[Trilinos-Users] Custom preconditioner with Tpetra

Martin Vymazal martin.vymazal at vki.ac.be
Thu Jan 7 07:07:40 EST 2016


Hello Mark and Mike,

 thank you for your answers. I looked for examples before posting on the 
mailing list, but somehow managed not to find lesson06_custom_operator.cpp. 
Sorry about that.

Best regards,

 Martin


On Monday, January 4, 2016 7:50:10 PM GMT Heroux, Mike wrote:
> Martin,
> 
> To follow up on Mark's comments, Trilinos provides two paths for users to
> provide their own linear operators.  This is true for Tpetra and Epetra,
> and for all of the preconditioner and solver packages.  These APIs are:
> 
> 
>   *   Operator: This basic interface enables users to define their own
> linear operator as an adaptor to the abstract interface used by Trilinos
> for applying the action of a linear operator on a vector (or multivector if
> you have more than one vector).  The example
> (tpetra/core/example/Lesson06-Custom-Operator/lesson06_custom_operator.cpp)
> that Mark points to shows how define your own Operator class that can be
> used with Belos.  The Operator interface is also implemented by Trilinos
> solvers and preconditioners, so each Trilinos solver and preconditioner
> "isa" Operator.  Belos permits you to register an Operator instance as the
> "A" in solving Ax = b or as the "P" to be used as the preconditioner. *  
> RowMatrix: This interface is related to Operator.  In fact, RowMatrix "isa"
> Operator in that any RowMatrix object "isa" Operator object.  RowMatrix
> enables you to provide matrix coefficients to Trilinos preconditioners and
> utility functions, in addition to the functionality needed by the Operator
> interface.  All Trilinos preconditioners access matrix coefficients via the
> RowMatrix interface, so if you define your own RowMatrix class, you can
> have Trilinos compute multigrid, ILU or other preconditioners for your
> RowMatrix object.  Since RowMatrix "isa" Operator, you can pass a RowMatrix
> object to Belos as the "A" or "P" needed for solving a linear system.
> 
> In addition to Mark's example, you may find the following example to be
> useful.  It shows how to define and  use both Operator and RowMatrix.  It
> is written in terms of Epetra and AztecOO, but the principles carry forward
> to Tpetra and Belos:
> 
> Trilinos/packages/aztecoo/example/AztecOO_UserOpUserMat
> 
> I hope this helps.
> 
> Mike
> 
> 
> On 1/3/16, 12:49 AM, "Trilinos-Users on behalf of Hoemmen, Mark"
> <trilinos-users-bounces at trilinos.org<mailto:trilinos-users-bounces at trilinos
> .org> on behalf of mhoemme at sandia.gov<mailto:mhoemme at sandia.gov>> wrote:
> 
> 
> 
> On 12/28/15, 11:03 PM,
> "trilinos-users-request at trilinos.org<mailto:trilinos-users-request at trilinos
> .org>"
> <trilinos-users-request at trilinos.org<mailto:trilinos-users-request at trilinos
> .org>> wrote: Message: 6
> Date: Mon, 28 Dec 2015 16:42:03 +0000
> From: Martin Vymazal
> <martin.vymazal at vki.ac.be<mailto:martin.vymazal at vki.ac.be>> To:
> trilinos-users at trilinos.org<mailto:trilinos-users at trilinos.org> Subject:
> [Trilinos-Users] Custom preconditioner with Tpetra
> Message-ID:
> <0982ff04998b5b3181f7c401386ec950 at vki.ac.be<mailto:0982ff04998b5b3181f7c401
> 386ec950 at vki.ac.be>> Content-Type: text/plain; charset=US-ASCII;
> format=flowed
> 
> Hello,
> 
>   I would like to build a custom preconditioner for a linear system using
> Tpetra matrix/vector and linear solvers provided by Belos. Could you
> please
> tell me where to start? Is there some basic example showing how to do
> this?
> 
> Yes, Tpetra has an example showing how to define a custom
> Tpetra::Operator.  Please refer to
> tpetra/core/example/Lesson06-Custom-Operator/lesson06_custom_operator.cpp.
> Belos also has examples showing how to use preconditioners.
> 
> The matrix I would like to use for preconditioning is P^(-1), so I
> suppose one
> simple way of applying the preconditioner would be to assemble P, invert
> it
> (or compute cheap approximation of P^(-1)) and then multiply my lhs and
> rhs by
> P^(-1), but perhaps there's a more proper way of applying the
> preconditioner?
> 
> Preconditioning is a whole field of study in itself.  I can't tell you
> what the "proper way" to apply your preconditioner is, because that
> depends on your preconditioner.  If you plan to write your own
> preconditioner rather than using an existing implementation (including the
> many preconditioners in various Trilinos packages), it's up to you to
> decide how to apply it.
> 
> mfh
> 
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