FairRoot and ALICE O2 Multithreading Simulation

To address the challenges of the major upgrade of the experiment, the ALICE simulations must be able to make efficient use of computing and opportunistic supercomputing resources available on the GRID. The Geant4 transport package, the performance of which has been demonstrated in a hybrid multithreading (MT) and multiprocessing (MPI) environment with up to 1/4 million threads, is therefore of a particular interest. The O2 simulation framework is based on FairRoot, which itself is based on the Virtual Monte Carlo (VMC). The integration of multithreading into the VMC design and its impact on the Geant4 VMC were presented at CHEP 2014. Geant4 VMC multithreading and the scaling behaviour of the computing time with the number of cores have then been tested using a simplified but realistic multithreaded simulation application. The focus was then put on the integration of multithreading in FairRoot classes as the necessary step towards multithreading in the FairRoot-based experimental frameworks. The new O2 framework is the first one of which the migration to multithreading is achieved for all actually included detectors. In this paper we present the progress with the integration of multithreading in FairRoot classes, the work for thread-safety in the O2 simulation classes and the experience with the integration of the multithreading mode in testing. We will also discuss plans for the further developments.