[petsc-users] Example of SNES using matrix free jacobian

[Daniel Pino Munoz]

Hi Barry,

Thanks for getting back to me!

I tried replacing KSPSetOperators(ksp, J, J); by 
SNESSetJacobian(snes,J,J, MatMFFDComputeJacobian)

and I get the same result = It works in Debug mode but not in Release. I 
also ran valgrind and it did not catch any memory problem.

Any ideas?

PS : You are right regarding the number of iterations of the non 
preconditioned problem. In the previous version of the code that only 
used a KSP, I already had to set -ksp_gmres_restart 100. But thanks for 
the heads up.

Best,

   Daniel

On 29/10/2024 20:01, Barry Smith wrote:
>     Don't call
>
>     KSPSetOperators(ksp, J, J);
>
>
>      instead call
>
>      SNESSetJacobian(snes,J,J, MatMFFDComputeJacobian)
>
>      but I am not sure that would explain the crash.
>
>      BTW: since you are applying no preconditioner if the matrix is ill-conditioned it may take many iterations or not converge. You can try something like -ksp_gmres_restart 100 or similar value to try to improve convergence (default is 30).
>
>      Barry
>
>
>
>
>> On Oct 29, 2024, at 12:37 PM, Daniel Pino Munoz <daniel.pino_munoz at mines-paristech.fr> wrote:
>>
>> Dear all,
>>
>> I have a linear problem that I am currently solving with a KSP matrix-free.
>>
>> I would like to move on to a non linear problem, so figure I could start by solving the same linear problem using SNES. So I am setting the problem as follows:
>>
>> SNESCreate(PETSC_COMM_WORLD, &snes);
>> MatCreateShell(PETSC_COMM_WORLD, n_dofs, n_dofs, PETSC_DETERMINE, PETSC_DETERMINE, &ctx, &J);
>> MatCreateShell(PETSC_COMM_WORLD, n_dofs, n_dofs, PETSC_DETERMINE, PETSC_DETERMINE, &ctx, &B);
>> MatShellSetOperation(J, MATOP_MULT, (void (*)(void))(Multiplication));
>> MatCreateVecs(J, &x_sol, &b);
>> VecDuplicate(x_sol, &r);
>> SNESSetFromOptions(snes);
>> SNESSetFunction(snes, r, &(computeResidual), &ctx);
>> SNESSetUseMatrixFree(snes, PETSC_FALSE, PETSC_TRUE);
>> SNESGetLineSearch(snes, &linesearch);
>> SNESGetKSP(snes, &ksp);
>> KSPSetOperators(ksp, J, J);
>> KSPSetInitialGuessNonzero(ksp, PETSC_TRUE);
>>
>> I tested it with a small problem (compiled in debug) and it works.
>>
>> When I compiled it in Release, it crashes with a segfault. I tried running the Debug version through valgrind, but even for a small problem, it is too slow. So I was wondering if you guys could see any rocky mistake on the lines I used above?
>>
>> Otherwise, is there any example that uses a SNES combined with a matrix free KSP operator?
>>
>> Thank you,
>>
>>    Daniel
>>