[petsc-users] Vec sizing using DMDA

[Matthew Knepley]

On Thu, Apr 23, 2020 at 12:12 PM Antoine Côté <Antoine.Cote3 at usherbrooke.ca>
wrote:

> Hi,
>
> I'm using a C++/PETSc program to do Topological Optimization. A finite
> element analysis is solved at every iteration of the optimization.
> Displacements U are obtained using KSP solver. U is a Vec created using a
> 3D DMDA with 3 DOF (ux, uy, uz). Boundary conditions are stored in Vec N,
> and forces in Vec RHS. They also have 3 DOF, as they are created using
> VecDuplicate on U.
>
> My problem : I have multiple load cases (i.e. different sets of boundary
> conditions (b.c.) and forces). Displacements U are solved for each load
> case. I need to extract rapidly the b.c. and forces for each load case
> before solving.
>
> One way would be to change the DOF of the DMDA (e.g. for 8 load cases, we
> could use 3*8=24 DOF). Problem is, prior solving, we would need to loop on
> nodes to extract the b.c. and forces, for every node, for every load case
> and for every iteration of the optimization. This is a waste of time, as
> b.c. and forces are constant for a given load case.
>
> A better way would be to assemble b.c. and forces for every load case
> once, and read them afterwards as needed. This is currently done using a
> VecDuplicate on U to create multiple vectors N and RHS (N_0, N_1, RHS_0,
> RHS_1, etc.). Those vectors are hard coded, and can only solve a set number
> of load cases.
>
> I'm looking for a way to allocate dynamically the number of N and RHS
> vectors. What I would like :
> Given nlc, the number of load cases and nn, the number of nodes in the
> DMDA. Create matrices N and RHS of size (DOF*nn lines, nlc columns). While
> optimizing : for every load case, use N[all lines, current load case
> column] and RHS[all lines, current load case column], solve with KSP,
> obtain displacement U[all lines, current load case].
>
> Would that be possible?
>

Why wouldn't you just allocate an array of Vecs, since you only use one at
a time?

  Thanks,

    Matt


> Best regards,
>
> Antoine Côté
>
>

-- 
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

https://www.cse.buffalo.edu/~knepley/ <http://www.cse.buffalo.edu/~knepley/>
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