[petsc-users] Accessing Global Vectors

[Matthew Knepley]

On Mon, May 19, 2014 at 8:50 PM, Andrew Cramer
<andrewdalecramer at gmail.com>wrote:

> Hi All,
>
> I'm new to PETSc and would like to use it as my linear elasticity solver
> within a structural optimisation program. Originally I was using GP-GPUs
> and CUDA for my solver but I would like to shift to using PETSc to leverage
> it's breadth of trustworthy solvers. We have some SMP servers and a couple
> compute clusters (one with GPUs, one without). I've been digging through
> the docs and I'd like some feedback on my plan and perhaps some pointers if
> at all possible.
>
> The plan is to keep the 6000 lines or so of current code and try as much
> as possible to use PETSc as a 'drop-in'. This would require giving one
> field (array) of densities and receiving a 3d field (array) of
> displacements back. Providing the density field would be easy with the
> usual array construction functions on one node/process but pulling the
> displacements back to the 'controlling' node would be difficult.
>
> I understand that this goes against the ethos of PETSc which is
> distributed all the way. My code is highly modular with differing objective
> functions and optimisers (some of which are written by other research
> groups) that I drop in and pull out. I don't want to throw all that away. I
> would need to relearn object oriented programming within PETSc (currently I
> use c++) and rewrite my entire code base. In terms of performance the
> optimisers typically rely heavily on tight loops of reductions once the
> solve is completed so I'm not sure that the speed-up would be too great
> rewriting them as distributed anyway.
>
> Sorry for the long winded post but I'm just not sure how to move forward,
> I'm sick of implementing every solver I want to try in CUDA especially
> knowing that people have done it better than I can in PETSc. But it's a
> framework that I don't know how to interface with, all the examples seem to
> have the solve as the main thing rather than one part of a broader program.
>

1) PETSc can do a good job on linear elasticity. GAMG is particularly
effective, and we have an example:


http://www.mcs.anl.gov/petsc/petsc-dev/src/ksp/ksp/examples/tutorials/ex56.c.html

2) You can use this function to go back and forth from 1 process


http://www.mcs.anl.gov/petsc/petsc-dev/docs/manualpages/Vec/VecScatterCreateToZero.html

3) The expense of pushing all that data to nodes can large. You might be
better off just using GAMG on 1 process, which is how I would start.

    Matt


> Andrew Cramer
> University of Queensland, Australia
> PhD Candidate
>



-- 
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
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