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CEA-Leti Makes a 96 core CPU from Six Chiplets

Chiplet design of processors is getting more popular due to many improvements and opportunities it offers. Some of the benefits include lower costs as the dies are smaller compared to one monolithic design, while you are theoretically able to stitch as much of the chiplets together as possible. During the ISSCC 2020 conference, CEA-Leti, a French research institute, created a 96 core CPU made from six 3D stacked 16 core chiplets. The chip is created as a demonstration of what this modular approach offers and what are the capabilities of the chiplet-based CPU design.

The chiplets are manufactured on the 28 nm FD-SOI manufacturing process from STMicroelectronics, while the active interposer die below them that is connecting everything is made using the 65 nm process. Each one of the six dies is housing 16 cores based on MIPS Instruction Set Architecture core. Each chiplet is split into four 4-core clusters that make up for a total of 16 cores per chiplet. When it comes to the core itself, it is a scalar MIPS32v1 core equipped with 16 KiB of L1 instruction and an L1 data cache. For L2 cache, there is 256 KiB per cluster, while the L3 cache is split into four 1 MiB tiles for the whole cluster. The chiplets are stacked on top of an active interposer which connects the chiplets and provides external I/O support.

AMD Gives Itself Massive Cost-cutting Headroom with the Chiplet Design

At its 2020 IEEE ISSCC keynote, AMD presented two slides that detail the extent of cost savings yielded by its bold decision to embrace the MCM (multi-chip module) approach to not just its enterprise and HEDT processors, but also its mainstream desktop ones. By confining only those components that tangibly benefit from cutting-edge silicon fabrication processes, namely the CPU cores, while letting other components sit on relatively inexpensive 12 nm, AMD is able to maximize its 7 nm foundry allocation, by making it produce small 8-core CCDs (CPU complex dies), which add up to AMD's target core-counts. With this approach, AMD is able to cram up to 16 cores onto its AM4 desktop socket using two chiplets, and up to 64 cores using eight chiplets on its SP3r3 and sTRX4 sockets.

In the slides below, AMD compares the cost of its current 7 nm + 12 nm MCM approach to a hypothetical monolithic die it would have had to build on 7 nm (including the I/O components). The slides suggest that the cost of a single-chiplet "Matisse" MCM (eg: Ryzen 7 3700X) is about 40% less than that of the double-chiplet "Matisse" (eg: Ryzen 9 3950X). Had AMD opted to build a monolithic 7 nm die that had 8 cores and all the I/O components of the I/O die, such a die would cost roughly 50% more than the current 1x CCD + IOD solution. On the other hand, a monolithic 7 nm die with 16 cores and I/O components would cost 125% more. AMD hence enjoys a massive headroom for cost-cutting. Prices of the flagship 3950X can be close to halved (from its current $749 MSRP), and AMD can turn up the heat on Intel's upcoming Core i9-10900K by significantly lowering price of its 12-core 3900X from its current $499 MSRP. The company will also enjoy more price-cutting headroom for its 6-core Ryzen 5 SKUs than it did with previous-generation Ryzen 5 parts based on monolithic dies.

AMD Doubles L3 Cache Per CCX with Zen 2 "Rome"

A SiSoft SANDRA results database entry for a 2P AMD "Rome" EPYC machine sheds light on the lower cache hierarchy. Each 64-core EPYC "Rome" processor is made up of eight 7 nm 8-core "Zen 2" CPU chiplets, which converge at a 14 nm I/O controller die, which handles memory and PCIe connectivity of the processor. The result mentions cache hierarchy, with 512 KB dedicated L2 cache per core, and "16 x 16 MB L3." Like CPU-Z, SANDRA has the ability to see L3 cache by arrangement. For the Ryzen 7 2700X, it reads the L3 cache as "2 x 8 MB L3," corresponding to the per-CCX L3 cache amount of 8 MB.

For each 64-core "Rome" processor, there are a total of 8 chiplets. With SANDRA detecting "16 x 16 MB L3" for 64-core "Rome," it becomes highly likely that each of the 8-core chiplets features two 16 MB L3 cache slices, and that its 8 cores are split into two quad-core CCX units with 16 MB L3 cache, each. This doubling in L3 cache per CCX could help the processors cushion data transfers between the chiplet and the I/O die better. This becomes particularly important since the I/O die controls memory with its monolithic 8-channel DDR4 memory controller.

On The Coming Chiplet Revolution and AMD's MCM Promise

With Moore's Law being pronounced as within its death throes, historic monolithic die designs are becoming increasingly expensive to manufacture. It's no secret that both AMD and NVIDIA have been exploring an MCM (Multi-Chip-Module) approach towards diverting from monolithic die designs over to a much more manageable, "chiplet" design. Essentially, AMD has achieved this in different ways with its Zen line of CPUs (two CPU modules of four cores each linked via the company's Infinity Fabric interconnect), and their own R9 and Vega graphics cards, which take another approach in packaging memory and the graphics processing die in the same silicon base - an interposer.
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