.. _solvers: Solvers ======= Parthenon does not yet provide an exhaustive set of plug and play solvers. Nevertheless, the building blocks required for implementing Krylov subspace methods (i.e. global reductions for vector dot products) like CG, BiCGStab, and GMRES are available. An example of a Parthenon based implementation of BiCGStab can be found in ``examples/poisson_gmg``. Additionally, the infrastructure required for implementing multigrid solvers is also included in Parthenon. The requisite hierarchy of grids is produced if ``parthenon/mesh/multigrid=true`` is set in the parameter input. An example of a multi-grid based linear solver in Parthenon is also given in ``examples/poisson_gmg`` (and also an example of using multi-grid as a preconditioner for BiCGStab). We plan to build wrappers that simplify the use of these methods in down stream codes in the future. Note that the example code does not currently rely on the Stencil and SparseMatrixAccessor code described below. Stencil ------- This class provides a very simple and efficient means of storing a sparse matrix with the special form that every row has identical entries relative to the matrix diagonal. A good example of this is in the straightforward finite difference discretization of the Poisson equation (see `here `__ for example usage). The ``Stencil`` object is extremely efficient at storing these sparse matrices because it only has to store the matrix values and offsets from the diagnonal for a single row. The ``Stencil`` class provides member functions to compute matrix vector products (``MatVec``) and Jacobi iterates (``Jacobi``). Both are designed to be called from within kernels and operate on a single matrix row at a time. SparseMatrixAccessor -------------------- This is a helper class that allows one to store a more general sparse matrix than ``Stencil`` provides. Like ``Stencil``, the ``SparseMatrixAccessor`` class assumes that the location of the nonzero matrix elements have fixed offsets from the diagonal in every row. Here, though, the values of the matrix elements can be different from row to row. The sparse matrix itself can be stored in a normal :ref:`cell var` with the number of components equal to the number of nonzero elements in a row of the matrix. The ``SparseMatrixAccessor`` class than associates each of these components with a particular matrix element. Like ``Stencil``, the ``SparseMatrixAccessor`` class provides ``MatVec`` and ``Jacobi`` member functions. A simple demonstration of usage can be found in the `Poisson example `__.