16 th Geant4 Collaboration Meeting SLAC, September 2011 P. Mato, CERN

 For the last 40 years HEP event processing frameworks have had the same structure ◦ initialize; loop events {loop modules {…} }; finalize ◦ O-O has not added anything substantial ◦ It is simple, intuitive, easy to manage, scalable  Current frameworks designed late 1990’s ◦ We know now better what is really needed ◦ Unnecessary complexity impacts on performance  Clear consensus that we need to adapt HEP applications to new generation CPUs ◦ Multi-process, multi-threads, GPU’s, vectorization, etc. ◦ The one job-per-core scales well but requires too much memory and sequential file merging P. Mato/CERN 2

 Covering a broad scope of applications and environments ◦ Single user desktop/laptop case to minimize latency ◦ Production (throughput computing) to minimize memory and other resources  Investigations are starting and prototypes are being developed ◦ Opportunity to collaborate and organize better the work P. Mato/CERN 3

 Universal framework for simulation, reconstruction, analysis, high level trigger applications  Common framework for use by any experiment  Decomposition of the processing of each event into ‘chunks’ that can be executed concurrently  Ability to process several events concurrently  Optimal scheduling and associated data structures  Minimize any processing requiring exclusive access to resources because it breaks concurrency  Supporting various hardware/software technologies  Facilitate the integration of existing LHC applications code (algorithmic part)  Quick delivery of running prototypes. The opportunity of the 18 months LHC shutdown P. Mato/CERN 4

 Current frameworks used by LHC experiments supports all data processing applications ◦ High-level trigger, reconstruction, analysis, etc. ◦ Nothing really new here  But, simulation applications are designed with a big ‘chunk’ in which all Geant4 processing is happening ◦ We to improve the full and fast simulation using the set common services and infrastructure ◦ See later the implications for Geant4  Running on the major platforms ◦ Linux, MacOSX, Windows P. Mato/CERN 5

 Frameworks can be shared between experiments ◦ E.g. Gaudi used by LHCb, ATLAS, HARP, MINERVA, GLAST, BES3, etc.  We can do better this time :-) ◦ Expect to work closely with LHC experiments ◦ Aim to support ATLAS and CMS at least  Special emphasis to requirements from: ◦ New experiments  E.g. Linear Collider, SuperB, etc. ◦ Different processing paradigms  E.g. fix target experiments, astroparticles P. Mato/CERN 6

 Framework with the ability to schedule concurrent tasks ◦ Full data dependency analysis would be required (no global data or hidden dependencies) ◦ Need to resolve the DAGs (Direct Acyclic Graphs)  Not much gain expected with today’s designed ‘chunks’ ◦ See CMS estimates at CHEP’10 (*)* ◦ Algorithm decomposition can be influenced by the framework capabilities  ‘Chunks’ could be processed by different hardware/software ◦ CPU, GPU, threads, process, etc. P. Mato/CERN 7 Time Input Processing Output

 Need to deal with tails of sequential processing ◦ See Rene’s presentation (*)*  Introducing Pipeline processing ◦ Never tried before! ◦ Exclusive access to resources can be pipelined e.g. file writing  Need to design a very powerful scheduler P. Mato/CERN 8 Time

 Concrete algorithms can be parallelized with some effort ◦ Making use of Threads, OpenMP, MPI, GPUs, etc. ◦ But difficult to integrate them in a complete application  E.g. MT-G4 with Parallel Gaudi ◦ Performance-wise only makes sense to parallelize the complete application and not only parts  Developing and validating parallel code is difficult ◦ ‘Physicists’ should be saved from this ◦ In any case, concurrency will limit what can and can not be done in the algorithmic code  At the Framework level you have the overall view and control of the application P. Mato/CERN 9

 It is not simple but we are not alone ◦ Technologies like the Apple’s Grand Central Dispatch (GCD) are designed to help write applications without having to fiddle directly with threads and locking (and getting it terribly wrong)  New paradigms for concurrency programming ◦ Developer needs to factor out the processing in ‘chunks’ with their dependencies and let the framework (system) to deal with the creation and management of a ‘pool’ of threads that will take care of the execution of the ‘chunks’ ◦ Tries to eliminates lock-based code and makes it more efficient P. Mato/CERN 10

 Geant4 [core] is a toolkit and should continue to be ◦ Facilitates the integration in existing applications/frameworks  However Geant4 applications should be based on a new and more modern framework ◦ Configuration, proper scripting, interactivity, I/O, analysis, etc. ◦ Plugins based (physics process/models, visualization drivers, etc.) ◦ Ability to run full and fast MC together using common infrastructure (e.g. geometry, conditions, etc.)  E.g. today’s frameworks allow to run different ‘tacking algorithms’ in the same program  Defining clearly the input and output types  Make use of the common set of foundation packages (math, vectors, utility classes, etc.) P. Mato/CERN 11

 With an approach like the GDC we could exercise different factorizations ◦ Processing each event (set of primary particles) could be the ‘chunk’ (same as GeantMT)  We could also go at the sub-event level ◦ Development of Rene’s ideas of ‘baskets’ of particles organized by particle type, volume shape, etc. ◦ Would need to develop an efficient summing (‘reduce’) of the results ◦ Would require to study the reproducibility of results (random number sequence) P. Mato/CERN 12

 Collaboration of CERN with FNAL, DESY and possible other Labs ◦ Start with small number of people (at the beginning) ◦ Open to people willing to collaborate ◦ Strong interactions with ATLAS and CMS (and others)  E.g. Instrumentation of existing applications to provide requirements ◦ Strong collaboration with Geant4 team  Quick delivery of running prototypes (I and II) ◦ First prototype in 12 months :-)  Agile project management with ‘short’ cycles ◦ Weekly meetings to review progress and update plans P. Mato/CERN 13

 We need to evaluate some of the existing technologies and design partial prototypes of critical parts ◦ See next slide  The idea would be to organize these R&D activities in short cycles ◦ Coordinating the interested people to cover all aspects ◦ Coming with conclusions (yes/no) within few months P. Mato/CERN 14

 Investigate current LHC applications to gather requirements ◦ Dependencies, data access patterns, opportunities for concurrency, etc.  Investigate design and implementations of state – of-the-art concurrency frameworks ◦ Scheduling (static, dynamic, adaptive), memory model, I/O  Prototype framework elements ◦ Identify ‘exemplar’ algorithms to be parallelized ◦ Data structures and memory allocation strategies ◦ New languages (C++11,…) and libraries (OpenCL,…)  Understanding I/O P. Mato/CERN 15

P. Mato/CERN LHC shutdown Today R&D, technology evaluation and design of critical parts Complete Prototype I Initial adaptation of LHC and Geant4 applications Complete Prototype II with experience of porting LHC applications First production quality release Project definition

 Presented a proposal for the development of the new generic data processing framework with concurrency to exploit new CPU/GPU architectures  Geant4 should be a main player providing simulation specific requirements and taking advantage of the new framework ◦ Would imply some re-engineering of Geant4  Need a R&D program to evaluate existing technologies and development of partial prototypes of critical parts  Some initial ideas for the project definition being outlined P. Mato/CERN 17