Add section to let CMake manage the Kokkos dependency

[pzehner]

Warning: this currently destroys the page layout.

[tpadioleau]

Two minor questions:

• Do you think it is a good practice to explicitly set CONFIG ?
• If we allow find_package to fail, should we put it in QUIET ?

I am still skeptical with this mixed approach though. If Kokkos is found, the options are already set. On the other hand, if it is not found, the user is responsible to select the options. We use a similar approach in DDC, with submodules instead, and I don't know if we should advertise about it.

[pzehner]

I'm not sure about CONFIG, to be honest. I started using it blindly when I saw it somewhere. So I tried the different options:

• find_package(Kokkos)
  • Kokkos installed:

    -- Enabled Kokkos devices: SERIAL;CUDA
    -- kokkos_launch_compiler is enabled globally. C++ compiler commands with -DKOKKOS_DEPENDENCE will be redirected to the appropriate compiler for Kokkos
    

  • Kokkos not installed:

    CMake Warning at CMakeLists.txt:8 (find_package):
    By not providing "FindKokkos.cmake" in CMAKE_MODULE_PATH this project has
    asked CMake to find a package configuration file provided by "Kokkos", but
    CMake did not find one.
    
    Could not find a package configuration file provided by "Kokkos" (requested
    version 4.3.1) with any of the following names:
    
      KokkosConfig.cmake
      kokkos-config.cmake
    
    Add the installation prefix of "Kokkos" to CMAKE_PREFIX_PATH or set
    "Kokkos_DIR" to a directory containing one of the above files.  If "Kokkos"
    provides a separate development package or SDK, be sure it has been
    installed.
    -- Setting default Kokkos CXX standard to 17
    -- Kokkos version: 4.3.1
    <snip>
    

    The bunch of warnings is not helpful;
• find_package(Kokkos CONFIG)
  • Kokkos installed:

    -- Enabled Kokkos devices: SERIAL;CUDA
    -- kokkos_launch_compiler is enabled globally. C++ compiler commands with -DKOKKOS_DEPENDENCE will be redirected to the appropriate compiler for Kokkos
    

  • Kokkos not installed:

    -- Could NOT find Kokkos (missing: Kokkos_DIR)
    -- Setting default Kokkos CXX standard to 17
    -- Kokkos version: 4.3.1
    <snip>
    

    I saw tutorial attendants being confused about the first line;
• find_package(Kokkos QUIET)
  • Kokkos installed:

    -- Enabled Kokkos devices: SERIAL;CUDA
    -- kokkos_launch_compiler is enabled globally. C++ compiler commands with -DKOKKOS_DEPENDENCE will be redirected to the appropriate compiler for Kokkos
    

  • Kokkos not installed:

    -- Setting default Kokkos CXX standard to 17
    -- Kokkos version: 4.3.1
    <snip>
    

So, QUIET seems to be more appropriate.

I see your point. The subdirectory (aka submodule) approach already in the cheat sheet does not provide the "find package otherwise use a subdirectory" approach.

Initially, I wanted to give a flexible solution. Note that this is what Daniel advised Keigo Nitadori (during the last Kokkos Café) to do when using CMake. But this is not the most flexible solution, which would be too long for this cheat sheet.

Eventually, I think I can limit the section to just using FetchContent, but I feel something would be missing. I would add a specific "Mixed approach" section which also warns about what you just said.

[pzehner]

Eventually, the Kokkos documentation proposes the 3 + 1 approaches altogether: https://kokkos.org/kokkos-core-wiki/get-started/integrating-kokkos-into-your-cmake-project.html

[pzehner]

Well, I used the approach of the documentation.

[tpadioleau]

In the end I don't think we should go into the details of how to use FetchContent etc. This is largely opinionated. I would rather describe the package name, the targets and the components available. Similar to the Targets tab in https://conan.io/center/recipes/ginkgo. We can also mention that Kokkos supports being added as a subdirectory, this implies for example that Kokkos::kokkos also exists which is not obvious.

[pzehner]

I don't know, I think it gives practical directions about "how to include Kokkos as a dependency to my project," which is unfortunately not trivial at this time.

You cannot simply do:

conan install kokkos