Solvers for Linear Algebraic Equations

We have implemented multigrid solvers for linear algebraic systems arising from various finite element methods. Here we briefly present the usage for a symmetric and positive definite matrix equation Ax = b.

x = A\b;
x = amg(A,b);
x = mg(A,b,elem);

Backslash \ is the build-in direct solver of MATLAB. It is suitable for small size problems. x = amg(A,b) implements algebraic multigrid solver. To acheive multigrid efficiency, a hierarchical ‘grids’ is generated from the graph of A. When the mesh is avaiable, x = mg(A,b,elem) implements geometric multigrid solvers. Inside mg, an coarsening algorithm is applied to the mesh given by elem.

More options is allowed in mg and amg. Try help mg and help amg for possible options including tolerance, V or W-cycles, number of smoothings steps, and print level etc.

Here we include several examples for discrete Poisson matrices. Solvers for other finite element methods and other equations can be found

Example: 2-D Linear Element

% setting
mesh.shape = 'square';
mesh.type = 'uniform';
mesh.size = 2e5;
pde = 'Poisson';
fem = 'P1';
% get the matrix
[eqn,T] = getfemmatrix(mesh,pde,fem);
% compare solvers
tic; disp('Direct solver'); x1 = eqn.A\eqn.b; toc;
tic; x2 = mg(eqn.A,eqn.b,T.elem); toc;
tic; x3 = amg(eqn.A,eqn.b); toc;
format shorte
fprintf('Difference between direct and mg, amg solvers %0.2g, %0.2g \n',...
         norm(x1-x2)/norm(eqn.b),norm(x1-x3)/norm(eqn.b));

Direct solver
Elapsed time is 1.293569 seconds.

 Multigrid V-cycle Preconditioner with Conjugate Gradient Method
#dof:   263169,  #nnz:  1303561, smoothing: (1,1), iter: 10,   err = 1.35e-09,   time =  1.1 s
Elapsed time is 1.186739 seconds.

 Algebraic Multigrid W-cycle Preconditioner with Conjugate Gradient Method
  nnz/N: 4.96,   level:  6,   coarse grid 169,   nnz/Nc 9.38
#dof:  263169,    iter: 14,   err = 5.5672e-09,   time = 4.65 s
 
Elapsed time is 4.334722 seconds.
Difference between direct and mg, amg solvers 1.4e-09, 7.8e-08

png

For problem size of $2.6 \times 10^5$, mg ties with direct solver \. But amg is aroud 3-4 times slover.

Example: 2-D Adaptive Meshes

%% Lshape adaptive grids
mesh.shape = 'Lshape';
mesh.type = 'adaptive';
mesh.size = 2e4;
pde = 'Poisson';
fem = 'P1';
% get the matrix
[eqn,T] = getfemmatrix(mesh,pde,fem);
% compare solvers
tic; disp('Direct solver'); x1 = eqn.A\eqn.b; toc;
tic; x2 = mg(eqn.A,eqn.b,T.elem); toc;
tic; x3 = amg(eqn.A,eqn.b); toc;
fprintf('Difference between direct and mg, amg solvers %0.2g, %0.2g \n',...
         norm(x1-x2)/norm(eqn.b),norm(x1-x3)/norm(eqn.b));

Direct solver
Elapsed time is 3.347438 seconds.

 Multigrid V-cycle Preconditioner with Conjugate Gradient Method
#dof:   738561,  #nnz:  3682043, smoothing: (1,1), iter: 10,   err = 6.29e-09,   time =  3.1 s
Elapsed time is 2.843210 seconds.

 Algebraic Multigrid W-cycle Preconditioner with Conjugate Gradient Method
  nnz/N: 4.99,   level:  6,   coarse grid 453,   nnz/Nc 9.72
#dof:  738561,    iter: 15,   err = 2.9982e-09,   time = 14.9 s
 
Elapsed time is 13.711571 seconds.
Difference between direct and mg, amg solvers 1.2e-08, 6e-08

png

The finest mesh is several uniform refinement of an adaptive mesh shown above. Now the multigrid outperforms the direct solver around the size of 7e5. amg is 4-5 times slower.

Example: 3-D Linear Element

% Cube uniform grids
mesh.shape = 'cube';
mesh.type = 'uniform';
mesh.size = 2e4;
pde = 'Poisson';
fem = 'P1';
% get the matrix
[eqn,T] = getfemmatrix3(mesh,pde,fem);
% compare solvers
tic; disp('Direct solver'); x1 = eqn.A\eqn.b; toc;
tic; x2 = mg(eqn.A,eqn.b,T.elem); toc;
tic; x3 = amg(eqn.A,eqn.b); toc;
fprintf('Difference between direct and mg, amg solvers %0.2g, %0.2g \n',...
         norm(x1-x2)/norm(eqn.b),norm(x1-x3)/norm(eqn.b));

Direct solver
Elapsed time is 0.443978 seconds.

 Multigrid V-cycle Preconditioner with Conjugate Gradient Method
#dof:    35937,  #nnz:   202771, smoothing: (1,1), iter: 11,   err = 5.50e-09,   time = 0.18 s
Elapsed time is 0.150138 seconds.

 Algebraic Multigrid W-cycle Preconditioner with Conjugate Gradient Method
  nnz/N: 5.81,   level:  4,   coarse grid 305,   nnz/Nc 30.80
#dof:   35937,    iter: 10,   err = 3.5316e-09,   time = 0.92 s
 
Elapsed time is 0.645183 seconds.
Difference between direct and mg, amg solvers 1e-09, 4.2e-09

png

For 3-D linear element, mg wins at an even smaller size $3.6\times 10^4$. Again amg is 3-4 times slower than mg.

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