Installation#

Dependencies#

CMake 3.24+.
SYCL 2020 with USM support: Tested with hipsycl 0.9.4 or Intel DPCPP 2024.1.0.
MPI 3.0: Tested with MPICH 4.0 or IntelMPI 2021.6. See known issues for Ubuntu 22.03 mpich.
HDF5 (parallel): (optional see CMake variable NESO_PARTICLES_ENABLE_HDF5) If particle trajectories are required - will execute without.

Using with CMake#

We provide a CMake interface for projects to access NESO-Particles as a package or as a submodule. To use NESO-Particles in package form projects should involve CMake implementation like:

find_package(NESO-Particles REQUIRED)
...
target_link_libraries(${EXECUTABLE} PUBLIC NESO-Particles::NESO-Particles)
...
add_sycl_to_target(TARGET ${EXECUTABLE} SOURCES ${EXECUTABLE_SOURCE})

To use NESO-Particles from a submodule within your project replace the find_package call as follows:

# Enable/disable options for NESO-Particles, e.g. HDF5
#option(NESO_PARTICLES_ENABLE_HDF5 OFF)
add_subdirectory(neso-particles)

where neso-particles is the directory within your project that contains NESO-Particles.

Note that we use the existence of the add_sycl_to_target function when find_package(NESO-Particles REQUIRED) is called to determine if a SYCL implementation has been found already. If a SYCL implementation is not already configured NESO-Particles will attempt to find one. This detection is relevant at build time, i.e. when the implementation using NESO-Particles is configured. These downstream implementations should pass CMake variables or compilers at configure time like:

# Intel DPCPP
cmake -DCMAKE_CXX_COMPILER=dpcpp .
# Hipsycl cpu via omp
cmake -DHIPSYCL_TARGETS=omp .
# Hipsycl cuda using nvcxx
cmake -DHIPSYCL_TARGETS=cuda-nvcxx .

When a SYCL implementation is passed to NESO-Particles itself then it is used to build the tests. Please read the section Device Aware MPI for more information relating to device aware MPI.

CXX Standard#

Although we explicitly set C++17 as the required C++ version on the NESO-Particles interface target, CMake may not pass this requirement down onto targets using NESO-Particles. Downstream projects may need to explicitly set the required C++ standard to C++17.

Installing#

To build NESO-Particles with tests:

git clone https://github.com/ExCALIBUR-NEPTUNE/NESO-Particles.git
cd NESO-Particles
mkdir build
cd  build
# Choose a SYCL implementation to build tests with
cmake -DHIPSYCL_TARGETS=omp.library-only -DCMAKE_INSTALL_PREFIX=<install location> ..
make
make install

The tests can then be run with:

# Intel DPCPP
SYCL_DEVICE_FILTER=host mpirun -n <nproc> bin/testNESOParticles

# hipsycl omp
OMP_NUM_THREADS=<nthreads> mpirun -n <nproc> bin/testNESOParticles

Note that typically the OpenMP default is to run a number of threads equal to the number of logical CPU cores (this is implementation defined). Hence with OpenMP SYCL implementations the number of threads should, almost always, be set when using MPI. If the number of threads is not set then each MPI rank will launch a thread per CPU core resulting in over-subscription of cores and slowdown. Intel SYCL implementations currently choose the number of workers (threads) based on the process affinity of each MPI rank which can be configured through environment variables and runtime options passed to the MPI launcher.

Notes:

Searching for and building against HDF5 can be disabled by passing -DNESO_PARTICLES_ENABLE_HDF5=OFF to CMake.
If an installation is required without tests built or SYCL configured then CMake can be called with -DNESO_PARTICLES_ENABLE_TESTS=OFF -DNESO_PARTICLES_ENABLE_FIND_SYCL=OFF. Tests should always be ran before any notion of trust is formed of outputs.

Device Aware MPI#

By default device aware MPI, i.e. passing device pointers to MPI, is disabled for compatibility. Device aware MPI functionality can be enabled by default by passing -DNESO_PARTICLES_DEVICE_AWARE_MPI=ON to cmake. At runtime device aware MPI can be enabled by setting the environment variable NESO_PARTICLES_DEVICE_AWARE_MPI to ON alternatively at runtime this functionality can be disabled by setting the same environment variable to OFF. The environment variable takes precedence over the cmake variable and in particular it is not required to enable device aware MPI at cmake time to later enable this functionality by using the environment variable. Note that when compiling for a CPU device it should always be safe to enable device aware MPI.

Known Issues and Workarounds#

Ubuntu 22.04 MPICH#

The build of MPICH found in the Ubuntu 22.04 APT repositories includes additional compiler flags, e.g. -flto=auto, relating to link time optimisation which break compilation workflows that involve compilers different to the system GCC. The symptoms of this issue are errors at compile time due to the unknown flags, e.g.

nvc++-Error-Unknown switch: -flto=auto
nvc++-Error-Unknown switch: -ffat-lto-objects

Or issues at link time due to the intermediate objects having types incompatible with the linker, e.g.

CMakeFiles/test_buffers.dir/main.cpp.o: file not recognized: file format not recognized

Two possible solutions are as follows

Build a separate installation of MPICH (ideally with the same compiler as used by the SYCL implementation).
Clear the offending variables from the CMake cache by first running CMake as normal then rerunning CMake as cmake -DMPI_CXX_COMPILE_OPTIONS="" -DMPI_CXX_COMPILE_OPTIONS="" ...

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