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ATLAS Note
Report number	ATL-SOFT-PROC-2020-030
Title	Multithreaded simulation for ATLAS: challenges and validation strategy
Author(s)	Bandieramonte, Marilena (University of Pittsburgh) ; Chapman, John Derek (Cavendish Laboratory, University of Cambridge) ; Gray, Heather (Lawrence Berkeley National Laboratory and University of California, Berkeley) ; Muskinja, Miha (Lawrence Berkeley National Laboratory and University of California, Berkeley) ; Chiu, Yu Him Justin (University of Victoria)
Corporate Author(s)	The ATLAS collaboration
Collaboration	ATLAS Collaboration
Publication	2020
Imprint	14 Mar 2020
Number of pages	10
In:	EPJ Web Conf. 245 (2020) 02001
In:	24th International Conference on Computing in High Energy and Nuclear Physics, Adelaide, Australia, 4 - 8 Nov 2019, pp.02001
DOI	10.1051/epjconf/202024502001
Subject category	Particle Physics - Experiment
Accelerator/Facility, Experiment	CERN LHC ; ATLAS
Free keywords	AthenaMT ; Geant4MT ; Multithreaded simulation ; Multithreaded validation
Abstract	Estimations of the CPU resources that will be needed to produce simulated data for the future runs of the ATLAS experiment at the LHC, indicate a compelling need to speed-up the process to reduce the computational time required. While different fast simulation projects are ongoing (FastCaloSim, FastChain, etc.), full Geant4 based simulation will still be heavily used and is expected to consume the biggest portion of the total estimated processing time. In order to run effectively on modern architectures and profit from multi-core designs a migration of the Athena framework to a multi-threading processing model has been performed in the last years. A multi-threaded simulation based on AthenaMT and Geant4MT, enables substantial decreases in the memory footprint of jobs, largely from shared geometry and cross-sections tables. This approach scales better with respect to the multi-processing approach (AthenaMP) especially on the architectures that are foreseen to be used in the next LHC runs. In this paper we will report about the status of the multithreaded simulation in ATLAS, focusing on the different challenges of its validation process. We will demonstrate the different tools and strategies that have been used for debugging multi-threaded runs versus the corresponding sequential ones, in order to have a fully reproducible and consistent simulation result.
Copyright/License	© 2020-2024 The Authors (License: CC-BY-4.0)

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