Have you run your
> code
> through a profiler to see where it's actually slow?
>
> Best
> W.
>
> --
> ------------------------------------------------------------------------
> Wolfgang Bangerth email: bange...@colostate.edu
> www: http://www.math.colostate.edu/~bangerth/
>
>
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#include <deal.II/grid/tria.h>
#include <deal.II/grid/tria_accessor.h>
#include <deal.II/grid/grid_generator.h>
#include <deal.II/grid/grid_tools.h>
#include <deal.II/fe/fe_q.h>
#include <deal.II/fe/fe_values.h>
#include <deal.II/dofs/dof_handler.h>
#include <deal.II/dofs/dof_tools.h>
#include <deal.II/numerics/data_out.h>
#include <cmath>
#include <fstream>
#include <iostream>
#include <typeinfo>
#include <chrono>
namespace Static
{
using namespace dealii;
using index_t = unsigned int;
using clock_t = std::chrono::steady_clock;
using ChildIt = FilteredIterator<typename DoFHandler<2, 3>::active_cell_iterator>;
class ActiveChild
{
public:
CellId parent;
ActiveChild(const CellId &parent)
: parent(parent) {}
template <typename Iterator>
bool operator()(const Iterator &cell) const
{
return parent.is_ancestor_of(cell->id());
}
};
class Child_Test
{
public:
Child_Test(
const index_t n_levels,
const index_t fe_degree = 1);
void run();
private:
void make_grid();
void test_method();
const index_t n_levels;
Triangulation<2, 3> tria;
FE_Q<2, 3> fe;
DoFHandler<2, 3> dof_handler;
};
Child_Test::Child_Test(
const index_t n_levels,
const index_t fe_degree) : n_levels(n_levels), fe(fe_degree), dof_handler(tria)
{ }
void Child_Test::make_grid()
{
{
// Create a 3D meash with the target surface.
Triangulation<3> cube_mesh;
GridGenerator::hyper_cube(cube_mesh, -1, 1);
const std::set<types::boundary_id> boundary_ids = {};
// Extract the surface.
GridGenerator::extract_boundary_mesh(
cube_mesh, tria, boundary_ids);
}
tria.refine_global(n_levels);
dof_handler.distribute_dofs(fe);
}
void Child_Test::test_method()
{
std::vector<index_t> global_dofs(fe.dofs_per_cell);
// Iterate through coarse cells.
for (index_t idx = 0; idx < tria.n_quads(n_levels - 1); ++idx)
{
// Define iterators over active cells in coarse cell.
const CellAccessor<2, 3> coarse_cell(&tria, n_levels - 1, idx);
// Method I: Iterate using indices.
for (index_t i = 0; i < coarse_cell.n_children(); ++i)
{
const index_t id_child = coarse_cell.child(i)->index();
}
// Method II: Iterate using filter.
// ChildIt cell(ActiveChild(coarse_cell.id()));
// ChildIt endc(ActiveChild(coarse_cell.id()), dof_handler.end());
// cell.set_to_next_positive(dof_handler.begin_active());
// for (; cell != endc; ++cell)
// {
// cell->get_dof_indices(global_dofs);
// }
// Method II: Iterate without using filter.
// for (const auto &cell : dof_handler.active_cell_iterators())
// {
// if (coarse_cell.id().is_ancestor_of(cell->id()))
// {
// cell->get_dof_indices(global_dofs);
// }
// }
}
}
void Child_Test::run()
{
clock_t::time_point start;
clock_t::time_point end;
make_grid();
start = clock_t::now();
test_method();
end = clock_t::now();
std::cout << "Time: " << std::chrono::duration_cast<std::chrono::milliseconds>(end - start).count() << " ms\n";
}
} // namespace Static
int main()
{
using namespace Static;
Child_Test basic(4);
basic.run();
return 0;
}
