How to construct a linear solver using the "Linear Solver Factory"

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The class "LinearSolverFactory" is designed to help in the construction of the Kratos Linear Solvers, and makes an attempt to unify the construction mechanism.

the essential idea is that the "settings" to be used in the construction of a linear solver are defined by constructing a new python class (with arbitrary name) which contains the settings needed for the construction of the solver.

Some examples of usage can be found in [[1]]


Within kratos there exist different classes of linear solvers.

Contents

Iterative Solvers available in the Kratos Core

A first group of iterative solvers is included within the Kratos core and is always available to the user. Available choices for "solver_type"

 BiConjugate gradient stabilized
 Conjugate gradient
 Deflated Conjugate gradient
 GMRES

This solvers can be used with or without a preconditioner, which is also available within the Kratos core. Available options for "preconditioner_type" are

 None
 DiagonalPreconditioner
 ILU0

in order to construct a "BiConjugate gradient stabilized" together with an ILU0 preconditioner using the factory class one shall write

   ##here we specify the settings to be used in the construction
   class custom_settings:
      solver_type = "BiConjugate gradient stabilized"
      scaling = True
      preconditioner_type = "DiagonalPreconditioner"
      max_iteration = 500
      tolerance = 1e-6
   
   ##here we actually construct a new linear solver using the "custom settings" just defined
   import linear_solver_factory
   new_linear_solver =  linear_solver_factory.ConstructSolver(custom_settings)

Note that specifying "True" at the scaling option implies that the matrix coefficients is normalized prior to the solution step

the "Deflated" solver is specific in that it has two optional additional parameters "assume_constant_structure" and "max_reduced_size". The first of the two tells if it is possible to assume that the graph of the system will remain the same through the simulation. Setting it to True when possible enables a faster solution. "max_reduced_size" determines the maximum size of the deflation space.

"External" Iterative Solvers (available in some of the applications)

The Kratos also provides interface to external libraries which provide more advanced preconditioners than the ones provided in the Kratos core. Available options are

  SuperLUIterativeSolver
  Pastix

SuperLU solver (available in the ExternalSolversApplication) provides a gmres + ILUT preconditioner can be used by creating a configure of the type

  class solver_configure
     solver_type = "SuperLUIterativeSolver"
     scaling = False
     tolerance = 1e-7
     max_iteration = 300
     gmres_krylov_space_dimension=100 
     DropTol=1e-4
     FillTol = 1e-2
     ilu_level_of_fill = 5

the user is invited to check the SuperLU documentation [[2]] for an explanation of the meaning of the different parameters

Iterative Pastix solver (available in the ExternalSolversApplication provided that compilation enabled it) can be obtained with a configure of the type

  class solver_configure
     solver_type = "PastixIterative"
     scaling = False
     tolerance = 1e-7
     max_iteration = 300     
     gmres_krylov_space_dimension = 100
     ilu_level_of_fill = 5
     verbosity = 0
     is_symmetric = False 

note that by setting the is_symmetric=True, incomplete Cholesky is used instead of ILU. Pastix solver is OpenMP parallel

Direct solvers included in the Kratos

Admissible values for "solver_type" are

  Skyline LU factorization
  SuperLUSolver --> requires the ExternalSolversApplication
  Parallel MKL Pardiso --> requires kratos to be compiled with Intel MKL and MKLSolversApplication
  PastixDirect --> requires the ExternalSolversApplication to be compiled together with the Pastix solver

such solvers are appropriate for the solution of relatively small systems of equations which can be conveniently solved by employing a direct solver technology. The Pardiso solver and the Pastix are OpenMP parallel. Since these solvers are direct, tolerance, preconditioner_type and max_iterations make no sense and are not required. a new solver of this type could be constructed as

   ##here we specify the settings to be used in the construction
   class other_settings:
       solver_type = "SuperLUSolver"
       scaling = False
   

An error is thrown if the required application is not loaded.

Pastix solver is a little specific in that it allows specifying more parameters to handle the case of symmetric matrices and to

   ##here we specify the settings to be used in the construction
   class other_settings:
       solver_type = "PastixDirect"
       scaling = False
       verbosity = 0
       is_symmetric = False

Algebraic Multigrid Solver

The kratos contains an interface to the AMGCL solver (courtesy of Dr. Denis Demidov). Such solver combines Algebraic Multigrid with krylov methods.

the configuration of the solver is as follows:

  class solver_configuration:
      solver_type = "AMGCL"
      scaling = True
      max_iteration = 50
      tolerance = 1e-5
      smoother_type = "ILU0"  #other options are "DAMPED_JACOBI" and "SPAI0"
      krylov_type = "GMRES" #other options are "BICGSTAB" and "CG"
      verbosity = 0 #OPTIONAL! set to 1 to have some output
      gmres_krylov_space_dimension = 50 #OPTIONAL! only read in case GMRES is chosen. 

and the construction is as in the other cases. The solver automatically detect block structure in the unkowns, and can be used effecitively for multidimensional problems.

Specialized Solvers

The kratos core also provides more advanced solvers, specialized to the case of mixed formulations. An example of this is the

   Block UP Solver

solver which implements a SIMPLE-like preconditioner for the monolithic Navier-Stokes equations combined with a GMRES solver. This solver combines different linear solvers to be used for the "U block" and for the "P block" and shall be in some sense optimal in the case of dominating intertia.

The solver implies choosing a solver for the velocity block and a different one for the pressure block. an example of construction can be

class block_UP_iterative_solver_configure:

   solver_type = "GMRES-UP Block"
   scaling = False
   tolerance = 1e-3
   max_iteration = 5     
   gmres_krylov_space_dimension = 5
   
   class velocity_block_configuration:
       solver_type = "PastixIterative"
       scaling = False
       tolerance = 1e-6
       max_iteration = 300     
       gmres_krylov_space_dimension = 100
       ilu_level_of_fill = 3
       verbosity = 0
       is_symmetric = False  
   
   class pressure_block_configuration:
       solver_type = "AMGCL"
       scaling = False
       max_iteration = 50
       tolerance = 1e-3
       smoother_type = "DAMPED_JACOBI"
       krylov_type = "GMRES"
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