Re: [petsc-users] Using SNESSHELL as a wrapper for a CFD solver.

On Wed, May 31, 2023 at 5:51 PM Barry Smith <[email protected]> wrote:

Sorry, I wrote to quickly in my last email. You will need to create a SNESSHELL its solve simply calls your solver (for its one iteration) SNESNRICHARSON handles the rest.

Yes, this is a great suggestion. It should work with your current SNESSHELL. I would also point out that we have the SNESMS, which is the multistage solver from Jameson. You can use this as a residual smooth with FAS to do something similar to what you have, although I do not know what Mach number preconditioning is (Jed probably knows). Thanks, Matt

On May 31, 2023, at 4:16 PM, Kenneth C Hall <[email protected]> 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. 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 <[email protected]> *Date: *Wednesday, May 31, 2023 at 3:48 PM *To: *Kenneth C Hall <[email protected]> *Cc: *[email protected] <[email protected]> *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 <[email protected]> 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

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