The simple solver example..
Introduction
This simple solver example should help you get started with Ginkgo. This example is meant for you to understand how Ginkgo works and how you can solve a simple linear system with Ginkgo. We encourage you to play with the code, change the parameters and see what is best suited for your purposes.
About the example
Each example has the following sections:
-
Introduction:This gives an overview of the example and mentions any interesting aspects in the example that might help the reader.
-
The commented program: This section is intended for you to understand the details of the example so that you can play with it and understand Ginkgo and its features better.
-
Results: This section shows the results of the code when run. Though the results may not be completely the same, you can expect the behaviour to be similar.
-
The plain program: This is the complete code without any comments to have an complete overview of the code.
The commented program
Include files
This is the main ginkgo header file.
#include <ginkgo/ginkgo.hpp>
Add the fstream header to read from data from files.
Add the C++ iostream header to output information to the console.
Add the STL map header for the executor selection
Add the string manipulation header to handle strings.
#include <string>
int main(int argc, char* argv[])
{
Use some shortcuts. In Ginkgo, vectors are seen as a gko::matrix::Dense with one column/one row. The advantage of this concept is that using multiple vectors is a now a natural extension of adding columns/rows are necessary.
using ValueType = double;
using IndexType = int;
The gko::matrix::Csr class is used here, but any other matrix class such as gko::matrix::Coo, gko::matrix::Hybrid, gko::matrix::Ell or gko::matrix::Sellp could also be used.
The gko::solver::Cg is used here, but any other solver class can also be used.
Print the ginkgo version information.
Print help on how to execute this example.
if (argc == 2 && (std::string(argv[1]) == "--help")) {
std::cerr << "Usage: " << argv[0] << " [executor] " << std::endl;
std::exit(-1);
}
Where do you want to run your solver ?
The gko::Executor class is one of the cornerstones of Ginkgo. Currently, we have support for an gko::OmpExecutor, which uses OpenMP multi-threading in most of its kernels, a gko::ReferenceExecutor, a single threaded specialization of the OpenMP executor and a gko::CudaExecutor which runs the code on a NVIDIA GPU if available.
- Note
- With the help of C++, you see that you only ever need to change the executor and all the other functions/ routines within Ginkgo should automatically work and run on the executor with any other changes.
const auto executor_string = argc >= 2 ? argv[1] : "reference";
std::map<std::string, std::function<std::shared_ptr<gko::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)();
Reading your data and transfer to the proper device.
Read the matrix, right hand side and the initial solution using the read function.
- Note
- Ginkgo uses C++ smart pointers to automatically manage memory. To this end, we use our own object ownership transfer functions that under the hood call the required smart pointer functions to manage object ownership. gko::share and gko::give are the functions that you would need to use.
auto A =
gko::share(gko::read<mtx>(std::ifstream(
"data/A.mtx"), exec));
auto b = gko::read<vec>(std::ifstream("data/b.mtx"), exec);
auto x = gko::read<vec>(std::ifstream("data/x0.mtx"), exec);
Creating the solver
Generate the gko::solver factory. Ginkgo uses the concept of Factories to build solvers with certain properties. Observe the Fluent interface used here. Here a cg solver is generated with a stopping criteria of maximum iterations of 20 and a residual norm reduction of 1e-7. You also observe that the stopping criteria(gko::stop) are also generated from factories using their build methods. You need to specify the executors which each of the object needs to be built on.
const RealValueType reduction_factor{1e-7};
auto solver_gen =
cg::build()
.with_criteria(gko::stop::Iteration::build().with_max_iters(20u),
.with_reduction_factor(reduction_factor))
.on(exec);
Generate the solver from the matrix. The solver factory built in the previous step takes a "matrix"(a gko::LinOp to be more general) as an input. In this case we provide it with a full matrix that we previously read, but as the solver only effectively uses the apply() method within the provided "matrix" object, you can effectively create a gko::LinOp class with your own apply implementation to accomplish more tasks. We will see an example of how this can be done in the custom-matrix-format example
auto solver = solver_gen->generate(A);
Finally, solve the system. The solver, being a gko::LinOp, can be applied to a right hand side, b to obtain the solution, x.
Print the solution to the command line.
std::cout << "Solution (x):\n";
To measure if your solution has actually converged, you can measure the error of the solution. one, neg_one are objects that represent the numbers which allow for a uniform interface when computing on any device. To compute the residual, all you need to do is call the apply method, which in this case is an spmv and equivalent to the LAPACK z_spmv routine. Finally, you compute the euclidean 2-norm with the compute_norm2 function.
auto one = gko::initialize<vec>({1.0}, exec);
auto neg_one = gko::initialize<vec>({-1.0}, exec);
auto res = gko::initialize<real_vec>({0.0}, exec);
A->apply(one, x, neg_one, b);
b->compute_norm2(res);
std::cout << "Residual norm sqrt(r^T r):\n";
}
Results
The following is the expected result:
Solution (x):
19 1
0.252218
0.108645
0.0662811
0.0630433
0.0384088
0.0396536
0.0402648
0.0338935
0.0193098
0.0234653
0.0211499
0.0196413
0.0199151
0.0181674
0.0162722
0.0150714
0.0107016
0.0121141
0.0123025
Residual norm sqrt(r^T r):
1 1
2.10788e-15
Comments about programming and debugging
The plain program
#include <ginkgo/ginkgo.hpp>
#include <fstream>
#include <iostream>
#include <map>
#include <string>
int main(int argc, char* argv[])
{
using ValueType = double;
using IndexType = int;
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_ptr<gko::Executor>()>>
exec_map{
{"cuda",
[] {
}},
{"hip",
[] {
}},
{"dpcpp",
[] {
}},
{"reference", [] { return gko::ReferenceExecutor::create(); }}};
const auto exec = exec_map.at(executor_string)();
auto A =
gko::share(gko::read<mtx>(std::ifstream(
"data/A.mtx"), exec));
auto b = gko::read<vec>(std::ifstream("data/b.mtx"), exec);
auto x = gko::read<vec>(std::ifstream("data/x0.mtx"), exec);
const RealValueType reduction_factor{1e-7};
auto solver_gen =
cg::build()
.with_criteria(gko::stop::Iteration::build().with_max_iters(20u),
.with_reduction_factor(reduction_factor))
.on(exec);
auto solver = solver_gen->generate(A);
std::cout << "Solution (x):\n";
auto one = gko::initialize<vec>({1.0}, exec);
auto neg_one = gko::initialize<vec>({-1.0}, exec);
auto res = gko::initialize<real_vec>({0.0}, exec);
A->apply(one, x, neg_one, b);
b->compute_norm2(res);
std::cout << "Residual norm sqrt(r^T r):\n";
}
1.8.16