Hello all,
Following up from a previous post 
<https://groups.google.com/forum/#!searchin/dealii/CUDA%7Csort:date/dealii/Fby9uclgSbU/luZlePmgCgAJ>
 
about where the GPU capabilities are, I've been going through the CUDA 
matrix-free matrix-vector product tests as Bruno suggested. For the most 
part I've been able to make sense of it and have found parallels between 
what is going on there versus step-48.

The one area that I don't really understand for 
matrix_free_matrix_vector_01.cu is how information flows into the 
HelmholtzOperator 
<https://github.com/dealii/dealii/blob/2f12a074e4645b4cff01335b773a7f88b83051dc/tests/cuda/matrix_vector_mf.h>::operator()
 
method. Inside the vmult method in MatrixFreeTest a HelmholtzOperator 
object is created and then it is passed into CUDAWrapper::MatrixFree 
cell_loop method along with two CUDAVectors. However, when I look at the 
HelmholtzOperator::operator() definition it 
takes CUDAWrappers::MatrixFree::Data * and CUDAWrappers::SharedData * types 
as inputs. Do the CUDAWrapper::MatrixFree internals take care of building 
objects with those types in a way that I can just ignore? I thought from 
the previous discussion that the user had to package what they needed into 
CUDAWrappers::MatrixFree::Data * and CUDAWrappers::SharedData * objects but 
maybe I misunderstood it. Obviously it would be great if this happened in 
the background.
 
On a related topic, I can see that the signature for 
HelmholtzOperator::operator() is notably different than 
SineGordonOperation:: local_apply in step-48. Where in the documentation 
should I be looking for the requirements for the functor passed into 
CUDAWrapper::MatrixFree::cell_loop? I'm reaching a dead end at the 
documentation entry for CUDAWrapper::MatrixFree::cell_loop. I can just copy 
the form from HelmholtzOperator::operator(), but I'm worried that I could 
get myself into trouble if I don't quite understand it.

Thanks in advance,
Steve DeWitt

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