Ginkgo: The inverse-iteration program (original) (raw)
The inverse iteration example..
This example depends on simple-solver, .
| Table of contents | |
|---|---|
| Introduction About the example The commented program | Results Comments about programming and debugging The plain program |
Introduction
This example shows how components available in Ginkgo can be used to implement higher-level numerical methods. The method used here will be the shifted inverse iteration method for eigenvalue computation which find the eigenvalue and eigenvector of A closest to z, for some scalar z. The method requires repeatedly solving the shifted linear system (A - zI)x = b, as well as performing matrix-vector products with the matrix A. Here is the complete pseudocode of the method:
x_0 = initial guess
for i = 0 .. max_iterations:
solve (A - zI) y_i = x_i for y_i+1
x_(i+1) = y_i / || y_i || # compute next eigenvector approximation
g_(i+1) = x_(i+1)^* A x_(i+1) # approximate eigenvalue (Rayleigh quotient)
if ||A x_(i+1) - g_(i+1)x_(i+1)|| < tol * g_(i+1): # check convergence
break
About the example
The commented program
Figure out where to run the code
if (argc == 2 && (std::string(argv[1]) == "--help")) {
std::cerr << "Usage: " << argv[0] << " [executor]" << std::endl;
std::exit(-1);
}
const auto executor_string = argc >= 2 ? argv[1] : "reference";
std::map<std::string, std::function<std::shared_ptrgko::Executor()>>
exec_map{
{"cuda",
[] {
}},
{"hip",
[] {
}},
{"dpcpp",
[] {
}},
{"reference", [] { return gko::ReferenceExecutor::create(); }}};
executor where Ginkgo will perform the computation
const auto exec = exec_map.at(executor_string)();
auto this_exec = exec->get_master();
linear system solver parameters
auto system_max_iterations = 100u;
auto system_residual_goal = real_precision{1e-16};
eigensolver parameters
auto max_iterations = 20u;
auto residual_goal = real_precision{1e-8};
auto z = precision{20.0, 2.0};
Read data
auto A = share(gko::read(std::ifstream("data/A.mtx"), exec));
Generate shifted matrix A - zI
- we avoid duplicating memory by not storing both A and A - zI, but compute A - zI on the fly by using Ginkgo's utilities for creating linear combinations of operators
auto one = share(gko::initialize({precision{1.0}}, exec));
auto neg_one = share(gko::initialize({-precision{1.0}}, exec));
auto neg_z = gko::initialize({-z}, exec);
one, A, gko::initialize({-z}, exec),
Generate solver operator (A - zI)^-1
solver_type::build()
.with_criteria(gko::stop::Iteration::build().with_max_iters(
system_max_iterations),
.with_reduction_factor(system_residual_goal))
.on(exec)
->generate(system_matrix);
inverse iterations
start with guess [1, 1, ..., 1]
auto x = [&] {
auto work = vec::create(this_exec, gko::dim<2>{A->get_size()[0], 1});
const auto n = work->get_size()[0];
for (int i = 0; i < n; ++i) {
work->get_values()[i] = precision{1.0} / sqrt(n);
}
return clone(exec, work);
}();
auto norm = gko::initialize<real_vec>({1.0}, exec);
auto inv_norm = clone(this_exec, one);
for (auto i = 0u; i < max_iterations; ++i) {
std::cout << "{ ";
(A - zI)y = x
system_matrix->apply(one, y, neg_one, x);
x->compute_norm2(norm);
std::cout << "\"system_residual\": "
<< clone(this_exec, norm)->get_values()[0] << ", ";
x->copy_from(y);
x = y / || y ||
x->compute_norm2(norm);
inv_norm->get_values()[0] =
real_precision{1.0} / clone(this_exec, norm)->get_values()[0];
x->scale(clone(exec, inv_norm));
g = x^* A x
A->apply(x, tmp);
x->compute_dot(tmp, g);
auto g_val = clone(this_exec, g)->get_values()[0];
std::cout << "\"eigenvalue\": " << g_val << ", ";
||Ax - gx|| < tol * g
auto v = gko::initialize({-g_val}, exec);
tmp->add_scaled(v, x);
tmp->compute_norm2(norm);
auto res_val = clone(exec->get_master(), norm)->get_values()[0];
std::cout << "\"residual\": " << res_val / g_val << " }," << std::endl;
if (abs(res_val) < residual_goal * abs(g_val)) {
break;
}
}
}
Results
This is the expected output:
{ "system_residual": +1.61736920e-14, "eigenvalue": (+2.03741410e+01,-1.17744356e-16), "residual": (+2.92231055e-01,+1.68883476e-18) },
{ "system_residual": +4.98014795e-15, "eigenvalue": (+1.94878474e+01,+1.25948378e-15), "residual": (+7.94370276e-02,-5.13395071e-18) },
{ "system_residual": +3.39296916e-15, "eigenvalue": (+1.93282121e+01,-1.19329332e-15), "residual": (+4.11149623e-02,+2.53837290e-18) },
{ "system_residual": +3.35953656e-15, "eigenvalue": (+1.92638912e+01,+3.28657016e-16), "residual": (+2.34717040e-02,-4.00445585e-19) },
{ "system_residual": +2.91474009e-15, "eigenvalue": (+1.92409166e+01,+3.65597737e-16), "residual": (+1.34709547e-02,-2.55962367e-19) },
{ "system_residual": +3.09863953e-15, "eigenvalue": (+1.92331106e+01,-1.07919176e-15), "residual": (+7.72060707e-03,+4.33212063e-19) },
{ "system_residual": +2.31198069e-15, "eigenvalue": (+1.92305014e+01,-2.89755360e-16), "residual": (+4.42106625e-03,+6.66143651e-20) },
{ "system_residual": +3.02771202e-15, "eigenvalue": (+1.92296339e+01,+8.04259901e-16), "residual": (+2.53081312e-03,-1.05848687e-19) },
{ "system_residual": +2.02954523e-15, "eigenvalue": (+1.92293461e+01,+7.81834016e-16), "residual": (+1.44862114e-03,-5.88985854e-20) },
{ "system_residual": +2.31762332e-15, "eigenvalue": (+1.92292506e+01,-1.11718775e-16), "residual": (+8.29183451e-04,+4.81741912e-21) },
{ "system_residual": +8.12541038e-15, "eigenvalue": (+1.92292190e+01,-6.55606254e-16), "residual": (+4.74636702e-04,+1.61823936e-20) },
{ "system_residual": +2.77259926e-15, "eigenvalue": (+1.92292085e+01,+4.30588140e-16), "residual": (+2.71701077e-04,-6.08403935e-21) },
{ "system_residual": +8.87888675e-14, "eigenvalue": (+1.92292051e+01,+9.67936313e-18), "residual": (+1.55539937e-04,-7.82937998e-23) },
{ "system_residual": +2.85077117e-15, "eigenvalue": (+1.92292039e+01,-4.52923128e-16), "residual": (+8.90457139e-05,+2.09737561e-21) },
{ "system_residual": +6.46865302e-14, "eigenvalue": (+1.92292035e+01,+1.58710681e-17), "residual": (+5.09805252e-05,-4.20774259e-23) },
{ "system_residual": +4.18913713e-15, "eigenvalue": (+1.92292034e+01,+1.06839590e-15), "residual": (+2.91887365e-05,-1.62175862e-21) },
{ "system_residual": +1.06421578e-11, "eigenvalue": (+1.92292034e+01,+3.26089685e-17), "residual": (+1.67126561e-05,-2.83413965e-23) },
{ "system_residual": +2.97434420e-13, "eigenvalue": (+1.92292034e+01,-7.85427712e-16), "residual": (+9.56961199e-06,+3.90876227e-22) },
{ "system_residual": +1.63230281e-11, "eigenvalue": (+1.92292033e+01,+3.69307000e-16), "residual": (+5.47975753e-06,-1.05241636e-22) },
{ "system_residual": +6.14939758e-14, "eigenvalue": (+1.92292033e+01,+1.36057865e-15), "residual": (+3.13794996e-06,-2.22028320e-22) },
Comments about programming and debugging
The plain program
#include
#include
#include
#include
#include
#include
#include
#include <ginkgo/ginkgo.hpp>
int main(int argc, char* argv[])
{
using precision = std::complex;
using std::abs;
using std::sqrt;
std::cout << std::scientific << std::setprecision(8) << std::showpos;
if (argc == 2 && (std::string(argv[1]) == "--help")) {
std::cerr << "Usage: " << argv[0] << " [executor]" << std::endl;
std::exit(-1);
}
const auto executor_string = argc >= 2 ? argv[1] : "reference";
std::map<std::string, std::function<std::shared_ptrgko::Executor()>>
exec_map{
{"cuda",
[] {
}},
{"hip",
[] {
}},
{"dpcpp",
[] {
}},
{"reference", [] { return gko::ReferenceExecutor::create(); }}};
const auto exec = exec_map.at(executor_string)();
auto this_exec = exec->get_master();
auto system_max_iterations = 100u;
auto system_residual_goal = real_precision{1e-16};
auto max_iterations = 20u;
auto residual_goal = real_precision{1e-8};
auto z = precision{20.0, 2.0};
auto A = share(gko::read(std::ifstream("data/A.mtx"), exec));
auto one = share(gko::initialize({precision{1.0}}, exec));
auto neg_one = share(gko::initialize({-precision{1.0}}, exec));
auto neg_z = gko::initialize({-z}, exec);
one, A, gko::initialize({-z}, exec),
solver_type::build()
.with_criteria(gko::stop::Iteration::build().with_max_iters(
system_max_iterations),
.with_reduction_factor(system_residual_goal))
.on(exec)
->generate(system_matrix);
auto x = [&] {
auto work = vec::create(this_exec, gko::dim<2>{A->get_size()[0], 1});
const auto n = work->get_size()[0];
for (int i = 0; i < n; ++i) {
work->get_values()[i] = precision{1.0} / sqrt(n);
}
return clone(exec, work);
}();
auto norm = gko::initialize<real_vec>({1.0}, exec);
auto inv_norm = clone(this_exec, one);
for (auto i = 0u; i < max_iterations; ++i) {
std::cout << "{ ";
system_matrix->apply(one, y, neg_one, x);
x->compute_norm2(norm);
std::cout << "\"system_residual\": "
<< clone(this_exec, norm)->get_values()[0] << ", ";
x->copy_from(y);
x->compute_norm2(norm);
inv_norm->get_values()[0] =
real_precision{1.0} / clone(this_exec, norm)->get_values()[0];
x->scale(clone(exec, inv_norm));
A->apply(x, tmp);
x->compute_dot(tmp, g);
auto g_val = clone(this_exec, g)->get_values()[0];
std::cout << "\"eigenvalue\": " << g_val << ", ";
auto v = gko::initialize({-g_val}, exec);
tmp->add_scaled(v, x);
tmp->compute_norm2(norm);
auto res_val = clone(exec->get_master(), norm)->get_values()[0];
std::cout << "\"residual\": " << res_val / g_val << " }," << std::endl;
if (abs(res_val) < residual_goal * abs(g_val)) {
break;
}
}
}