[petsc-users] Using SNESSHELL as a wrapper for a CFD solver.
Barry Smith
bsmith at petsc.dev
Wed May 31 15:56:35 CDT 2023
> On May 31, 2023, at 4:16 PM, Kenneth C Hall <kenneth.c.hall at duke.edu> wrote:
>
> Matt,
>
> Thanks for your quick reply. I think what you say makes sense.
>
> You asked what my code does. The MySolver program performs one iteration of a CFD iteration. The CFD scheme is an explicit scheme that uses multigrid, Mach number preconditioning, and residual smoothing. Typically, I have to call MySolver on the order of 40 to 100 times to get acceptable convergence.
>
> And in fact, I have another version of this Petsc code that uses SNESNGMRES to solve the problem with MySolver providing the residuals as R = N(x) - x.
So you use SNESSetFunction( a function you wrote that computes R = N(x) - x )?
Have you considered using the SNESSetNPC() mechanism? That is, use your solver as a nonlinear preconditioner for any of the PETSc solvers?
In particular, you could use it with SNESNRICHARDSON which provides a lineseach, monitor, and convergence testing in a loop. I think this is exactly the loop structure you want, so no need for SNESSHELL.
> But I would like a version where I am using just MySolver, without any other operations applied to it. So I am trying to plug MySolver into the PETSc system to provide monitoring and other features, and for consistency and comparison to these other (more appropriate!) uses of PETSc.
>
> Thanks.
> Kenneth
>
>
> From: Matthew Knepley <knepley at gmail.com <mailto:knepley at gmail.com>>
> Date: Wednesday, May 31, 2023 at 3:48 PM
> To: Kenneth C Hall <kenneth.c.hall at duke.edu <mailto:kenneth.c.hall at duke.edu>>
> Cc: petsc-users at mcs.anl.gov <mailto:petsc-users at mcs.anl.gov> <petsc-users at mcs.anl.gov <mailto:petsc-users at mcs.anl.gov>>
> Subject: Re: [petsc-users] Using SNESSHELL as a wrapper for a CFD solver.
>
> On Wed, May 31, 2023 at 3:21 PM Kenneth C Hall <kenneth.c.hall at duke.edu <mailto:kenneth.c.hall at duke.edu>> wrote:
> Hi,
>
> I am doing a number of problems using PETSc/SLEPc, but I also work on some non-PETSc/SLEPc flow solvers. I would like to use PETSc as a wrapper for this non-PETSc flow solver for compatibility, so I can use the tolerance monitoring, options, viewers, and for direct comparison to PETSc methods I am using.
>
> Here is what I am trying to do… I have a CFD solver that iterates with a nonlinear iterator of the form x := N(x). This can be expressed in a fortran routine of the form,
>
> SUBROUTINE MySolver(x)
> or
> SUBROUTINE MySolver(x,y)
>
> In the first case, x is over written. In the second, y = N(x). In any event, I want to do something like what is shown in the subroutine at the bottom of this email.
>
> The code below “works” in the sense that MySolver is called, but it is called exactly *once*. But MyMonitor and MyConverged are *not* called. Again, I want to iterate so MySolver should be called many times, as should MyMonitor and MyConverged.
>
> The SNESolve() method is called once per nonlinear solve, just as KSPSolve() is called once per linear solve. There may be iteration inside the method, but that is handled inside the particular implementation. For example, both Newton's method and Nonlinear Conjugate Gradient iterate, but the iteration is internal to both, and they both call the monitor and convergence test at each internal iterate.
>
> So, if your nonlinear solver should iterate, it should happen inside the SNESSolve call for the SNESSHELL object. Does this make sense? What does your solver do?
>
> Thanks,
>
> Matt
>
> The SNESView before and after SNESSolve looks like this:
>
> SNES Object: 1 MPI process
> type: shell
> SNES has not been set up so information may be incomplete
> maximum iterations=50, maximum function evaluations=10000
> tolerances: relative=1e-50, absolute=1e-10, solution=1e+06
> total number of function evaluations=0
> norm schedule ALWAYS
>
> SNES Object: 1 MPI process
> type: shell
> maximum iterations=50, maximum function evaluations=10000
> tolerances: relative=1e-50, absolute=1e-10, solution=1e+06
> total number of function evaluations=0
> norm schedule ALWAYS
>
> Any suggestions on how to do what I am trying to accomplish?
>
> Thanks.
> Kenneth Hall
>
>
>
> #include <petsc/finclude/petsc.h>
> #include "macros.h"
>
> MODULE SolveWithSNESShell_module
> USE MyPetscModule
> CONTAINS
> !
> !====================================================================================================
> SUBROUTINE MySolver(snes, x, ierr)
> !====================================================================================================
> !!
> !!
> !====================================================================================================
> !
> USE MyPetscModule
> IMPLICIT NONE
> !
> !.... declared passed variables
> SNES :: snes
> Vec :: x
> PetscErrorCode :: ierr
> !
> !.... code to find residual x := N(x)
> !.... (or alternatively y := N(x))
>
> END SUBROUTINE MySolver
> !
> !====================================================================================================
> SUBROUTINE MyMonitor(snes, its, rnorm, ierr)
> !====================================================================================================
> !!
> !!
> !====================================================================================================
> !
> USE MyPetscModule
> IMPLICIT NONE
> !
> !.... Declare passed variables
> SNES :: snes
> PetscInt :: its
> PetscReal :: rnorm
> PetscErrorCode :: ierr
> !
> !.... Code to print out convergence history
> !.... Code to print out convergence history
>
> END SUBROUTINE MyMonitor
>
> !====================================================================================================
> SUBROUTINE MyConverged(snes, it, xnorm, ynorm, znorm, reason, ierr)
> USE MyPetscModule
> IMPLICIT NONE
>
> SNES :: snes
> PetscInt :: it,ctx
> PetscReal :: xnorm, ynorm, znorm
> KSPConvergedReason :: reason
> PetscErrorCode :: ierr
>
> ! ... add convergence test here ...
> ! set reason to a positive value if convergence has been achieved
>
>
> END SUBROUTINE MyConverged
> END MODULE SolveWithSNESShell_module
> !
> !====================================================================================================
> SUBROUTINE SolveWithSNESShell
> !====================================================================================================
> !!
> !!
> !====================================================================================================
> !
> USE SolveWithSNESShell_module
> IMPLICIT NONE
> !
> !.... Declare passed variables
> INTEGER :: level_tmp
> !
> !.... Declare local variables
> INTEGER :: iz
> INTEGER :: imax
> INTEGER :: jmax
> INTEGER :: kmax
> SNES :: snes
> KSP :: ksp
> Vec :: x
> Vec :: y
> PetscViewer :: viewer
> PetscErrorCode :: ierr
> PetscReal :: rtol = 1.0D-10 !! relative tolerance
> PetscReal :: atol = 1.0D-50 !! absolute tolerance
> PetscReal :: dtol = 1.0D+06 !! divergence tolerance
> PetscInt :: maxits = 50
> PetscInt :: maxf = 10000
> character(len=1000):: args
> !
> !.... count the number of degrees of freedom.
> level = level_tmp
> n = 0
> DO iz = 1, hb(level)%nzone
> imax = hb(level)%zone(iz)%imax - 1
> jmax = hb(level)%zone(iz)%jmax - 1
> kmax = hb(level)%zone(iz)%kmax - 1
> n = n + imax * jmax * kmax
> END DO
> n = n * neqn
> !
> !.... Initialize PETSc
> PetscCall(PetscInitialize(PETSC_NULL_CHARACTER, ierr))
> !
> !.... Log
> PetscCall(PetscLogDefaultBegin(ierr))
> !
> !.... Hard-wired options.
> ! PetscCall(PetscOptionsInsertString(PETSC_NULL_OPTIONS, "command line style option here" , ierr))
> !
> !.... Command line options.
> call GET_COMMAND(args)
> PetscCall(PetscOptionsInsertString(PETSC_NULL_OPTIONS, args, ierr))
> !
> !.... view command line table
> PetscCall(PetscViewerASCIIOpen(PETSC_COMM_SELF, PETSC_VIEWER_STDOUT_SELF, viewer, ierr))
> PetscCall(PetscOptionsView(PETSC_NULL_OPTIONS, viewer, ierr))
> PetscCall(PetscViewerDestroy(viewer, ierr))
> !
> !.... Create PETSc vectors
> PetscCall(VecCreateSeq(PETSC_COMM_SELF, n, x, ierr))
> PetscCall(VecCreateSeq(PETSC_COMM_SELF, n, y, ierr))
> PetscCall(VecSet(x, 0.0d0, ierr))
> PetscCall(VecSet(y, 0.0d0, ierr))
>
> !.... SNES context
> PetscCall(SNESCreate(PETSC_COMM_SELF, snes, ierr))
> PetscCall(SNESSetType(snes, SNESSHELL, ierr))
> PetscCall(SNESShellSetSolve(snes, MySolver, ierr))
>
> !!!! PetscCall(SNESSetFunction(snes, x, MySolver, PETSC_NULL_INTEGER, ierr))
> !!!! this line causes a segmentation error if uncommented.
>
> PetscCall(SNESSetConvergenceTest(snes, MyConverged, 0, PETSC_NULL_FUNCTION, ierr))
> !
> !.... Set SNES options
> PetscCall(SNESSetFromOptions(snes, ierr))
>
> !.... Set tolerances
> PetscCall(SNESSetTolerances(snes, rtol, atol, dtol, maxits, maxf, ierr))
> !
> !.... SNES montior
> PetscCall(SNESMonitorSet(snes, MyMonitor, PETSC_NULL_INTEGER, PETSC_NULL_FUNCTION,ierr))
> !
> !.... Set the initial solution
> CALL HBToVecX(x)
> !
> !.... View snes context
> PetscCall(SNESView(snes, viewer, ierr))
> !
> !.... Solve SNES problem
> PetscCall(SNESSolve(snes, PETSC_NULL_VEC, x, ierr))
> !
> !.... View snes context
> PetscCall(SNESView(snes, viewer, ierr))
> !
> !.... dump the logs
> ! call PetscLogDump(ierr) ! Why does this cause error
> !
> !.... Destroy PETSc objects
> PetscCall(SNESDestroy(snes, ierr))
> PetscCall(VecDestroy(x, ierr))
> PetscCall(VecDestroy(y, ierr))
> PetscCall(PetscViewerDestroy(viewer, ierr))
> !
> !.... Finish
> PetscCall(PetscFinalize(ierr))
>
> END SUBROUTINE SolveWithSNESShell
>
>
>
> --
> What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead.
> -- Norbert Wiener
>
> https://www.cse.buffalo.edu/~knepley/ <https://urldefense.com/v3/__http:/www.cse.buffalo.edu/*knepley/__;fg!!OToaGQ!sE2W1qI_dqcEHL1dOCnJ3Rdv9TATFVDBiBqx_tlQsOnjvGF7StDjsmVcm9Qkfe4XcTFOBtVjtFl5om07Rdjw$>
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