Speaker
Description
VecGeom is a modern C++ geometry modeling library specifically designed to accelerate particle detector simulation by leveraging Single Instruction Multiple Data (SIMD) vectorization. It offers optimized geometric primitives, developed in collaboration with the USolids project. Since Geant4 10.5, users can replace native Geant4 geometry primitives with VecGeom solids. This feature has already been adopted by several LHC experiments, with CMS simulations reporting a significant 7–13% CPU speed improvement from code improvements alone.
The ALICE O2 (Online-Offline) computing system uses a simulation framework based on ROOT's TGeo for geometry definition and Geant4 for particle transport via the Virtual Monte Carlo (VMC) interface, employing G4Root for geometry navigation. This geometry stack is currently suboptimal for modern hardware and vectorization.
This paper presents the integration and performance evaluation of VecGeom solids within the ALICE O2 simulation framework. We detail the necessary integration effort within the core packages, Geant4 VMC and VGM, to enable the transparent use of VecGeom's accelerated primitives while preserving the existing TGeo-defined geometry and G4Root-based navigation. We report on the results of performance benchmarks using realistic ALICE O2 simulation workflows, comparing the baseline configuration against the new configuration utilizing VecGeom solids. The evaluation focuses on quantifying the resulting reduction in total CPU time for the full simulation. We expect to confirm a measurable
efficiency gain, likely in the few-percent range, on the total simulation time.
Integrating VecGeom into the core VMC packages will allow other VMC-based experiments to benefit from improved simulation speed with minimal further implementation effort.