This title appears in the Scientific Report : 2016 

JUQUEEN Extreme Scaling Workshop 2016
Brömmel, Dirk
Frings, Wolfgang / Wylie, Brian J. N. (Corresponding author)
Jülich Supercomputing Center; JSC
67 p.
Internal Report
Supercomputer Facility
Computational Science and Mathematical Methods
JSC Internal Report
Please use the identifier: in citations.
Feedback from last year's very successful workshop motivated the organisation of a three-day workshop 1-3 February 2016, during which the entire 28-rack JUQUEEN BlueGene/Q system with 458,752 cores was reserved for over 50 hours. Eight code teams were selected to use this opportunity to investigate and improve their application scalability, assisted by staff from JSC Simulation Laboratories and Cross-sectional Teams. Code_Saturne from Daresbury Lab and Seven-League Hydro from HITS (Heidelberg) were both able to display good strong scalability and thereby become candidates for High-Q Club membership. Both used 4 OpenMP threads per MPI process, over 1.8 million threads in total. Existing members, CIAO from RWTH-ITV and iFETI from University of Cologne and TU Freiberg, were able to show that they had additional solvers which also scaled acceptably. In-situ interactive visualisation was demonstrated with a CIAO simulation using 458,752 MPI processes running on 28 racks coupled via JUSITU to VisIt. Two adaptive mesh refinement libraries, p4est from University of Bonn and IciMesh from Ecole Central de Nantes, showed that they could respectively scale to run with 917,504 and 458,752 MPI ranks, but both encountered problems loading large meshes. Parallel file I/O limitations also prevented large-scale executions of the FZJ IEK-6/Amphos21 PFLOTRAN subsurface flow and reactive transport code, however, a NEST-import HDF5 module developed by the EPFL Blue Brain Project could be optimised to use collective MPI file reading calls to load and connect 1.9TB of neuron and synapse data and enable large-scale data-driven neuronal network simulations with 458,752 threads. Detailed reports are provided by each code-team, and additional comparative analysis to the 25 High-Q Club member codes. Despite more mixed results than the previous workshop, we learnt more about application file I/O limitations and inefficiencies which continue to be the primary inhibitor to large-scale simulations, and all of the participants found the workshop to have been very valuable.