[petsc-dev] TS confusion

[Jed Brown]

Barry Smith <bsmith at mcs.anl.gov> writes:

> If no ijacobian is provided and not imex TS still seems to produce two matrices 
>
>     Mat Arhs = NULL,Brhs = NULL;
>     MatStructure flg2;
>     if (rhsjacobian) {
>       ierr = TSGetRHSMats_Private(ts,&Arhs,&Brhs);CHKERRQ(ierr);
>       ierr = TSComputeRHSJacobian(ts,t,U,&Arhs,&Brhs,&flg2);CHKERRQ(ierr);
>     }
>
> while TSComputeRHSJacobian
>
>   ierr = TSGetIJacobian(ts,&A,&B,NULL,NULL);CHKERRQ(ierr);
>   if (Arhs) {
>     if (!ts->Arhs) {
>       ierr = MatDuplicate(A,MAT_DO_NOT_COPY_VALUES,&ts->Arhs);CHKERRQ(ierr);
>     }
>     *Arhs = ts->Arhs;
>   }

Where is this code?  I don't see it in TSComputeRHSJacobian.

>     } else if (Arhs) {          /* Both IJacobian and RHSJacobian */
>       MatStructure axpy = DIFFERENT_NONZERO_PATTERN;
>       if (!ijacobian) {         /* No IJacobian provided, but we have a separate RHS matrix */
>         ierr = MatZeroEntries(*A);CHKERRQ(ierr);
>         ierr = MatShift(*A,shift);CHKERRQ(ierr);
>         if (*A != *B) {
>           ierr = MatZeroEntries(*B);CHKERRQ(ierr);
>           ierr = MatShift(*B,shift);CHKERRQ(ierr);
>         }
>       }
>       ierr = MatAXPY(*A,-1,Arhs,axpy);CHKERRQ(ierr);

Do we agree that if the user provides both IJacobian and RHSJacobian,
and if we are using non-imex assembly, then we need both matrices and
need the MatAXPY?

> The end result is two sets of matrices. A and Arhs and then MatAXPY costs me
>
> MatAXPY               47 1.0 7.1141e+00 1.0 0.00e+00 0.0 0.0e+00 0.0e+00 0.0e+00 22  0  0  0  0  22  0  0  0  0     0
>
> when it doesn't even need to be done.  The code needs some refactoring but I don't understand enough about its convoluted logic.
> What is going on here. Why does it feel it needs to create the non RHS Jacobian matrix?

I suspect the logic just wasn't optimized.  Can you think of a way to
simplify the logic?
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