[petsc-dev] Which approach to modifying vectors from DMComplex is better?

[Chris Eldred]

This makes sense- thanks!

On Wed, Sep 26, 2012 at 8:32 AM, Matthew Knepley <knepley at gmail.com> wrote:
> On Wed, Sep 26, 2012 at 10:18 AM, Chris Eldred <chris.eldred at gmail.com>
> wrote:
>>
>> I have a vector (created from a PetscSection) that holds a set of
>> values distributed over various points in my mesh. It represents
>> multiple fields with different dofs. I was wondering which approach
>> for modifying (or just getting) the values for a given field from the
>> vector was preferred? The vector is created using
>> DMCreateLocal/GlobalVector.
>>
>> Approach 1 (get the underlying array, find the points using
>> PetscSection, do whatever, restore the array)
>
>
> Yes, this is the way I work. However, it can be simpler, if you are making
> more
> structured changes, to use
>
>   call DMComplexVecGet/SetClosure()
>
>>
>> call VecGetArrayF90( varvector , varptr , ierr)
>> CHKERRQ(ierr)
>> call DMComplexGetDepthStratum(model_mesh, vardepth , pStart, pEnd, ierr)
>> CHKERRQ(ierr)
>
>
> Here if you want the entire section its simpler to use
>
>   call PetscSectionGetChart(varsection, pStart, pEnd, ierr)
>
>>
>> do p=pStart,pEnd-1
>> call PetscSectionGetFieldOffset( varsection, p, varnum , poffset, ierr)
>> CHKERRQ(ierr)
>> call PetscSectionGetFieldDof( varsection, p, varnum , ndof, ierr)
>> CHKERRQ(ierr)
>> do something with the values (modify them, read them, etc.)
>> enddo
>> call VecRestoreArrayF90( varvector , varptr , ierr)
>> CHKERRQ(ierr)
>>
>> Approach 2 (find the points using PetscSection, get the values using
>> VecGetValues, do whatever, set the values using VecSetValues (if
>> needed), call VecAssemblyBegin/End )
>
>
> Definitely do not do this. Its cumbersome and adds nothing.
>
>>
>> call DMComplexGetDepthStratum(model_mesh, vardepth , pStart, pEnd, ierr)
>> CHKERRQ(ierr)
>> do p=pStart,pEnd-1
>> call PetscSectionGetFieldOffset( varsection, p, varnum , poffset, ierr)
>> CHKERRQ(ierr)
>> call PetscSectionGetFieldDof( varsection, p, varnum , ndof, ierr)
>> CHKERRQ(ierr)
>> add points to a list (points are a contiguos list so its easy)- ie for
>> each dof add poffset+dof to the list
>> enddo
>> call VecGetValues( varvector, npts, ptslist, vals, ierr)
>> CHKERRQ(ierr)
>> do something with the values (modify them, read them, etc.)
>> call VecSetValues( varvector, npts, ptslist, vals, INSERT_MODE, ierr)
>> !if need to
>> CHKERRQ(ierr)
>> call VecAssemblyBegin/End (if i set values)
>> CHKERRQ(ierr)
>>
>> Instead of VecAssemblyBegin/End, should I be using the DMGlobaltoLocal
>> / DMLocaltoGlobal routines?
>
>
> With this setup there should never be a need for VecAssembly. Just use
> GlobalToLocal or LocalToGlobal for communication.
>
>>
>> On a related note, should I be calling VecAssemblyBegin/End (or
>> DMGlobaltoLocal, etc.) when I modify the values in the array using
>> Approach 1?
>
>
> No need, since anything you can modify in the array is process local.
>
>>
>> Is there another, third approach I should be using?
>>
>> -Chris
>> --
>> Chris Eldred
>> DOE Computational Science Graduate Fellow
>> Graduate Student, Atmospheric Science, Colorado State University
>> B.S. Applied Computational Physics, Carnegie Mellon University, 2009
>> chris.eldred at gmail.com
>
>
>
>
> --
> What most experimenters take for granted before they begin their experiments
> is infinitely more interesting than any results to which their experiments
> lead.
> -- Norbert Wiener



-- 
Chris Eldred
DOE Computational Science Graduate Fellow
Graduate Student, Atmospheric Science, Colorado State University
B.S. Applied Computational Physics, Carnegie Mellon University, 2009
chris.eldred at gmail.com