Thursday, August 24th 2017
AMD Defends MCM Approach for Building 32-core EPYC: Huge Cost Savings of ~41%
AMD, presenting at a HotChips talk, packed with VLSI gurus, defended its decision to make AMD EPYC enterprise processors in the socket SP3r2 package a multi-chip module (MCM) of four 8-core "Summit Ridge" dies, rather than building a monolithic 32-core die. For starters, it stressed on the benefits of a single kind of silicon, which it can use across its performance-desktop, high-end desktop, and enterprise product-stacks; which translates into higher yields. "Summit Ridge" is the only CPU silicon based on the "Zen" micro-architecture, with the company giving finishing touches to the second silicon, codenamed "Raven Ridge," which will power mobile and desktop APUs. AMD has to pick the best-performing dies out of a common bin. The top 5% dies go into powering the company's Ryzen Threadripper HEDT processors, and a higher percentile go into making EPYC.
The relatively smaller 8-core common die has an inherently higher yield than a larger chip due to the rule of inverse-exponential reduction in yield with increasing die-size. This, coupled with the R&D costs that would have gone into developing the hypothetical monolithic 32-core "Zen" based silicon, works out to a significant cost saving for the company. A 4-die/32-core MCM is 0.59X the cost of a hypothetical monolithic 32-core die, according to the company, which is a cost-saving that enables the company to aggressively price its products. The slide AMD used in its presentation also confirms that each 8-core "Summit Ridge" die features four external Infinity Fabric links, besides the one that connects the two CCX units with each other. On a 4-die EPYC MCM, three out of four of those external IF links wire out to the neighboring dies, and one link per die probably wires out to a neighboring socket on 2P machines.
The relatively smaller 8-core common die has an inherently higher yield than a larger chip due to the rule of inverse-exponential reduction in yield with increasing die-size. This, coupled with the R&D costs that would have gone into developing the hypothetical monolithic 32-core "Zen" based silicon, works out to a significant cost saving for the company. A 4-die/32-core MCM is 0.59X the cost of a hypothetical monolithic 32-core die, according to the company, which is a cost-saving that enables the company to aggressively price its products. The slide AMD used in its presentation also confirms that each 8-core "Summit Ridge" die features four external Infinity Fabric links, besides the one that connects the two CCX units with each other. On a 4-die EPYC MCM, three out of four of those external IF links wire out to the neighboring dies, and one link per die probably wires out to a neighboring socket on 2P machines.