LAMMPS

LAMMPS molecular dynamics on the SpaceMiT X60 — a whole-application throughput probe with an RVV-vectorized Kokkos build, not a drop-in library A/B like GROMACS FFT or QE BLAS.

Benchmark source: opensolvers/benchmarks/lammps — run-lammps-bench.sh runs the five upstream bench/ inputs × three modes (serial / Kokkos-OpenMP / MPI).

The binary is genuinely vectorized (Tag_RISCV_arch includes v1p0 / zve64d; ~53k vsetvli, ~12k vf* FMAs in liblammps.so.0). Speedups below are parallel scaling (1→8 cores) on that vectorized build — not a vector-vs-scalar A/B.

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Five upstream benches

Same default problem size everywhere: 32000 atoms, 100 steps.

Bench	Potential / physics	Kernel character
lj	Lennard-Jones (Melt)	short-range pair — canonical MD baseline
eam	embedded-atom metal (Cu)	many-body metal — heavier per-pair
chain	bead-spring polymer (FENE)	bonded + short-range — communication-bound
chute	granular chute flow	granular contact, gravity-driven
rhodo	rhodopsin (CHARMM + PPPM)	full biomolecular: bonded + long-range Coulomb

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Performance — Orange Pi RV2 (8× X60 @ ~1.6 GHz)

LAMMPS Loop time and Performance: (katom-step/s). Speedup vs that bench’s serial run.

Benchmark	Mode	Loop (s)	katom-step/s	vs serial
lj	serial	15.48	206.7	1.00×
	kokkos8	2.496	1282	6.20×
	mpi8	3.003	1065	5.15×
eam	serial	41.69	76.8	1.00×
	kokkos8	5.784	553.3	7.21×
	mpi8	7.559	423.4	5.52×
chain	serial	8.950	357.5	1.00×
	kokkos8	2.213	1446	4.04×
	mpi8	1.578	2028	5.67×
chute	serial	6.491	493.0	1.00×
	kokkos8	1.547	2069	4.20×
	mpi8	1.325	2414	4.90×
rhodo	serial	321.79	9.9	1.00×
	kokkos8	59.10	54.1	5.44×
	mpi8	54.17	59.1	5.94×

Kokkos = -k on t 8 -sf kk / OMP_NUM_THREADS=8. MPI = mpirun -np 8.

Two regimes on 8 cores

• Compute-bound pair potentials (lj, eam) favour Kokkos/OpenMP — RVV force kernels + shared-memory threads hit 6.2× / 7.2×, beating MPI. eam is the best threaded scaler (7.21×).
• Communication / bonded-heavy (chain, chute, rhodo) favour MPI — domain decomposition wins when the kernel is lighter and neighbor/ghost exchange dominates (chain 5.67×, rhodo 5.94×).

rhodo is the absolute-cost outlier (~20× slower than lj serial for the same atom count) — full CHARMM + PPPM — but still reaches a usable ~54 s under either parallel back-end.

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Where the RVV work lives

On single-thread Kokkos lj (-k on t 1 -sf kk), LAMMPS timers and gdb stack sampling agree:

Phase (LAMMPS timer)	% of wall	Kernel
Pair	84.4%	PairLJCutKokkos — autovectorized lj/cut force
Neigh	11.8%	neighbor-list build
Comm / Modify / other	~3.8%	ghosts, NVE, output

gdb leaf samples put PairComputeFunctor<PairLJCutKokkos>::operator() first (~46%, plus stripped leaves on the same path). Kokkos is header-only and inherits -march=…v, so the ELF’s vf* FMAs run inside this pair functor — not in FFTW/OpenBLAS (lj has no FFT/BLAS symbols; FFTW only matters for rhodo PPPM).

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Building on RISC-V

No upstream LAMMPS module in dev.eessi.io/riscv yet. This binary used a custom easyconfig (LAMMPS 22Jul2025 update4, Kokkos 4.6.2, GCC 14.3, OpenMPI 5.0.8) against foss-2025b. Five RISC-V-specific fixes were required — details in the benchmarks README (kokkos_arch=EASYBUILD_GENERIC, CVMFS find_* storm, pin binutils, USE_SPGLIB=OFF, drop MDI).

Gotchas

• Speedups are parallel scaling on an RVV build, not RVV-vs-scalar uplift.
• ctest blocks EasyBuild install (3/571 fail for non-compute reasons) — use --skip-test-step or install the built tree manually.
• Detached shells need a login shell for modules; put set -u after sourcing EESSI/Lmod.
• Prefer $EBROOTOPENMPI/bin/mpirun over bare mpirun on PATH.
• Stage data.{chain,chute,rhodo} and Cu_u3.eam next to the in.* inputs.

Reproduce

# once upstreamed:
module load LAMMPS/22Jul2025_update4-foss-2025b-kokkos
# until then, point LMP / MPIRUN at a manual prefix

cp <lammps-src>/bench/in.{lj,eam,chain,chute,rhodo} .
cp <lammps-src>/bench/Cu_u3.eam <lammps-src>/bench/data.{chain,chute,rhodo} .
./run-lammps-bench.sh

Toolchain: LAMMPS 22Jul2025 update4 / foss-2025b (custom easyconfig on EESSI dev.eessi.io/riscv), Kokkos 4.6.2, GCC 14.3.0, OpenMPI 5.0.8, Orange Pi RV2 (SpaceMiT X60).