[petsc-users] (no subject)

Mark Adams mfadams at lbl.gov
Sun Dec 1 06:43:09 CST 2019

FM matrices are slow and meant for debugging mostly (I thought,
although the docs just give this warning if coloring is not available).

I would check the timings from -log_view and verify that the time is spent
in MatFDColoringApply. Running with -info should print the number of colors
(C). The cost of an FD matrix is about C x cost of an exact Jacobian.
Roughly. You could check that.


On Sun, Dec 1, 2019 at 3:58 AM Li Luo <li.luo at kaust.edu.sa> wrote:

> Dear Developers,
> I tried to use the multi-color finite-difference (MC-FD) method for
> constructing the Jacobians. However, I find it is very slow compared to the
> exact Jacobian.
> My implementation of MC-FD Jacobian is posted below, would you please
> check whether I am correct? Anything missed? Thank you!
> ////////// Setup phase:
>           MatStructure flag;
>           ISColoring   iscoloring;
>           ierr = MatGetColoring(Jac,MATCOLORINGSL,&iscoloring);
>           ierr = MatFDColoringCreate(Jac,iscoloring,&this->matfdcoloring);
>           ierr =
> MatFDColoringSetFunction(this->matfdcoloring,(PetscErrorCode
> (*)(void))__libmesh_petsc_snes_residual,(void *)this);
>           ierr = MatFDColoringSetFromOptions(this->matfdcoloring);
>           ierr = ISColoringDestroy(&iscoloring);
> //////////// Apply:
>           ierr = MatZeroEntries(*jac);CHKERRQ(ierr);
>           ierr =
> MatFDColoringApply(*jac,solver->matfdcoloring,x,msflag,snes);
> Best regards,
> Li Luo
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