Advancements in Multi-threading for Geant4: Challenges and Solutions
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This summary presents an overview of advancements in multi-threading for Geant4, highlighting the performance benefits observed with 24-core systems, achieving a speedup of approximately 24. Key aspects include revised prototypes, enhancements to thread safety, and the implementation of user interface parallelism using MPI. The document outlines essential techniques for multi-threading, such as creating thread-specific data and ensuring code readability for maintainability. Challenges in synchronization, memory management, and the diversity of solutions are addressed, along with proposals for future improvements in parallel processing.
Advancements in Multi-threading for Geant4: Challenges and Solutions
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Presentation Transcript
Multi-threading and other parallelism options J. Apostolakis Summary of parallel session. Original title was “Technical aspects of proposed multi-threading approach” J. Apostolakis, Geant4 Workshop 2010
Recent news • Geant4 Multi-threaded • Demonstrated good scaling performance • With 24 cores, see speedup of about 24 • Revised prototype with Geant4 9.4 beta • Proposal to move UI-parallel ‘MPI’ into G4 source J. Apostolakis, Geant4 Workshop 2010
Recipe for Geant4 Multi-Threaded • Add __thread to globals, static data members • Identify data members which need to be separate for each thread • Create new classes to hold these data members for multi-threading • And methods to initialize worker data by copying master • Make scheme’s implementation more readable and simpler • So that it can be maintained by G4 developers • Create tools to check code, run-time integrity J. Apostolakis, Geant4 Workshop 2010
The Design of Geant4 Multi-Threading J. Apostolakis, Geant4 Workshop 2010
Thread safety and STL • Is thread safety of STL an issue? • Nearly all classes using STL are thread-private • Important objects which are shared must be treated carefully, and are • IonTable now initialized with 3000 stable isotopes at the start • A previous implementation used locks to ensure that table insertion is thread-safe. • Which parts of STL could have issue with thread-safety J. Apostolakis, Geant4 Workshop 2010
Tools used by Geant4 MT • Parser g++ to find/check static data • Urdb • Tool to use checkpointing to ‘go backwards’ when running your debugger session. • Protecting the master thread heap J. Apostolakis, Geant4 Workshop 2010
Types of parallel Geant4 “Older style” Aiming to reduce memory footprint Multi-process G4 fork, to use copy-on-write Multi-threaded G4 • Farming events using “TopC” • Can use native, MPI or distributed memory • Facility for Joint results files • UI mediated parallelism • Splitting job using MPI • Separate result files must be arranged J. Apostolakis, Geant4 Workshop 2010
Challenges • Four potential solutions for one issue is a large number • Need to consider effort for documentation, maintenance • Identify which solutions are appropriate for which use cases • Search for existing and further commonalities J. Apostolakis, Geant4 Workshop 2010
Trying out G4 multi-threaded • You can find the latest version • /afs/cern.ch/user/x/xdong/public/phase2/source4.9.4.b01.forsequentialclhep.tar.gz • And a recent version using ‘MT-revised’ CLHEP • /afs/cern.ch/user/x/xdong/public/phase2/geant4.9.4.b01mt.tar.gz J. Apostolakis, Geant4 Workshop 2010
Ideas for improvement & concerns • Web site for G4MT prototype • Need to clarify use cases • including constraints, performance needs, current and future • Adopt solutions to address these. • And guide users to use corresponding solutions. • My concern(s) • Diversity of solutions is a challenge • Consolidation must be considered • E.g. must have only ONE way to ‘install’ MPI version(s). J. Apostolakis, Geant4 Workshop 2010
