![Kokkos](https://avatars2.githubusercontent.com/u/10199860?s=200&v=4)

# Kokkos Spack

This gives instructions for using Spack to install Kokkos and developing packages that depend on Kokkos.

## Getting Started

Make sure you have downloaded [Spack](https://github.com/spack/spack).
The easiest way to configure the Spack environment is:
````bash
> source spack/share/spack/setup-env.sh
````
with other scripts available for other shells.
You can display information about how to install packages with:
````bash
> spack info kokkos
````
This will print all the information about how to install Kokkos with Spack.
For detailed instructions on how to use Spack, see the [User Manual](https://spack.readthedocs.io).

## Setting Up Spack: Avoiding the Package Cascade
By default, Spack doesn't 'see' anything on your system - including things like CMake and CUDA.
This can be limited by adding a `packages.yaml` to your `$HOME/.spack` folder that includes CMake (and CUDA, if applicable).  For example, your `packages.yaml` file could be:
````yaml
packages:
  cuda:
    buildable: false
    externals:
    - prefix: /opt/local/ppc64le-pwr8-nvidia/cuda/10.1.243
      spec: cuda@10.1.243
    - modules:
      - cuda/10.1.243
      spec: cuda@10.1.243
  cmake:
    buildable: false
    externals:
    - prefix: /opt/local/ppc64le/cmake/3.16.8
      spec: cmake@3.16.8
    - modules:
      - cmake/3.16.8
      spec: cmake@3.16.8
````
The `modules` entry is only necessary on systems that require loading Modules (i.e. most DOE systems).
The `buildable` flag is useful to make sure Spack crashes if there is a path error,
rather than having a type-o and Spack rebuilding everything because `cmake` isn't found.
You can verify your environment is set up correctly by running `spack graph` or `spack spec`.
For example:
````bash
> spack graph kokkos +cuda
o  kokkos
|\
o |  cuda
 /
o  cmake
````
Without the existing CUDA and CMake being identified in `packages.yaml`, a (subset!) of the output would be:
````bash
o  kokkos
|\
| o  cmake
| |\
| | | |\
| | | | | |\
| | | | | | | |\
| | | | | | | | | |\
| | | | | | | o | | |  libarchive
| | | | | | | |\ \ \ \
| | | | | | | | | |\ \ \ \
| | | | | | | | | | | | |_|/
| | | | | | | | | | | |/| |
| | | | | | | | | | | | | o  curl
| | |_|_|_|_|_|_|_|_|_|_|/|
| |/| | | |_|_|_|_|_|_|_|/
| | | | |/| | | | | | | |
| | | | o | | | | | | | |  openssl
| |/| | | | | | | | | | |
| | | | | | | | | | o | |  libxml2
| | |_|_|_|_|_|_|_|/| | |
| | | | | | | | | | |\ \ \
| o | | | | | | | | | | | |  zlib
|  / / / / / / / / / / / /
| o | | | | | | | | | | |  xz
|  / / / / / / / / / / /
| o | | | | | | | | | |  rhash
|  / / / / / / / / / /
| | | | o | | | | | |  nettle
| | | | |\ \ \ \ \ \ \
| | | o | | | | | | | |  libuv
| | | | o | | | | | | |  autoconf
| | |_|/| | | | | | | |
| | | | |/ / / / / / /
| o | | | | | | | | |  perl
| o | | | | | | | | |  gdbm
| o | | | | | | | | |  readline
````

## Configuring Kokkos as a Project Dependency
Say you have a project "SuperScience" which needs to use Kokkos.
In your `package.py` file, you would generally include something like:
````python
class SuperScience(CMakePackage):
  ...
  depends_on("kokkos")
````
Often projects want to tweak behavior when using certain features, e.g.
````python
  depends_on("kokkos+cuda", when="+cuda")
````
if your project needs CUDA-specific logic to configure and build.
This illustrates the general principle in Spack of "flowing-up".
A user requests a feature in the final app:
````bash
> spack install superscience+cuda
````
This flows upstream to the Kokkos dependency, causing the `kokkos+cuda` variant to build.
The downstream app (SuperScience) tells the upstream app (Kokkos) how to build.

Because Kokkos is a performance portability library, it somewhat inverts this principle.
Kokkos "flows-down", telling your application how best to configure for performance.
Rather than a downstream app (SuperScience) telling the upstream (Kokkos) what variants to build,
a pre-built Kokkos should be telling the downstream app SuperScience what variants to use.
Kokkos works best when there is an "expert" configuration installed on your system.
Your build should simply request `-DKokkos_ROOT=<BEST_KOKKOS_FOR_MY_SYSTEM>` and configure appropriately based on the Kokkos it finds.

Kokkos has many, many build variants.
Where possible, projects should only depend on a general Kokkos, not specific variants.
We recommend instead adding for each system you build on a Kokkos configuration to your `packages.yaml` file (usually found in `~/.spack` for specific users).
For a Xeon + Volta system, this could look like:
````yaml
 kokkos:
  variants: +cuda +openmp +cuda_lambda +wrapper ^cuda@10.1 cuda_arch=70
  compiler: [gcc@7.2.0]
````
which gives the "best" Kokkos configuration as CUDA+OpenMP optimized for a Volta 70 architecture using CUDA 10.1.
It also enables support for CUDA Lambdas.
The `+wrapper` option tells Kokkos to build with the special `nvcc_wrapper` (more below).
Note here that we use the built-in `cuda_arch` variant of Spack to specify the archicture.
For a Haswell system, we use
````yaml
 kokkos:
  variants: +openmp std=14 target=haswell
  compiler: [intel@18]
````
which uses the built-in microarchitecture variants of Spack.
Consult the Spack documentation for more details of Spack microarchitectures
and CUDA architectures.
Spack does not currently provide an AMD GPU microarchitecture option.
If building for HIP or an AMD GPU, Kokkos provides an `amd_gpu_arch` similar to `cuda_arch`.
````yaml
 kokkos:
  variants: +hip amd_gpu_arch=vega900
````

Without an optimal default in your `packages.yaml` file, it is highly likely that the default Kokkos configuration you get will not be what you want.
For example, CUDA is not enabled by default (there is no easy logic to conditionally activate this for CUDA-enabled systems).
If you don't specify a CUDA build variant in a `packages.yaml` and you build your Kokkos-dependent project:
````bash
> spack install superscience
````
you may end up just getting the default Kokkos (i.e. Serial).
Before running `spack install <package>` we recommend running `spack spec <package>` to confirm your dependency tree is correct.
For example, with Kokkos Kernels:
````bash
kokkos-kernels@3.0%gcc@8.3.0~blas build_type=RelWithDebInfo ~cblas~complex_double~complex_float~cublas~cuda cuda_arch=none ~cusparse~diy+double execspace_cuda=auto execspace_openmp=auto execspace_serial=auto execspace_threads=auto ~float~lapack~lapacke+layoutleft~layoutright memspace_cudaspace=auto memspace_cudauvmspace=auto +memspace_hostspace~mkl+offset_int+offset_size_t~openmp+ordinal_int~ordinal_int64_t~serial~superlu arch=linux-rhel7-skylake_avx512
    ^cmake@3.16.2%gcc@8.3.0~doc+ncurses+openssl+ownlibs~qt arch=linux-rhel7-skylake_avx512
        ^kokkos@3.0%gcc@8.3.0~aggressive_vectorization~amdavx~armv80~armv81~armv8_thunderx~armv8_tx2~bdw~bgq build_type=RelWithDebInfo ~carrizo~compiler_warnings+cuda cuda_arch=none +cuda_lambda~cuda_ldg_intrinsic~cuda_relocatable_device_code~cuda_uvm~debug~debug_bounds_check~debug_dualview_modify_check~deprecated_code~diy~epyc~examples~explicit_instantiation~fiji~gfx901~hpx~hpx_async_dispatch~hsw~hwloc~kaveri~kepler30~kepler32~kepler35~kepler37~knc~knl~maxwell50~maxwell52~maxwell53~memkind~numactl+openmp~pascal60~pascal61~power7~power8~power9+profiling~profiling_load_print~pthread~qthread~rocm~ryzen~serial~skx~snb std=14 ~tests~turing75~vega+volta70~volta72+wrapper~wsm arch=linux-rhel7-skylake_avx512
                ^cuda@10.1%gcc@8.3.0 arch=linux-rhel7-skylake_avx512
                        ^kokkos-nvcc-wrapper@old%gcc@8.3.0 build_type=RelWithDebInfo +mpi arch=linux-rhel7-skylake_avx512
                                    ^openmpi@4.0.2%gcc@8.3.0~cuda+cxx_exceptions fabrics=none ~java~legacylaunchers~memchecker patches=073477a76bba780c67c36e959cd3ee6910743e2735c7e76850ffba6791d498e4 ~pmi schedulers=none ~sqlite3~thread_multiple+vt arch=linux-rhel7-skylake_avx512
````
The output can be very verbose, but we can verify the expected `kokkos`:
````bash
kokkos@3.0%gcc@8.3.0~aggressive_vectorization~amdavx~armv80~armv81~armv8_thunderx~armv8_tx2~bdw~bgq build_type=RelWithDebInfo ~carrizo~compiler_warnings+cuda cuda_arch=none +cuda_lambda~cuda_ldg_intrinsic~cuda_relocatable_device_code~cuda_uvm~debug~debug_bounds_check~debug_dualview_modify_check~deprecated_code~diy~epyc~examples~explicit_instantiation~fiji~gfx901~hpx~hpx_async_dispatch~hsw~hwloc~kaveri~kepler30~kepler32~kepler35~kepler37~knc~knl~maxwell50~maxwell52~maxwell53~memkind~numactl+openmp~pascal60~pascal61~power7~power8~power9+profiling~profiling_load_print~pthread~qthread~rocm~ryzen~serial~skx~snb std=11 ~tests~turing75~vega+volta70~volta72+wrapper~wsm arch=linux-rhel7-skylake_avx512
````
We see that we do have `+volta70` and `+wrapper`, e.g.

### Spack Environments
The encouraged way to use Spack is with Spack environments ([more details here](https://spack-tutorial.readthedocs.io/en/latest/tutorial_environments.html#dealing-with-many-specs-at-once)).
Rather than installing packages one-at-a-time, you add packages to an environment.
After adding all packages, you concretize and install them all.
Using environments, one can explicitly add a desired Kokkos for the environment, e.g.
````bash
> spack add kokkos +cuda +cuda_lambda +volta70
> spack add my_project +my_variant
> ...
> spack install
````
All packages within the environment will build against the CUDA-enabled Kokkos,
even if they only request a default Kokkos.

## NVCC Wrapper
Kokkos is a C++ project, but often builds for the CUDA backend.
This is particularly problematic with CMake. At this point, `nvcc` does not accept all the flags that normally get passed to a C++ compiler.
Kokkos provides `nvcc_wrapper` that identifies correctly as a C++ compiler to CMake and accepts C++ flags, but uses `nvcc` as the underlying compiler.
`nvcc` itself also uses an underlying host compiler, e.g. GCC.

In Spack, the underlying host compiler is specified as below, e.g.:
````bash
> spack install package %gcc@8.0.0
````
This is still valid for Kokkos. To use the special wrapper for CUDA builds, request a desired compiler and simply add the `+wrapper` variant.
````bash
> spack install kokkos +cuda +wrapper %gcc@7.2.0
````
Downstream projects depending on Kokkos need to override their compiler.
Kokkos provides the compiler in a `kokkos_cxx` variable,
which points to either `nvcc_wrapper` when needed or the regular compiler otherwise.
Spack projects already do this to use MPI compiler wrappers.
````python
def cmake_args(self):
  options = []
  ...
  options.append("-DCMAKE_CXX_COMPILER=%s" % self.spec["kokkos"].kokkos_cxx)
  ...
  return options
````
Note: `nvcc_wrapper` works with the MPI compiler wrappers.
If building your project with MPI, do NOT set your compiler to `nvcc_wrapper`.
Instead set your compiler to `mpicxx` and `nvcc_wrapper` will be used under the hood.
````python
def cmake_args(self):
  options = []
  ...
  options.append("-DCMAKE_CXX_COMPILER=%s" % self.spec["mpi"].mpicxx)
  ...
  return options
````
To accomplish this, `nvcc_wrapper` must depend on MPI (even though it uses no MPI).
This has the unfortunate consequence that Kokkos CUDA projects not using MPI will implicitly depend on MPI anyway.
This behavior is necessary for now, but will hopefully be removed later.
When using environments, if MPI is not needed, you can remove the MPI dependency with:
````bash
> spack add kokkos-nvcc-wrapper ~mpi
````

## Developing With Spack

Spack has historically been much more suited to *deployment* of mature packages than active testing or developing.
However, recent features have improved support for development.
Future releases are likely to make this even easier and incorporate Git integration.
The most common commands will do a full build and install of the packages.
If doing development, you may wish to merely set up a build environment.
This allows you to modify the source and re-build.
In this case, you can stop after configuring.
Suppose you have Kokkos checkout in the folder `kokkos-src`:
````bash
> spack dev-build -d kokkos-src -u cmake kokkos@develop +wrapper +openmp
````
This sets up a development environment for you in `kokkos-src` which you can use (Bash example shown):
Note: Always specify `develop` as the version when doing `dev-build`, except in rare cases.
You are usually developing a feature branch that will merge into `develop`,
hence you are making a new `develop` branch.

````bash
> cd kokko-src
> source spack-build-env.txt
> cd spack-build
> make
````
Before sourcing the Spack development environment, you may wish to save your current environment:
````bash
> declare -px > myenv.sh
````
When done with Spack, you can then restore your original environment:
````bash
> source myenv.sh
````
