Hypre

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The Hypre module makes available some features of the high-performance preconditioning library Hypre, in particular its algebraic multigrid for use instead of Gerris' native multigrid solver for Poisson equations.

There are some special notes on installing Hypre for use with Gerris.

The parameters of the Hypre module can be tuned using an optional parameter block. For example

GModule hypre {
  solver_type      = boomer_amg
  precond_type     = none
  relax_type       = gs-j
  coarsening_type  = cgc_e
  cycle_type       = 1
  nlevel           = 0
  verbose          = 0
}

where the values are set to their default. The parameters are

solver_type
the type of solver to be used,
relax_type
the type of relaxation to use for the AMG solver,
coarsening_type
the coarsening algorithm to use for the AMG solver,
relax_type
the type of cycle to use for the AMG solver,
nlevel
the maximum number of multigrid levels for the AMG solver (setting this to zero will use the depth of the quad/octree),
verbose
selects whether Hypre should print its own statistics.

So far, only two types of solvers are available:

boomer_amg
the AMG solver,
pcj
a preconjugate gradient solver.

For the AMG solver various types of cycles can be used and several algorithms are available for relaxation and coarsening. Please refer to the documentation page of the Hypre library for more details on the different algorithms. The keywords for the different relaxation algorithms are:

jacobi
Jacobi or CF-Jacobi,
gauss_seidel 
Gauss-Seidel, sequential (very slow!),
sor-j-forward 
Hybrid: SOR-J mix off-processor, SOR on-processor with outer relaxation parameters (forward solve),
sor-j-backward 
Hybrid: SOR-J mix off-processor, SOR on-processor with outer relaxation parameters (backward solve),
ssor-j 
Hybrid: SSOR-J mix off-processor, SSOR on-processor with outer relaxation parameters,
gs-j 
Hybrid: GS-J mix off-processor, chaotic GS on-node,
matvec-jacobi 
Jacobi (uses Matvec), only needed in CGNR.

The keywords for the different coarsening algorithms are:

cljp 
CLJP-coarsening - a parallel coarsening algorithm using independent sets,
ruge_stueben 
Classical Ruge-Stueben coarsening on each processor, followed by a third pass, which adds coarse points on the boundaries,
falgout 
Falgout coarsening (uses 1 first, followed by CLJP using the interior coarse points generated by 1 as its first independent set) ,
pmis 
PMIS-coarsening (a parallel coarsening algorithm using independent sets, generating lower complexities than CLJP, might also lead to slower convergence),
hmis 
HMIS-coarsening (uses one pass Ruge-Stueben on each processor independently, followed by PMIS using the interior C-points generated as its first independent set),
cgc 
CGC coarsening by M. Griebel, B. Metsch and A. Schweitzer,
cgc_e 
CGC-E coarsening by M. Griebel, B. Metsch and A. Schweitzer.

For compatibility with Gerris' default solver, the Hypre module also uses the following parameters, set using GfsApproxProjectionsParams and GfsProjectionParams:

tolerance 
the convergence threshold,
nitermin
the minimum number of iterations,
nitermax
the maximum number of iterations,
nrelax
the number of relaxation sweeps on each level.

Note that Hypre uses the RMS-norm of the residual to define its convergence tolerance criteria (rather than the maximum for Gerris). It also calculates the RMS-norm in a different way than Gerris. This is partly related to the fact that the Hypre module solves a linear probem for which each stencil has been non-dimensionalised by the size of the diagonal cell, whereas the residual norm in Gerris takes into account the volume of all the cells of the domain. Also, whereas Gerris calculates the norm of the residual only over all the cells of the domain, Hypre takes into accounts the ghost cells at the boundaries of the domain. To account partially for the difference, a correction factor defined as <math> \sqrt \right( \frac{ \sum_1^n cell_volume }{n} \left) </math> where n is the number of cells in the domain is used to rescale the norm residual calculated by Hypre.

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