ppkMHD

What is it ?

ppkMHD stands for Performance Portable Kokkos for Magneto-HydroDynamics (MHD) solvers.

Here a small list of numerical schemes implementations:

• second order MUSCL-HANCOCK scheme for hydrodynamics and MHD
• high-order MOOD (hydrodynamics only)
• high-order Spectral Difference Method schemes: hydrodynamics only

All scheme are available in 2D and 3D using Kokkos+MPI implementation, and support at least the OpenMP and CUDA kokkos backends (other backends may be used, but not tested).

Dependencies

• Kokkos library will either be built by ppkMHD by using cmake option -DPPKMHD_BUILD_KOKKOS=ON, either be detected if you already have it installed.
• cmake with version >= 3.X (3.X is chosen to meet Kokkos own requirement for cmake; i.e. it might increase in the future)

For beginner user, wu suggest that you build ppkMHD with -DPPKMHD_BUILD_KOKKOS=ON so that Kokkos is built together, with the same flags as the main application.

Build

A few example builds, with minimal configuration options.

If you already have Kokkos installed

If you build kokkos yourself, we advise you to always active HWLOC TPLS.

Just make sure that your env variable CMAKE_PREFIX_PATH point to the location where Kokkos where installed. More precisely if Kokkos is installed in KOKKOS_ROOT, you add $KOKKOS_ROOT/lib/cmake to your CMAKE_PREFIX_PATH; this way kokkos will be found automagically by cmake, and the right Kokkos hardware backend will be selected.

# cd into ppkMHD toplevel sources
cmake -S . -B _build/default -DPPKMHD_BUILD_KOKKOS=OFF
cmake --build _build/default -j 6

Build ppkMHD and kokkos without MPI activated for Kokkos-openmp backend

• Create a build directory, configure and make

# cd into ppkMHD toplevel sources
cmake -S . -B _build/openmp -DPPKMHD_BUILD_KOKKOS=ON -DPPKMHD_USE_MPI=OFF -DPPKMHD_BACKEND=OpenMP -DKokkos_ENABLE_HWLOC=ON
cmake --build _build/openmp -j 6

Add variable CXX on the cmake command line to change the compiler (clang++, icpc, pgcc, ....).

Build ppkMHD and kokkos without MPI activated for Kokkos-cuda backend

• Create a build directory, configure and make

# cd into ppkMHD toplevel sources
cmake -S . -B _build/cuda -DPPKMHD_BUILD_KOKKOS=ON -DPPKMHD_USE_MPI=OFF -DPPKMHD_BACKEND=Cuda
# note: GPU architecture will be detected when building on a host with a GPU; if you're building on a host, and running on another machine
# you'll need to tell kokkos what is the target architecture, e.g. add flag like '-DKokkos_ARCH_TURING75=ON' on the cmake configure line
cmake --build _build/cuda -j 6

Build ppkMHD with MPI activated for Kokkos-cuda backend

Please make sure to use a CUDA-aware MPI implementation (OpenMPI or MVAPICH2) built with the proper flags for activating CUDA support.

It may happen that eventhough your MPI implementation is actually cuda-aware, cmake MPI detection (through a call to find_package(MPI) macro) may fails to detect if your MPI implementation is cuda aware. In that case, you can enforce cuda awareness by turning option PPKMHD_USE_MPI_CUDA_AWARE_ENFORCED to ON.

You don't need to use mpi compiler wrapper mpicxx, cmake should be able to correctly populate MPI_CXX_INCLUDE_PATH, MPI_CXX_LIBRARIES which are passed to all final targets by using the alias library MPI::MPI_CXX.

• Create a build directory, configure and make

mkdir build; cd build
cmake -DPPKMHD_USE_MPI=ON -DPPKMHD_BACKEND=Cuda -DKokkos_ARCH_TURING75=ON ..
make -j 4

Example command line to run the application (1 GPU used per MPI task)

mpirun -np 4 ./ppkMHD ./test_implode_2D_mpi.ini

Additionnal features

In order to activate building SDM (Spectral Difference Method) schemes, use cmake option -DPPKMHD_USE_SDM=ON.

The MOOD numerical scheme require some linear algebra (QR decomposition) on the host (not device). This is done using a Blas/Lapack implementation using the C language interface named Lapacke.

Please note that Atlas doesn't provide Lapackage. Currently (March 2017), on Ubuntu 16.04, package libatlas-dev is not compatible with package Lapacke (generate errors at link time). So please either Netlib/Lapack or OpenBLAS implementation.

If you want to enforce the use of OpenBLAS, just use a recent cmake (>=3.6) and add -DBLA_VENDOR on the cmake command line. This will tell the cmake build system (through the call to find_package(BLAS) ) to only look for the OpenBLAS implementation.

On a recent Ubuntu, if atlas is not installed, but OpenBLAS is, you don't need to have a bleeding edge cmake, current cmake will find OpenBLAS.

See also

• ppkMHD Wiki
• Implementing Spectral Difference Methods (SDM) for Compressible Euler flow simulations using performance portable library kokkos
• Implementing Spectral Difference Methods (SDM) for Compressible Euler flow simulations using performance portable library kokkos (astrosim 2018)