The Tiny Chip That Could Disrupt Exascale Computing
March 12, 2015 Nicole Hemsoth
When Rex Computing CEO, Thomas Sohmers, was working with embedded computing systems for military applications at MIT at age 13, his thoughts turned to much larger scale systems. For exascale-class computing, he realized there were many lessons to be carried over from embedded computing that could potentially have an impact on the toughest challenges that lie ahead—balancing the performance demands with overall power efficiency and scalability of both the hardware and software.
The result of his research is an up and coming chip called Neo, which brings to bear a new architecture, instruction set, and core design that scraps a great deal of what Sohmers deems unnecessary about current cache architecture, snaps in a new interconnect, and if his assumptions are correct, can do this in a power envelope and performance target that goes beyond the current Department of Energy requirements for exascale computing goals, which they hope to realize in the 2020 to 2023 timeframe.
Sohmers says that the national labs are already expressing early interest in the 64-bit Neo cores, which are 1/145 the size of a fourth generation Haswell core and 1/27 the size of a 32-bit ARM Cortex A-15. He expects to deliver a 256 core chip by the end of 2016 at the earliest using a 28 nanometer process, which will offer 65 gigaflops per watt. Successive generations will use 10 nanometer or 7 nanometer processes as those roll out. “Current proposals for exascale in 2022 are for 20 megawatts, but it’s definitely possible to do better than that within five years,” he noted.
But how is it possible to see the required performance within these kinds of power constraints? A solid place to start is at the current bottlenecks and one in particular that captured his attention. The inherent inefficiencies of cache.
“Caches and virtual memory as they are currently implemented are some of the worst design decisions that have ever been made,” Sohmers boldly told a room of HPC-focused attendees at the Open Compute Summit this week. “A lot of people are thinking about the HPC problem incorrectly. Caches are great when you’re thinking of things from a pure memory and latency point, but as time went on, so many extra features were layered into the cache systems, especially virtual memory, that while making the programmers lives easier (especially before compilers), it added a lot of new inefficiencies.”
Neo is a 256 core MIMD processor that will offer 256 gigaflops at double precision, 512 for single precision that is able to multitask remarkably well. The concept is that for every cycle, there is an ALU operation, a double-precision floating operation, a load store from the local memory to registers (or vice versa) and a DMA operation—all happening at once to push the pipeline.
To do this, there is what Sohmers calls a “scatchpad” that is a physically addressed SRAM block. In other words, there is no virtual memory translation happening, which in theory, will significantly cut latency (and hence boost performance and efficiency). This means that there is one cycle to address the SRAM, so “this saves half the power right off the bat just by getting rid of address translation from virtual memory.”
At the core level it manages all of this by using quad-issue VLIW, although the system is functionally superscalar and eschews doing some of the new things like using variable lengths where one can change the number of instructions. “The key is to have a full quad-issue pipeline that is kept full at all times and doing an operation every single cycle.” To do that, the design takes the complexity out of the instruction decoder, which saves around 5-10 percent area on the core.
When you think about how this is laid out on a chip, let’s say that core 0 has its own 128 KB of SRAM and the first 128 KB of that address space. When we go to core 1 (for example), it is just a continuation of that address space. When core 1 needs to access something in another core, it uses a DMA operation, takes one cycle to get there, and can do an operation there. The key is that we can guarantee the number of cycles it takes to go from any core—so while a core can do a floating point operation, use the ALU and do load store, it can also be doing DMA every cycle to every other piece of memory on the chip or every other interconnected chip.”
Of course, central to all of this is that interconnect, which Sohmers and his partners have designed. There are three bi-directional interconnects (six interfaces), which provide a parallel point-to-point interface that allows up to 48 GB/s per direction (so 96 GB/s bi-directional), leading to a total aggregate bandwidth of 288 GB/s per chip. The idea for future exascale systems built on these chips is to set them up in a grid and be able to communicate free from the constraints of PCI or NVlink. In fact, Sohmers says he anticipates a 2X improvement over PCI and 4X over NVlink.
Even with all of the potential for performance and efficiency, however, without the ability for programmers to port and make use of Neo, all will be in vain. And while power and performance are key considerations for future exascale systems the biggest challenge of all, as some might argue, is the massive parallel programming challenge ahead.
Sohmers says that in his observations of how other chip upstarts have rolled into the game, they expect the software to follow after the hardware is refined and ready. There has to be a “holistic approach where the hardware and software are considered together” he argued, pointing to work his co-founder is doing to make sure the ISA and portability are produced in lockstep with the chip. “The idea is to make it programmable and have an easy way to port over to our architecture. It’s a VLIW approach, which is not that different from existing architectures beyond the cache. So porting will be reliant on predicting where the memory will be and placing it on the grid.” We will follow up to explore the software challenges in a coming article, but in essence, Rex Computing plans to open its reference architecture and begin pushing development tools out to early testers as early as next year.
As we noted, Rex Computing was presenting at Open Compute Summit this week for an audience of a special HPC working group within the project. He says that opening the architecture is critical to add new thinking and possibilities in semiconductors, especially in the context of the current price/performance paradigms of the current HPC offerings, a comparison of which he offers below.
Aside from the hardware and software plans for the near term, Rex Computing is looking at a few other areas, including a new project called Unum, which seeks to reduce the number of bits that are moved down to 29 for the same amount of work. They are also exploring how they might wrap a 2D mesh from each side of the system, which Sohmers says is something they’ve figured out a “trick” to do, even if it’s still loaded with wires.
He showed how Neo might fit into an Open Compute system using Penguin’s 1U box outfitted with 16 of the Rex Computing chips.
“Based on the other component costs, including DRAM, the efficiencies at rack scale as demonstrated here are within the exascale power limits. A single one of these 1U tall sleds is around 80 watts with the compute portion consuming 50 to 80 of those units.
These are big claims for a…okay, we won’t call him a kid. But as Sohmers openly said to his HPC-centric audience this week, his youth is an advantage. He is clearly excited, giddy even, when talking about Seymour Cray and the various incremental developments in supercomputing architectures. But he says that not having made the mistakes of those previous generations and approaching large-scale computing with an entirely fresh perspective is exactly what is needed when challenges like actual power-efficient exascale computing are at the fore.