[petsc-users] Problem with NVIDIA compiler and OpenACC

[Satish Balay]

On Thu, 4 Apr 2024, Frank Bramkamp wrote:

> Dear PETSC Team,
> 
> I found the following problem:
> I compile petsc 3.20.5 with Nvidia compiler 23.7.
> 
> 
> I use a pretty standard configuration, including
> 
> --with-cc=mpicc --with-cxx=mpicxx --with-fc=mpifort COPTFLAGS="-O2 -g" CXXOPTFLAGS="-O2 -g" FOPTFLAGS="-O2 -g"  --with-debugging=0 --with-log=1 --download-fblaslapack --with-cuda=0
> 
> I exclude cuda, since I was not sure if the problem was cuda related. 

Can you try using (to exclude cuda): --with-cudac=0

> 
> 
> The problem is now, if I have s simple fortran program where I link the petsc library, but I actually do not use petsc in that program
> (Just for testing). I want to use OpenACC directives in my program, e.g. !$acc parallel loop .
> The problem is now, as soon I link with the petsc library, the openacc commands do not work anymore.
> It seems that openacc is not initialised and hence it cannot find a GPU.
> 
> The problem seems that you link with -lnvc.
> In “petscvariables” => PETSC_WITH_EXTERNAL_LIB you include “-lnvc”.
> If I take this out, then openacc works. With “-lnvc” something gets messed up.
> 
> The problem is also discussed here:
> https://urldefense.us/v3/__https://forums.developer.nvidia.com/t/failed-cuda-device-detection-when-explicitly-linking-libnvc/203225/1__;!!G_uCfscf7eWS!dlXNyKBzSbximQ13OXxwO506OF71yRM_H5KEnarqXE75D6Vg-ePZr2u6SJ5V3YpRETatvb9pMOUVmpyN0-19SFlbug$  <https://urldefense.us/v3/__https://forums.developer.nvidia.com/t/failed-cuda-device-detection-when-explicitly-linking-libnvc/203225/1__;!!G_uCfscf7eWS!dlXNyKBzSbximQ13OXxwO506OF71yRM_H5KEnarqXE75D6Vg-ePZr2u6SJ5V3YpRETatvb9pMOUVmpyN0-19SFlbug$ >
> 
> My understanding is that libnvc is more a runtime library that does not need to be included by the linker.
> Not sure if there is a specific reason to include libnvc (I am not so familiar what this library does).
> 
> If I take out -lnvc from “petscvariables”, then my program with openacc works as expected. I did not try any more realistic program that includes petsc.
> 
> 
> 
> 2)
> When compiling petsc with cuda support, I also found that in the petsc library the library libnvJitLink.so.12
> Is not found. On my system this library is in $CUDA_ROOT/lib64
> I am not sure where this library is on your system ?! 

Hm - good if you can send configure.log for this. configure attempts '$CC -v' to determine the link libraries to get c/c++/fortran compatibility libraries. But it can grab other libraries that the compilers are using internally here.

To avoid this - you can explicitly list these libraries to configure. For ex: for gcc/g++/gfortran

./configure CC=gcc CXX=g++ FC=gfortran LIBS="-lgfortran -lstdc++"

Satish

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> Thanks a lot, Frank Bramkamp
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