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

[Barry Smith]

   Hmm, cut and paste the output when it crashes.

    Also run with -malloc_debug and see what happens


    Barry


> On Oct 29, 2024, at 3:13 PM, Daniel Pino Munoz <daniel.pino_munoz at mines-paristech.fr> wrote:
> 
> 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
>>>