# Backends¶

A backend in AMGCL is a class that defines matrix and vector types together with several operations on them, such as creation, matrix-vector products, elementwise vector operations, inner products etc. The <amgcl/backend/interface.hpp> file defines an interface that each backend should extend. The AMG hierarchy is moved to the specified backend upon construction. The solution phase then uses types and operations defined in the backend. This enables transparent acceleration of the solution phase with OpenMP, OpenCL, CUDA, or any other technologies.

In order to use a backend, user must include its definition from the corresponding file inside amgcl/backend folder. On the user side of things, only the types of the right-hand side and the solution vectors should be affected by the choice of AMGCL backend. Here is an example of using the builtin backend. First, we need to include the appropriate header:

#include <amgcl/backend/builtin.hpp>


Then, we need to construct the solver and apply it to the vector types supported by the backend:

typedef amgcl::backend::builtin<double> Backend;

typedef amgcl::make_solver<
amgcl::amg<Backend, amgcl::coarsening::aggregation, amgcl::relaxation::spai0>,
amgcl::solver::gmres<Backend>
> Solver;

Solver solve(A);

std::vector<double> rhs, x; // Initialized elsewhere

solve(rhs, x);


Now, if we want to switch to a different backend, for example, in order to accelerate the solution phase with a powerful GPU, we just need to include another backend header, and change the definitions of Backend, rhs, and x. Here is an example of what needs to be done to use the VexCL backend.

#include <amgcl/backend/builtin.hpp>


Change the definition of Backend:

typedef amgcl::backend::vexcl<double> Backend;


Change the definition of the vectors:

vex::vector<double> rhs, x;


That’s it! Well, almost. In case the backend requires some parameters, we also need to provide those. In particular, the VexCL backend should know what VexCL context to use:

// Initialize VexCL context on a single GPU:
vex::Context ctx(vex::Filter::GPU && vex::Filter::Count(1));

// Create backend parameters:
Backend::params backend_prm;
backend_prm.q = ctx;

// Pass the parameters to the solver constructor:
Solver solve(A, Solver::params(), backend_prm);


## Builtin¶

#include <amgcl/backend/builtin.hpp>

template <typename ValueType>
struct amgcl::backend::builtin

The builtin backend does not have any dependencies except for the Boost libraries, and uses OpenMP for parallelization. Matrices are stored in the CRS format, and vectors are instances of std::vector<value_type>. There is no usual overhead of moving the constructed hierarchy to the builtin backend, since the backend is used internally during setup.

Public Types

typedef amgcl::detail::empty_params params

The backend has no parameters.

struct provides_row_iterator

Inherits from true_type

## VexCL¶

#include <amgcl/backend/vexcl.hpp>

template <typename real, class DirectSolver = solver::vexcl_skyline_lu<real>>
struct amgcl::backend::vexcl

The backend uses the VexCL library for accelerating solution on the modern GPUs and multicore processors with the help of OpenCL or CUDA technologies. The VexCL backend stores the system matrix as vex::SpMat<real> and expects the right hand side and the solution vectors to be instances of the vex::vector<real> type.

struct params

The VexCL backend parameters.

Public Members

template<>
std::vector<vex::backend::command_queue> q

Command queues that identify compute devices to use with VexCL.

template<>
bool fast_matrix_setup

Do CSR to ELL conversion on the GPU side.

This will result in faster setup, but will require more GPU memory.

struct provides_row_iterator

Inherits from false_type