Professor Steve Furber’s SpiNNaker project is in the news again. I wrote about Furber’s massively parallel brain-emulation project back on March 30 after listening to his keynote at this year’s DATE (Design Automation and Test Europe) conference in Grenoble, France. (See “The incredible vanishing power of a machine instruction. Is this the way to the brain?”) Furber’s DATE keynote title says it all: “Biologically-inspired massively-parallel architectures—computing beyond a million processors.” Furber and his team are referencing nature to help them tackle the really hard processing problems we need to solve in the future through massively parallel, brain-like computing. Brain-like computing—go slow, go wide, go massively parallel—seems to offer a proven, low-power approach to solving some of these big computational problems.
The SpiNNaker project is again in the news at EETimes Europe (see “A million ARM cores to host brain simulator”) and the idea of harnessing one million ARM processor cores is certainly a big idea. It excites me. However, we’re still at the humble beginnings of the project.
The SpiNNaker project’s first test chip harnesses 18 ARM9 cores on one 130nm chip manufactured by UMC in Taiwan. This is a 100M-transistor chip and, like most many-processor SoCs, the SpiNNaker SoC mostly consists of memory. The memory needs to be close to the processors for speed and for low-power consumption and there are 55 32Kbyte SRAM blocks on the SpiNNaker die. That’s 14 million bits of SRAM and, frankly speaking, that’s really not very much SRAM. Eighteen processors isn’t really a large number of processors either when your stated goal is one million.
The ARM processors on the SpiNNaker chip use packet communications to emulate the electrical spike communications that occur among the neurons in human and animal brains. From a hardware perspective, I think it’s easy to conceive of a system-level design like this and even conceptually scaling the design to a million connected ARM9 processors isn’t really hard, as long as you don’t try to enumerate all of the processors in your mind. However, with 18 processors per chip, you’ll need approximately 55,600 chips to build an interconnected network of one million processors. That’s still a mighty big box of hardware. More on that in a bit.
The rub is that we really don’t have many good ideas for programming such a massively parallel system. The SpiNNaker project seems to be mostly a hardware endeavor with the explicitly stated intent of developing a hardware testbed for brain researchers who will use SpiNNaker systems for studying various theories of brain function. Presumably, we’ll learn more about massively parallel programming by working with these systems and no doubt we will. As Furber says in a quote published in the EETimes Europe article, “We don’t know how the brain works as an information-processing system, and we do need to find out. We hope that our machine will enable significant progress towards achieving this understanding.”
Each SpiNNaker chip in the current design is bundled with a 166MHz, 1Gbit DDR SDRAM and packaged in a 300-pin BGA package. But we’re not going to be building million-processor testbeds with 18 processors per packaged chip. I’m almost absolutely, positively certain about that. This first SpiNNaker prototype just doesn’t scale to one million processors very easily. So the question is, how to get there?
Well, possible clues to answer that question can be found in two recent blogs that I wrote on the EDA360 Insider blog. First, Samsung has just announced successful tapeout of a 20nm test chip incorporating an ARM Cortex-M0 processor core. (See “Samsung 20nm test chip includes ARM Cortex-M0 processor core. How many will fit on the head of a pin?”) Now an ARM Cortex-M0 processor is not as powerful as an ARM9 processor, but then it’s not supposed to be. It’s designed for control-oriented applications and its 3-stage execution pipeline isn’t designed to get maximum speed from any given process technology. However, we’re building a system that emulates a brain that operates at a few hundred Hertz (that’s Hertz, not kilohertz, megahertz, or gigahertz) so I really don’t think the clock speed is all that critical when you’re talking about a million processors. The ARM Cortex-M0 processor core is still a 32-bit RISC processor and I am guessing with a high degree of confidence that it’s fully up to the task of executing the required electrical-spike calculations, albeit not quite as quickly as an ARM9 processor.
What’s interesting about a 12-to-14Kgate ARM Cortex-M0 processor implemented in 20nm process technology is that my calculations suggest that more than half a million ARM Cortex-M0 processors would fit on a chip the size of an Intel “Tukwila” Itanium processor (OK, that’s a big chip, but it’s a commercial one) and that calculation is based on the published number for the area required by an ARM Cortex-M0 implemented in 90nm process technology, not 20nm. Now there’s a lot of slop in this calculation. First, there’s the disparity of using 90nm numbers instead of 20nm numbers. Then there’s the disparity caused by putting no memory at all into the calculation. I just mentally tiled processors edge to edge. Ditto, there’s no on-chip interconnect.
So you probably won’t get half a million ARM Cortex-M0 processor cores on one 20nm chip. But you might get 100,000 or 200,000 ARM Cortex-M0 processor cores on a chip along with an interesting amount of memory and the required interconnect. Now we’re talking about only a handful of chips to get to one million processors. We’re talking about a tabletop box. Now we’re getting into the realm of the feasible for million-processor systems.
The second related blog entry I recently wrote in EDA360 Insider that also bears on this very interesting endeavor was about an announcement from Imec, a global research company. Just days ago, Imec announced that it and its partners successfully assembled a custom logic chip with two DRAMs in a stacked 3D configuration. (See “3D Thursday: IMEC prototypes 3D chip stack, finds some thermal surprises”.) This 3D stacked-chip prototype allowed Imec to test out some process ideas for manufacturing 3D stacked chip assemblies and to make some critical thermal tests to verify thermal models that will be so necessary when 3D assembly goes mass market. The 3D chip stack uses copper-tin micro-bumps and compression bonding for the electrical and mechanical assembly of the chip stack and you can see photos of the assembled stack below.
Here’s a photo of the overall chip stack:
And here’s a close-up of the edge of the chip stack to show the three stacked die.
The 3D Stack’s base chip is approximately 750µm thick. The two top components in the chip stack are each 25µm thick. There’s more technical info in the referenced EDA360 Insider blog.
I am convinced that 3D stacking of logic and RAM chips will be absolutely essential to developing massively parallel, low-power systems like the ones envisioned by the SpiNNaker project. First, the only way to feed data and instructions to massively parallel processing chips is through large amounts of on-chip memory and through high-bandwidth, low-energy channels connected to large off-chip memories. 3D assembly techniques permit both Wide I/O and high-speed serial I/O channels to work most effectively and at minimal energy levels and I expect to see rapid adoption of 3D assembly—even and perhaps especially in high-volume, cost-sensitive applications such as mobile phone handsets—in the next few years. This is precisely the sort of manufacturing technology we require to think seriously about million-processor systems.
Now all we need to do is figure out how to program them.