[petsc-users] kokkos gpu/cpu copy

[Junchao Zhang]

Hi, Steven,
   Thanks for the test, which helped me easily find the petsc bug. I have a
fix at https://urldefense.us/v3/__https://gitlab.com/petsc/petsc/-/merge_requests/8272__;!!G_uCfscf7eWS!dEECoiAgY3gt4OtiEoIEoGuacFHbL1whaXexphGauRP-CI_sNm-iHlr3Aqh1kQRUkVGWcgNgtRJsaiMGB0Mt_63EjYXr$ .
   VecKokkos does not use offloadmask for its gpu/cpu sync state, while
VecCUDA/VecHIP do.  My expectation is users should not use
VecGetOffloadMask(), because it is too low level. We have bad API design
here.

  Thank you!
--Junchao Zhang


On Wed, Apr 2, 2025 at 7:11 PM Steven Dargaville <
dargaville.steven at gmail.com> wrote:

> Hi
>
> I have some code that does a solve with a PCMAT preconditioner. The mat
> used is a shell and inside the shell MatMult it calls VecPointwiseDivide
> with a vector "diag" that is the diagonal of a matrix assigned outside the
> shell.
>
> If I use mat/vec type of cuda, this occurs without any gpu/cpu copies as I
> would expect. If however I use mat/vec type kokkos, at every iteration of
> the solve there is a gpu/cpu copy that occurs. It seems this is triggered
> by the offloadmask in the vector "diag", as it stays as 1 and hence a copy
> occurs in VecPointwiseDivide.
>
> I would have expected the offload mask to be 256 (kokkos) after the first
> iteration, as the offload mask of "diag" changes to 3 when using cuda after
> the first iteration.
>
> Is this the expected behaviour with Kokkos, or is there something I need
> to do to trigger that "diag" has its values on the gpu to prevent copies? I
> have example c++ code that demonstrates this below. You can see the
> difference when run with petsc 3.23.0 and either "-log_view -mat_type
> aijcusparse -vec_type cuda" or "-log_view -mat_type aijkokkos -vec_type
> kokkos".
>
> Thanks for your help
> Steven
>
> Example c++ code:
>
> // ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> static char help[] = "Tests Kokkos for SHELL matrices\n\n";
>
> #include <iostream>
> #include <petscksp.h>
> #include <petsclog.h>
>
> typedef struct _n_User *User;
> struct _n_User {
>   Mat A;
>   Vec diag;
> };
>
> static PetscErrorCode MatMult_User(Mat A, Vec X, Vec Y)
> {
>   User user;
>
>   PetscFunctionBegin;
>   PetscCall(MatShellGetContext(A, &user));
>
>   // Print the offload mask inside the matmult
>   PetscOffloadMask offloadmask;
>   PetscCall(VecGetOffloadMask(X, &offloadmask));
>   std::cout << "offload inside X " << offloadmask << std::endl;
>   PetscCall(VecGetOffloadMask(Y, &offloadmask));
>   std::cout << "offload inside Y " << offloadmask << std::endl;
>   PetscCall(VecGetOffloadMask(user->diag, &offloadmask));
>   std::cout << "offload inside diag " << offloadmask << std::endl;
>
>   PetscCall(VecPointwiseDivide(Y, X, user->diag));
>   PetscFunctionReturn(PETSC_SUCCESS);
> }
>
> int main(int argc, char **args)
> {
>   const PetscScalar xvals[] = {11, 13}, yvals[] = {17, 19};
>   const PetscInt    inds[]  = {0, 1};
>   PetscScalar       avals[] = {2, 3, 5, 7};
>   Mat               S1, A;
>   Vec               X, Y, diag;
>   KSP               ksp;
>   PC                pc;
>   User              user;
>   PetscLogStage  stage1, gpu_copy;
>
>   PetscFunctionBeginUser;
>   PetscCall(PetscInitialize(&argc, &args, NULL, help));
>
>   // Build a matrix and vectors
>   PetscCall(MatCreateFromOptions(PETSC_COMM_WORLD, NULL, 1, 2, 2, 2, 2,
> &A));
>   PetscCall(MatSetUp(A));
>   PetscCall(MatSetValues(A, 2, inds, 2, inds, avals, INSERT_VALUES));
>   PetscCall(MatAssemblyBegin(A, MAT_FINAL_ASSEMBLY));
>   PetscCall(MatAssemblyEnd(A, MAT_FINAL_ASSEMBLY));
>   PetscCall(MatCreateVecs(A, NULL, &X));
>   PetscCall(VecCreateFromOptions(PETSC_COMM_WORLD, NULL, 1, 2, 2, &X));
>   PetscCall(VecSetValues(X, 2, inds, xvals, INSERT_VALUES));
>   PetscCall(VecDuplicate(X, &Y));
>   PetscCall(VecDuplicate(X, &diag));
>   PetscCall(VecSetValues(Y, 2, inds, yvals, INSERT_VALUES));
>   PetscCall(VecAssemblyBegin(Y));
>   PetscCall(VecAssemblyEnd(Y));
>
>   // Create a shell matrix
>   PetscCall(MatGetDiagonal(A, diag));
>   PetscCall(PetscNew(&user));
>   user->A = A;
>   user->diag = diag;
>   PetscCall(MatCreateShell(PETSC_COMM_WORLD, 2, 2, 2, 2, user, &S1));
>   PetscCall(MatSetUp(S1));
>   PetscCall(MatShellSetOperation(S1, MATOP_MULT, (void
> (*)(void))MatMult_User));
>   PetscCall(MatAssemblyBegin(S1, MAT_FINAL_ASSEMBLY));
>   PetscCall(MatAssemblyEnd(S1, MAT_FINAL_ASSEMBLY));
>
>   // Do a solve
>   PetscCall(KSPCreate(PETSC_COMM_WORLD,&ksp));
>   // Give the ksp a pcmat as the preconditioner and the mat is the shell
>   PetscCall(KSPSetOperators(ksp,A, S1));
>   PetscCall(KSPSetType(ksp, KSPRICHARDSON));
>   PetscCall(KSPSetFromOptions(ksp));
>   PetscCall(KSPGetPC(ksp, &pc));
>   PetscCall(PCSetType(pc, PCMAT));
>   PetscCall(KSPSetUp(ksp));
>
>   // Print the offload mask before our solve
>   PetscOffloadMask offloadmask;
>   PetscCall(VecGetOffloadMask(X, &offloadmask));
>   std::cout << "offload X " << offloadmask << std::endl;
>   PetscCall(VecGetOffloadMask(Y, &offloadmask));
>   std::cout << "offload Y " << offloadmask << std::endl;
>   PetscCall(VecGetOffloadMask(user->diag, &offloadmask));
>   std::cout << "offload diag " << offloadmask << std::endl;
>
>   // Trigger any gpu copies in the first solve
>   PetscCall(PetscLogStageRegister("gpu_copy",&gpu_copy));
>   PetscCall(PetscLogStagePush(gpu_copy));
>   PetscCall(KSPSolve(ksp, X, Y));
>   PetscCall(PetscLogStagePop());
>
>   // There should be no copies in this solve
>   PetscCall(PetscLogStageRegister("no copy",&stage1));
>   PetscCall(PetscLogStagePush(stage1));
>   PetscCall(KSPSolve(ksp, X, Y));
>   PetscCall(PetscLogStagePop());
>
>   PetscCall(MatDestroy(&S1));
>   PetscCall(VecDestroy(&X));
>   PetscCall(VecDestroy(&Y));
>   PetscCall(PetscFinalize());
>   return 0;
> }
>
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