[petsc-users] Error while running with -pc_type gamg on NVIDIA GPU

[Barry Smith]

> On Sep 10, 2023, at 5:54 AM, Maruthi NH <maruthinh at gmail.com> wrote:
> 
> Hi all,
> 
> I am trying to accelerate the linear solver with PETSc GPU backend. For testing I have a simple 1D heat diffusion solver, here are some observations. 
> 1. If I use -pc_type gamg it throws the following error 
>  ** On entry to cusparseCreateCsr() parameter number 5 (csrRowOffsets) had an illegal value: NULL pointer
> 
> [0]PETSC ERROR: --------------------- Error Message --------------------------------------------------------------
> [0]PETSC ERROR: GPU error
> [0]PETSC ERROR: cuSPARSE errorcode 3 (CUSPARSE_STATUS_INVALID_VALUE) : invalid value
> [0]PETSC ERROR: See https://petsc.org/release/faq/ for trouble shooting.
> [0]PETSC ERROR: Petsc Development GIT revision: v3.19.4-959-g92f1e92e88  GIT Date: 2023-08-13 19:43:04 +0000
> 
   Can you share the code that triggers this?

> 2. Default pc ilu takes about 1.2 seconds on a single CPU and it takes about 105.9 seconds on a GPU. Similar observations with pc_type asm 
> I have NVIDIA RTX A2000 8GB Laptop GPU  

  This is expected. The triangular solves sequentialize on the GPU so naturally are extremely slow since they cannot take advantage of the massive parallelism of the GPU.

> 
> 3. What I could be missing? Also, are there any general guidelines for better GPU performance using PETSc? 
> 
> Regards,
> Maruthi

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