Optimizing the throughput of the ATLAS Geant4 detector simulation

For Run 3 at the Large Hadron Collider the Monte Carlo Detector Simulation performed with Geant4 (Full Simulation) within the ATLAS experiment has undergone significant improvements to enhance its computational performance and overall efficiency. Here we provide an overview of the physics and technical optimizations implemented in the ATLAS Full Simulation for Run 3. Notable physics developments include use of Geant4’s range cuts and G4GammaGeneralProcess for gammas, and variance reduction techniques such as neutron/photon Russian Roulette, and Woodcock Tracking in the Electromagnetic Endcap Calorimeter (EMEC). Technical developments include optimizations of the geometric representation of the particularly complex EMEC detector to assist navigation, tailored switch-off of the magnetic field, and use of static linking and Link-Time Optimization within a single shared library. These enhancements collectively resulted in a major achievement, increasing the throughput of the simulation by a factor of 2 compared to the baseline simulation configuration used in Run 2. Further optimizations are under development, and we provide an overview of this continuing programme covering both near and longer term features.