[petsc-users] Copy stiffness matrix to jacobian matrix in form jacobian

[Barry Smith]

> On Jun 10, 2021, at 3:29 PM, Kaushik Vijaykumar <kaushikv318 at gmail.com> wrote:
> 
> Hi everyone,
> 
> I am trying to copy the stiffness matrix that I generated in form function to the jacobian matrix jac in form jacobian. The piece of code for that:
> 
> 
   If you have created jac and B in your main program and passed them with SNESSetJacobian() then you should be able to just use

> ierr = MatCopy(ptr->K,jac,SAME_NONZERO_PATTERN);CHKERRQ(ierr);

Generally jac and B are the same matrix so you don't need the second copy. 
> ierr = MatCopy(ptr->K,B,SAME_NONZERO_PATTERN);CHKERRQ(ierr);  

If they are sometimes the same and sometime not you can do

if (jac != B) {ierr = MatCopy(ptr->K,B,SAME_NONZERO_PATTERN);CHKERRQ(ierr);  }


   The following won't work because you are creating new local matrices in your form jacobian that don't exist outside of that routine.

> 
> ierr = MatDuplicate(ptr->K,MAT_COPY_VALUES,&jac);CHKERRQ(ierr);
> ierr = MatDuplicate(ptr->K,MAT_COPY_VALUES,&B);CHKERRQ(ierr);
> 
> ierr = MatCopy(ptr->K,jac,SAME_NONZERO_PATTERN);CHKERRQ(ierr);
> ierr = MatCopy(ptr->K,B,SAME_NONZERO_PATTERN);CHKERRQ(ierr);  
> 
> 
   If the matrix you are creating in form function is the Jacobian then you don't need to use the memory and time to make copies. You can just form the matrix directly into the Jacobian you will use for SNES. You can obtain this with a call 

   SNESGetJacobian(snes,&jac,NULL,NULL,NULL); 

in your form function and just put the matrix values directly into jac. Then your form Jacobian does not need to do anything but return since the jac already contains the Jacobian.

On the off-change you are using the iteration A(x^n) x^{n+1} = b(x^n} and not using Newtons' method (which is why you compute A in form function, then you might be better off using SNESSetPicard() instead of SNESSetFunction(), SNESSetJacobian().

> 
> Thanks
> Kaushik