Resumen
Jülich Supercomputing Centre (JSC) started running (extreme) scaling workshops with its first IBM Blue Gene supercomputer, finally spanning three generations each seeing an increase in the number of cores and available threads. Over the years, this workshop series attracted numerous international code teams and resulted in many applications capable of running on all available cores of each system.This article reviews some of the knowledge gained with running and tuning highly-scalable applications, focussing on JUQUEEN, the IBM Blue Gene/Q at JSC. The ability to execute successfully on all 458752 cores with up to 1.8 million processes or threads may qualify codes for the High-Q Club, which serves as a showcase for diverse codes scaling to the entire 28 racks, effectively defining a collection of the highest scaling codes on JUQUEEN. The intention was to encourage other developers to invest in tuning and scaling their codes while identifying the necessary key aspects for that goal.As this era closes, it is timely to compare the characteristics of the 32 High-Q Club member codes, considering their strong and/or weak scaling, exploitation of hardware threading, and whether/how intra-node multi-threading is employed combined with message-passing. We also identify the obstacles for scaling such as inefficient use of limited compute node memory and file I/O as key governing factors. Overall, the analysis provides guidance as to how applications may (need to) be designed in future to exploit expected exa-scale computer systems.