[petsc-users] Using SNES to solve Non linear elasticity, passing matrix to form function

[Barry Smith]

> ierr = MatSeqAIJGetArray(ptr->K,&Kc);CHKERRQ(ierr); 
> ierr = MatGetLocalSize(ptr->K,&m,&n); CHKERRQ(ierr);
> for(i=0;i<m;i++)
>     { 
>       for(j=0;j<n;j++)
>       {
>         v  = *(Kc + (i*n+j));
>         ierr = MatSetValues(Kloc,1,&i,1,&j,&v,INSERT_VALUES);CHKERRQ(ierr);
>       }
>     } 
>     

I don't understand the purpose of this code fragment. Are you wanting to copy your K into Kloc? What is the purpose of Kloc? With standard usage of SNES one only needs to provide PETSc a global matrix, one does not need to work with "ghosted" matrices at all. 

Also normally one fills the f vector in FormFunction and the Jacobian in FormJacobian and there is no need to access the Jacobians at all in SNES https://www.mcs.anl.gov/petsc/documentation/faq.html#functionjacobian <https://www.mcs.anl.gov/petsc/documentation/faq.html#functionjacobian>  nor do you need to pass into FormFunction in the context the F or Jacobian matrix. 

Maybe if you explained your use case a bit more we could make suggestions on how to accomplish it.

Barry



> On May 19, 2021, at 10:00 AM, Kaushik Vijaykumar <kaushikv318 at gmail.com> wrote:
> 
> Hi everyone,
> 
> We are in the process of integrating nonlinear solution to our FE code through SNES. The first steps that I understood that need to be done is to be able to pass the assembled stiffness matrix K and force vector F to the "formfunction" to calculate the residual and "formjacobian" to calculate the tangent stiffness matrix. To do so, I have defined a struct:
> 
> extern PetscErrorCode FormJacobian(SNES,Vec,Mat,Mat,void*);
> extern PetscErrorCode FormFunction(SNES,Vec,Vec,void*);
> 
> // define a struct type to pass K and f as "user context"
> typedef struct
>     {
>       Mat K;
>       Vec f;
>     } K_and_f;
> 
> In the main program, the struct is declared
> int main()
> {
> K_and_f main_K_and_f; // declare the struct
> 
> // SNES - Populate K and f into the struct
> main_K_and_f.K = K; // K matrix
> main_K_and_f.f = f; // f vector 
> ....
> ....
> }
> In form function
> 
> PetscErrorCode FormFunction(SNES snes, Vec x, Vec F, void* ctx) {
>     PetscErrorCode ierr;
>     PetscReal *ax,*c;
>     PetscReal *aF;
>     PetscScalar *Kc,v; 
>     PetscInt nlocal,m,n,i,j,index[100000];
> 
>     // Create local Vec and Mat
>     Mat Kloc;
>     Vec Floc, Uloc, KUloc, resloc;
>     
>     K_and_f* ptr = (K_and_f*) ctx; // cast the pointer to void into pointer to struct
>     // Get local F array, FLOC
>     ierr = VecGhostGetLocalForm(ptr->f,&Floc);CHKERRQ(ierr);
> 
> 
> I am able to get the f array from the main program using " VecGhostGetLocalForm" and the vec is correct. However, I am having trouble with reading the K matrix in formfunction. 
> 
> The following trial gives me incorrect K values in Kloc:
> ierr = MatSeqAIJGetArray(ptr->K,&Kc);CHKERRQ(ierr); 
> ierr = MatGetLocalSize(ptr->K,&m,&n); CHKERRQ(ierr);
> for(i=0;i<m;i++)
>     { 
>       for(j=0;j<n;j++)
>       {
>         v  = *(Kc + (i*n+j));
>         ierr = MatSetValues(Kloc,1,&i,1,&j,&v,INSERT_VALUES);CHKERRQ(ierr);
>       }
>     } 
>     
>     When I compare K and Kloc, they are not identical 
> 
> 
> Please let me know if there is an equivalent function like VecGhostGetLocalForm() for Matrices. If not, is there a better way to do this.
> 
> Any guidance/help is greatly appreciated.
> 
> Thanks
> Kaushik
> 
> 
> 
> 

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