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Exascale challenges


The current buzz in the world of high-performance computing is exascale systems. The next major milestone is HPC systems that will be capable of executing an exaflop – a billion billion floating point operations per second, writes John Barr


E


xtrapolating the performance delivered by the fastest systems in the world in recent years suggests that an exascale system could be


built in 2018. However, the approach that has oſten led to the development of the next generation of supercomputers – more of the same but bigger and faster – is no longer tenable. Additional compute power cannot be achieved by cranking up the processor clock-speed because this also cranks up the power consumption. Te best option for an exascale system is to use a massive number of very efficient processors that have very low power consumption, augmented by a small number of higher- performing processors for tackling segments of applications that do not scale well. Steve Oberlin, CTO for accelerated computing at GPU manufacturer Nvidia, explains that the industry has had it easy in recent years, with CMOS scaling giving greater density of transistors on a chip and faster clock speeds as well as power reductions. Oberlin believes that by moving to simpler, more efficient processor architectures and by focusing on issues such as data locality, the industry can stay on the linear performance projection, even if we are approaching physical limits of scaling CMOS devices.


When will the first exascale system be operational? Before this question can be answered, we really need to understand exactly what an ‘exascale system’ is. If we mean a system that is as usable for a broad range of scientific


24 SCIENTIFIC COMPUTING WORLD


applications as current petascale systems are, and consumes less than 25 MW power, then the general consensus is that we will have to wait until well into the next decade. If our definition is more liberal, and includes systems whose theoretical peak performance exceeds one exaflop/s – and we ignore whether or not it can stay up long enough to run real applications, and if we don’t care how much power it consumes – then many HPC experts think that we will see the first exascale system around 2019. However, it may even be a challenge


to keep the machine operational long enough to run the Linpack benchmark


PRODUCING AN


EARLY EXASCALE SYSTEM COULD HAVE SIGNIFICANT BENEFITS


and demonstrate a sustained performance in excess of one exaflop/s. Alex Ramirez, manager of the heterogeneous architectures group at the Barcelona Supercomputer Centre, suggests that the first exascale systems may be motivated by politics or national interests and may be both unreliable and have an unreasonable power requirement – possibly up to 100MW. Producing an early exascale system could


have significant benefits, even if the system’s usefulness is limited. It could be a statement of intent by the country that funded such an


exercise, or the companies that developed the key technologies, pulling more skilled staff and further investment into the programme to develop a more usable, second generation of exascale systems. Also, having a full-scale testbed would allow the HPC industry to run experiments that demonstrated which aspects of the system were up to the job, and which could benefit from taking a new approach. Professor Tomas Lippert, director of the


Institute for Advanced Simulation and head of Jülich Supercomputing Centre, points out that the first petaflop/s system arrived earlier than expected (at the Los Alamos National Laboratory in 2008) and used IBM’s Cell processors – a technology that is no longer available in HPC systems. He suggests that


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