Monday, 31 July 2017

HPC Getting More Choices - Technology Diversity

HPC has been easy for a while ...

When buying new workstations or personal computers, it is easy to adopt the simple mantra that a newer processor or higher clock frequency means your application will run faster. It is not totally true, but it works well enough. However, with High Performance Computing, HPC, it is more complicated.

HPC works by using parallel computing – the use of many computing elements together. The nature of these computing elements, how they are combined, the hardware and software ecosystems around them, and the challenges for the programmer and user vary significantly – between products and across time. Since HPC works by bringing together many technology elements, the interaction between those elements becomes as important as the elements themselves.

Whilst there has always been a variety of HPC technology solutions, there has been a strong degree of technical similarity of the majority of HPC systems in the last decade or so. This has meant that (i) code portability between platforms has been relatively easy to achieve and (ii) attention to on-node memory bandwidth (including cache optimization) and inter-node scaling aspects would get you a long way towards a single code base that performs well on many platforms.

Increase in HPC technology diversity

However, there is a marked trend of an increase in diversity of technology options over the last few years, with all signs that this is set to continue for the next few years. This includes breaking the near-ubiquity of Intel Xeon processors, the use of many-core processors for the compute elements, increasing complexity (and choice) of the data storage (memory) and movement (interconnect) hierarchies of HPC systems, new choices in software layers, new processor architectures, etc.

This means that unless your code is adjusted to effectively exploit the architecture of your HPC system, your code may not run faster at all on the newer system.

It also means HPC clusters proving themselves where custom supercomputers might have previously been the only option, and custom supercomputers delivering value where commodity clusters might have previously been the default.

Many-core processors, often referred to as accelerators, are processors that use a high degree of parallel processing within the chip – such as GPUs and Xeon Phi – and require different programming techniques to achieve the best performance. In the case of GPUs, a different language might be required (e.g., CUDA or OpenCL). It is likely that code written for one accelerator will be non-trivial to port and achieve good performance on another accelerator (maybe even when both accelerators are from the same family).

Even away from accelerators, there are strong signs that credible competitors to Intel’s Xeon CPU family are back in play. One or more of AMD’s EPYC x86 processors, ARM architecture candidates, or IBM’s OpenPower could take market share from Intel. This adds a further portability and performance tuning challenge to programmers.

Not just CPUs and GPUs

The data storage hierarchy that an executing HPC code must consider now spans registers, multiple levels of on-chip cache (some shared, some dedicated), off-chip cache or local high speed memory, on-node memory, memory on remote nodes (maybe split into topologically near and far nodes!), fast storage layers (e.g., cache buffers), and disk. Between each level of this hierarchy is a substantial sharp step in capacity, bandwidth and latency (and cost). A good compiler might manage the registers and lower cache levels, whilst some parallelisation strategies might minimize the performance effects of the off-node memories. Use of libraries or software frameworks might help manage the hierarchies.

However, the combination of processor diversity and data hierarchy means that software developers, and application buyers, must pay attention to the hardware details in a way that has not mattered this strongly for several years.

The rules of this new game are often reduced to simple sounding ideals: expose more parallelism; avoid data movement; extra compute is cheaper than data movement; etc. The reality of effective implementation turns out to be somewhat harder in practice.

What increased HPC technology diversity means

The diversity is good, because it brings competition, which helps reduce prices, and - perhaps more importantly - drives continued innovation among the suppliers. The diversity is hard, because it adds decision risk in determining the optimum technology path for buyers, and adds complexity for software developers seeking portability and performance.

Ultimately, technology diversity keeps HPC alive and fun - and it keeps our consulting business going, as we provide impartial expert advice on the technology choices and mitigating decision risk!

Are you ready to seize the opportunity performance or cost opportunities? How are you managing the decision risk associated with finding the optimum technology path for you? Are you comfortable with the potential lost performance of not exploring the technology diversity?

To discuss this further contact me at @hpcnotes or, or use the comments box below.

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