Abstract
In the context of the common online-offline computing infrastructure for Run3 (ALICE-O2), ALICE is reorganizing its detector simulation software to be based on FairRoot, offering a common toolkit to implement simulation based on the Virtual-Monte-Carlo (VMC) scheme. Recently, FairRoot has been augmented by ALFA, a software framework developed in collaboration between ALICE and FAIR, offering portable building blocks to construct message-based and loosely-coupled multiprocessing systems. We will report here on the implementation of a scalable and asynchronous de-tector simulation system which is based on ALFA. The system offers paral-lelization at the primary-track level, going beyond the usual inter-event parallelism of Geant4-MT, and the possibility to asynchronously and simultaneously process simulation data for the purpose of digitization and clusterization. Core advantages of our implementation are an ideal reduction of the processing time per event as well as a reduction of the memory footprint, allowing us to make significantly better use of opportunistic resources, such as HPC backfills. Moreover, the track-level parallelism opens up the interesting possibility to use different simulation engines (such as Geant4 and Fluka) concurrently, based on simple selection filters on the primary particles. The integration of fast MC processes, such as machine learning kernels running on a dedicated GPU, are a natural extension to the system.
Highlights
Introduction and MotivationDetector simulation, transforming computer generated collisions into simulated hardware/readout signals, is a pillar of any software stack in high-energy physics (HEP) experiments
This work describes a generic and scalable system achieving these goals based on message passing and which is obtained through easy migration of our existing code
In HEP, detector simulation is built around the Geant4 toolkit [1] natively or using the so called Virtual Monte-Carlo (VMC) layer [2]
Summary
Detector simulation, transforming computer generated collisions into simulated hardware/readout signals, is a pillar of any software stack in high-energy physics (HEP) experiments. In HEP, detector simulation is built around the Geant toolkit [1] natively or using the so called Virtual Monte-Carlo (VMC) layer [2]. The latter enables the possibility to use other simulation engines such as Geant3 [3] and Fluka [4]. For LHC Run, ALICE and FAIR are using common software components, in particular FairRoot [6], reducing much the boilerplate code to setup a VMC detector simulation. To requiring considerable resources to treat such a workable chunk, this is typically bad or sub-optimal for scheduling in case of large events. It is currently impossible to access and benefit from any opportunistic computing resource that offers a time window considerably smaller than what such a single atomic task needs
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