Solving sparse systems

There are many methods available for solving sparse systems. We compare some of them here.

Figure 1: Matrix of the discretized PDE.

Mathematica has the following methods available (https://reference.wolfram.com/language/ref/LinearSolve.html#DetailsAndOptions)

• direct: banded, cholesky, multifrontal (direct sparse LU)
• iterative: Krylov

Matlab has the following methods:

• direct: https://www.mathworks.com/help/matlab/ref/mldivide.html#bt42omx_head
• iterative: https://www.mathworks.com/help/matlab/math/systems-of-linear-equations.html#brzoiix, including bicgstab, gmres

Eigen has the following methods: (https://eigen.tuxfamily.org/dox-devel/group__TopicSparseSystems.html)

• direct: sparse LU
• iterative: bicgstab, cg

Solving a simple sparse system $A x = b$ for steady space of heat equation in 1d with $n$ nodes, results in a matrix shown in Figure Figure 1.

The following timings of solvers are given in seconds:

$n = 10^6$	Matlab	Mathematica	Eigen
Banded	0.16	0.28	0.04
SparseLU	/	1.73	0.82
BICGStab / Krylov	0.33	0.39	0.53

Incomplete LU preconditioner was used for BICGStab. Without the preconditioner BICGStab does not converge.

BICGStab can be run in parallel, as explain in the general parallelism: https://eigen.tuxfamily.org/dox/TopicMultiThreading.html, and specifically

"When using sparse matrices, best performance is achied for a row-major sparse matrix format.
Moreover, in this case multi-threading can be exploited if the user code is compiled with OpenMP enabled".

Eigen uses number of threads specified my OopenMP, unless Eigen::setNbThreads(n); was called. Minimal working example:

Figure 2: Matrix of the discretized PDE.

1. include <iostream>
2. include <vector>
3. include "Eigen/Sparse"
4. include "Eigen/IterativeLinearSolvers"

using namespace std; using namespace Eigen;

int main(int argc, char* argv[]) {

   assert(argc == 2 && "Second argument is size of the system.");
   stringstream ss(argv[1]);
   int n;
   ss >> n;
   cout << "n = " << n << endl;

   // Fill the matrix
   VectorXd b = VectorXd::Ones(n) / n / n;
   b(0) = b(n-1) = 1;
   SparseMatrix<double, RowMajor> A(n, n);
   A.reserve(vector<int>(n, 3));  // 3 per row
   for (int i = 0; i < n-1; ++i) {
       A.insert(i, i) = -2;
       A.insert(i, i+1) = 1;
       A.insert(i+1, i) = 1;
   }
   A.coeffRef(0, 0) = 1;
   A.coeffRef(0, 1) = 0;
   A.coeffRef(n-1, n-2) = 0;
   A.coeffRef(n-1, n-1) = 1;

   // Solve the system
   BiCGSTAB<SparseMatrix<double, RowMajor>, IncompleteLUT<double>> solver;
   solver.setTolerance(1e-10);
   solver.setMaxIterations(1000);
   solver.compute(A);
   VectorXd x = solver.solve(b);
   cout << "#iterations:     " << solver.iterations() << endl;
   cout << "estimated error: " << solver.error()      << endl;
   cout << "sol: " << x.head(6).transpose() << endl;

   return 0;

}

was compiled with g++ -o parallel_solve -O3 -fopenmp solver_test_parallel.cpp Figure Figure 2. was produced when the program above was run as ./parallel_solve 10000000.