[petsc-dev] using typedef for each different memory space the variable lives in

Mark Adams mfadams at lbl.gov
Sun Dec 13 21:23:10 CST 2020

On Sun, Dec 13, 2020 at 4:34 PM Barry Smith <bsmith at petsc.dev> wrote:

> On Dec 13, 2020, at 8:39 AM, Mark Adams <mfadams at lbl.gov> wrote:
> So how do I declare
> struct myMat {
>  PetscScalar *a;
>  PetscInt        *i, *j, nrows;
> }
> so that I can use, or give me an alternative idiom,
> struct myMat h_mat, *d_mat;
> ....
> cudaMemcpy(          d_mat,   &h_mat,  sizeof(myMat),
> cudaMemcpyHostToDevice))
>    This will copy the pointers (not the entries in the arrays) which means
> in both d_mat and h_mat a and i,j refer to Kokkos (or CUDA etc) memory so

Yes. Note, h_mat is, in my usage, a temporary container. It is a mix of
host and device memory and is just there to stuff device pointers and raw
(meta) data, to be copied whole onto the device.

> struct myMat {
>  PetscKokkosScalar *a;
>  PetscKokkosInt        *i, *j;
>    PetscInt                    nrows;
> }
>    the nrows is annoying because in d_mat it refers to device memory while
> in h_mat it refers to host memory so we cannot label it clearly.

Kokkos wraps all this ugliness, but in raw Cuda I do it this manual way.
It's low level and you have to keep track in your head (names help) what is
going on. But this mixed h_mat is just around in a local scope. A better
name for it might be h_temp_mat.

I think your PetscKokkosScalar is a Scalar in the
default execution's memory space.

>    Sorry, by code reader I meant someone reading or maintaining the code.
>    To me the above struct is clearer, I know immediately that *a and *i,
> *j point to Kokkos space. While labeled as PetscScalar *a I have to figure
> out somehow by examining other code where it points (this is a waste of
> time I think).

I think Junchao is doing something like this in aijkokkosimpl.hpp, as I
said. He and I use variable names to make it clear where they are (eg,
d_mat for a Mat on the device). Junchoa had to use Kokkos data type to talk
to Kokkos, but we both use naming conventions.

Here is a whole method that does this idiom. I have a Mat with a "data"
pointer (this is a bad name but this was a void* at first) to its device
"self".  I'm not sure how your suggestion would impact this.

PETSC_EXTERN PetscErrorCode LandauCUDACreateMatMaps(P4estVertexMaps *maps,
pointInterpolationP4est (*points)[LANDAU_MAX_Q_FACE], PetscInt Nf, PetscInt
  P4estVertexMaps h_maps;
  h_maps.num_elements =maps->num_elements;
  h_maps.num_face = maps->num_face;
  h_maps.num_reduced = maps->num_reduced;
  h_maps.deviceType = maps->deviceType;
  h_maps.Nf = Nf;
  h_maps.Nq = Nq;
  CUDA_SAFE_CALL(cudaMalloc((void **)&h_maps.c_maps,
maps->num_reduced  * sizeof *points));
  CUDA_SAFE_CALL(cudaMemcpy(          h_maps.c_maps, maps->c_maps,
maps->num_reduced  * sizeof *points, cudaMemcpyHostToDevice));
  CUDA_SAFE_CALL(cudaMalloc((void **)&h_maps.gIdx,
maps->num_elements * sizeof *maps->gIdx));
  CUDA_SAFE_CALL(cudaMemcpy(          h_maps.gIdx, maps->gIdx,
maps->num_elements * sizeof *maps->gIdx, cudaMemcpyHostToDevice));
  CUDA_SAFE_CALL(cudaMalloc((void **)&maps->data, sizeof(P4estVertexMaps)));
  CUDA_SAFE_CALL(cudaMemcpy(          maps->data,   &h_maps,
sizeof(P4estVertexMaps), cudaMemcpyHostToDevice));

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