Section: New Results

Exascale Systems

Participant : Franck Cappello.

Over the last 20 years, the open-source community has provided more and more software on which the world's high-performance computing systems depend for performance and productivity. The community has invested millions of dollars and years of effort to build key components. Although the investments in these separate software elements have been tremendously valuable, a great deal of productivity has also been lost because of the lack of planning, coordination, and key integration of technologies necessary to make them work together smoothly and efficiently, both within individual petascale systems and between different systems. A repository gatekeeper and an email discussion list can coordinate open-source development within a single project, but there is no global mechanism working across the community to identify critical holes in the overall software environment, spot opportunities for beneficial integration, or specify requirements for more careful coordination. It seems clear that this completely uncoordinated development model will not provide the software needed to support the unprecedented parallelism required for peta/exascale computation on millions of cores, or the flexibility required to exploit new hardware models and features, such as transactional memory, speculative execution, and GPUs. We presented a rational promoting that the community must work together to prepare for the challenges of exascale computing, ultimately combing their efforts in a coordinated International Exascale Software Project.

Over the past few years resilience has became a major issue for high-performance computing (HPC) systems, in particular in the perspective of large petascale systems and future exascale systems. These systems will typically gather from half a million to several millions of central processing unit (CPU) cores running up to a billion threads. From the current knowledge and observations of existing large systems, it is anticipated that exascale systems will experience various kind of faults many times per day. It is also anticipated that the current approach for resilience, which relies on automatic or application level checkpoint/restart, will not work because the time for checkpointing and restarting will exceed the mean time to failure of a full system. This set of projections leaves the community of fault tolerance for HPC systems with a difficult challenge: finding new approaches, which are possibly radically disruptive, to run applications until their normal termination, despite the essentially unstable nature of exascale systems. Yet, the community has only five to six years to solve the problem. In order to start addressing this challenge, we synthesized the motivations, observations and research issues considered as determinant of several complimentary experts of HPC in applications, programming models, distributed systems and system management.

As a first step to adress the resilience challenge, we conducted a comprehensive study of the state of the art . The emergence of petascale systems and the promise of future exascale systems have reinvigorated the community interest in how to manage failures in such systems and ensure that large applications, lasting several hours or tens of hours, are completed successfully. Most of the existing results for several key mechanisms associated with fault tolerance in high-performance computing (HPC) platforms follow the rollback-recovery approach. Over the last decade, these mechanisms have received a lot of attention from the community with different levels of success. Unfortunately, despite their high degree of optimization, existing approaches do not fit well with the challenging evolutions of large-scale systems. There is room and even a need for new approaches. Opportunities may come from different origins: diskless checkpointing, algorithmic-based fault tolerance, proactive operation, speculative execution, software transactional memory, forward recovery, etc. We provided the following contributions: (1) we summarize and analyze the existing results concerning the failures in large-scale computers and point out the urgent need for drastic improvements or disruptive approaches for fault tolerance in these systems; (2) we sketch most of the known opportunities and analyze their associated limitations; (3) we extract and express the challenges that the HPC community will have to face for addressing the stringent issue of failures in HPC systems.