[petsc-users] Boomeramg as a solver

[Barry Smith]

On Nov 1, 2010, at 2:46 PM, jibum kim wrote:

> Are you saying that boomeramg with richardson iterations is same as the multigrid without a preconditioner?

   I do not know what you mean by multigrid with or without a preconditioner?  Multigrid doesn't "take a preconditioner", it is an iterative solver that can be used as a preconditioner but it cannot use other preconditioners.

> and if it the case, how can I set an extra preconditioner like ILU ?

   What do you mean by an "extra preconditioner"? I assume you mean use ILU as a smoother instead of SOR or Jacobi. BoomerAMG has no mechanism to use ILU as a smoother so you cannot do this.

   Because ML is more flexible it is possible to use PETSc's ILU as a smoother for it.

   Barry

> 
> Thanks,
> 
> - Jibum
> 
> ----- Original Message ----- From: "Barry Smith" <bsmith at mcs.anl.gov>
> To: "PETSc users list" <petsc-users at mcs.anl.gov>
> Sent: Monday, November 01, 2010 2:25 PM
> Subject: Re: [petsc-users] Boomeramg as a solver
> 
> 
> 
> On Nov 1, 2010, at 12:35 PM, jibum kim wrote:
> 
>> Hello,
>> 
>> I've used  Boomeramg as a preconditioner.
>> 
>> Now, I am wondering whether I can use Boomeramg as a solver ? and is it possible that I use Boomeramg as both a preconditioner and a solver?
>> 
>> I saw one slide saying that I can use KSPSetType(ksp, KSPRICHARDSON);
>> 
>> But when I try it and use -ksp_view, it shows that my solver is Richardson not  Boomeramg.
>> 
>> Could u clarify this issue ?
> 
> Richardson is our name for the iterative process   u^{n+1} = u^n + B(b - A u^n) for n=1,...max_it where B is the application of the preconditioner (in other words running a linear iterative method without any Krylov acceleration). In BoomerAMG's case B is one application of the V or W cycle (depending on your choice of BoomerAMG options).  So even though it says the "solver" is Richardson it is using BoomerAMG for the entire solution process, that is  algebraic multigrid with multiple V (or W cycles).
> 
>  Barry
> 
> 
> 
> 
>> 
>> Thank you.
>> 
>> - Jibum
> 
>