Steve,
Just to add on what Bruno said: Explicit time integration typically
requires only a subset of what you need for running an iterative solver
(no decision making like in convergence test of conjugate gradient, no
preconditioner), so running that should be pretty straight-forward.
Best,
Martin
On 05.11.18 21:06, Bruno Turcksin wrote:
Steve
On Monday, October 29, 2018 at 10:43:48 AM UTC-4, Stephen DeWitt wrote:
So far I've read through the "CUDA Support" issue
<https://github.com/dealii/dealii/issues/7037>, the "Roadmap for
inclusion of GPU implementation of matrix free in Deal.II" issue
<https://github.com/dealii/dealii/issues/2351>, the Doxygen
documentation for classes in the CUDAWrappers namespace
<https://www.dealii.org/9.0.0/doxygen/deal.II/group__CUDAWrappers.html>,
andthe manuscript by Karl Ljungkvist
<http://scs.org/wp-content/uploads/2017/06/4_Final_Manuscript-1.pdf>.
Are there any other pages I should be looking at?
No that's pretty much it.
My understanding from these pages is that deal.II has partial
support for using CUDA with matrix free calculations. Currently,
calculations can be done with scalar variables (but not vector
variables) and adaptive meshes.
That's correct with the weird exception that you cannot impose
constraints (and thus have hanging nodes) if dim equals 2 and
fe_degree is even. There is a bug in that case but we don't know why.
A few (somewhat inter-related) questions:
1). Do all of the tools exist to create a GPU version of step-48?
Has anyone done so?
I think so but nobody as tried so I am not 100% sure. Note that we
don't have MPI support yet.
2). What exactly would be involved in creating a GPU version of
step-48? Is it just changing the CPU Vector, MatrixFree, and
FEEvaluation classes to their GPU counterparts, plus packaging
some data (plus local apply functions?) into a
CUDAWrappers::MatrixFree< dim, Number >::Data
<https://www.dealii.org/9.0.0/doxygen/deal.II/structCUDAWrappers_1_1MatrixFree_1_1Data.html>
struct?
Yes, pretty much. You can take a look at the matrix_free_matrix_vector
tests here <https://github.com/dealii/dealii/tree/master/tests/cuda>.
These tests are doing a matrix-vector multiplication using an
Helmholtz operator. As you can see here
<https://github.com/dealii/dealii/blob/master/tests/cuda/matrix_vector_mf.h>
the main difference between the GPU and the CPU code is that the body
loop over the quadrature points needs to be encapsulated in a functor
when using GPU. The tests are based on the CPU tests matrix_vector
here <https://github.com/dealii/dealii/tree/master/tests/matrix_free>.
So you should have a good idea of the modifications required to use
the GPU code.
3). Most of the discussions seemed to revolve around linear
solves. For something like step-48 with explicit updates, will the
current paradigm work well? Or would that require shuttling data
between the GPU and CPU every time step, causing too much
overhead? (I know that in general GPUs can work very well for
explicit codes.)
That should work fine. Most people are interested in using matrix-free
with a solver but I don't see a reason why it would be slow in step
48. The communication between the CPU and the GPU should be limited in
step-48.
Best,
Bruno
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