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AMD Takes EPYC Server CPUs to Another Level With 3D Cache

Nov. 9, 2021
AMD said Microsoft plans to roll out a new cloud service based on its new EPYC CPUs with 3D V-Cache, which deliver a more than 50% performance uplift for workloads such as electronic design automation.

AMD is raising the bar in its battle against Intel in the data center with a new lineup of EPYC CPUs that use its 3D packaging technology to triple the L3 cache, giving them a significant hike in performance.

The Santa Clara, California-based company is upping the ante in data centers with a new class of its EPYC CPUs, code-named Milan X, which uses its 3D V-Cache packaging technology. AMD said V-Cache stacks up to 512 MB of additional cache memory on top of the CPU, resulting in a more than 50% uplift in performance for workloads such as computational fluid dynamics, structural analysis, and electronic design automation.

Microsoft will also use Milan-X in a new offering from its Azure cloud computing service, the firms said.

 AMD will beat Intel to the market with a server processor that uses 3D chip packaging with plans to launch the Milan-X CPUs in the first quarter of 2022. Intel is banking on its advanced packaging prowess to help it regain its leadership in the data center and other areas, but it is not using its 3D Foveros technology in the latest Sapphire Rapids server CPUs, which instead use Intel’s 2.5D chip packaging technology called EMIB.

AMD introduced Milan-X and several other chips, including its latest server GPU to take on Nvidia, during its Accelerated Data Center event on Monday. It also revealed new details about its future “Genoa” server CPUs.

AMD also said it landed Meta Platforms, the firm formerly known as Facebook, as a buyer for its EPYC CPUs, cementing its market share gains against Intel. For AMD, the win means that its server chips are designed into data centers by ten of the world’s largest hyperscale companies, including top US cloud computing firms—AWS, Microsoft, Google, IBM, Oracle—and their Chinese counterparts: Baidu, Alibaba, and Tencent.

For CEO Lisa Su, winning customers such as Microsoft and Meta has been a major part of her turnaround plan for AMD. As Intel's struggle to move to more advanced chip production hobbled its ability to compete in recent years, AMD has re-engineered its lineup of server processors, which can sell for up to thousands of dollars each. It has rolled out server chips that match or beat Intel’s Xeons on performance benchmarks.

The Milan-X processors will have the same capabilities and features as EPYC 7003 server chips introduced in March, code-named Milan, which come with up to 64 cores manufactured on the 7-nm process by TSMC.

The “Zen 3” architecture at the heart of the EPYC CPUs brings improvements in clock speeds, latency, cache, and memory bandwidth. The processor is disaggregated into up to eight compute dies—also called chiplets or tiles—that contain up to eight cores each. The flagship processor features eight compute dies each with up to 32 MB of shared L3 cache directly on the die, for a total of 64 cores and 256 MB of L3 cache. The L3 cache serves the chip’s central repository where data is stored for fast, repeated access by the CPU cores.

The compute tiles are co-packaged with a central I/O tile based on the 14-nm node from GlobalFoundries that coordinates data traveling between the compute tiles surrounding it. The I/O tile supports up to eight DDR4 channels clocked at up to 3.2 GHz and up to 128 lanes of PCIe Gen 4. All the die are assembled with TSMC’s 2.5D chip packaging technology on a substrate that resembles a very compact circuit board (PCB).

The new Milan-X CPUs will feature up to 64 cores, the same amount as the existing third-generation EPYC CPU lineup and they also will be “fully compatible” with existing EPYC server platforms with a BIOS upgrade.

AMD said V-Cache adds another 64 MB of SRAM on top of the 32 MB present on every compute tile in the current third-generation EPYC CPUs, giving Milan-X up to 96 MB of L3 cache per compute die. The V-Cache is manufactured by TSMC on 7-nm and measures 6-mm by 6-mm. With a maximum of eight compute die as part of Milan-X’s processor architecture, that translates into up to 768 MB of shared L3 cache in the CPU.

“This additional L3 cache relieves memory bandwidth pressure and reduces latency and that in turn speeds up application performance dramatically,” Su said.

AMD said that opens the door for customers to buy dual-socket servers with more than 1.5 GB of L3 cache. When adding the L2 and L1 caches, the Milan-X processors will have a total of 804 MB of cache per socket.

AMD uses TSMC’s SoIC 3D packaging technology to place the memory on top of the compute die with direct copper-to-copper bonding of the through silicon vias (TSVs) that connect the die, slashing the resistance of the interconnects. AMD said V-Cache works without the use of copper pads capped in solder called micro bumps, hiking power efficiency, interconnect density, and signaling routing, while limiting heat dissipation.

AMD said TSMC’s SoIC technology permanently bonds the interconnects in V-Cache to the CPU, closing the distance between the die, resulting in 2 TB/s of communications bandwidth. As a result, the interconnects in the Milan-X CPUs have up to three times the power efficiency by consuming one-third the energy-per-bit and 200 times more interconnect density than the 2D chiplet packaging used by its third-generation EPYC CPUs.

AMD has previously unveiled plans to use V-Cache technology in its Ryzen CPUs for the PC market.

While the V-Cache is physically further from the CPU than the L3 cache that runs through the middle of the compute tile like a spinal cord, AMD said the performance penalty is limited. The company said that it takes less latency to travel through the interconnects and into the stacked die than it takes to leave the CPU, travel through the I/O tile in the 2.5D package, access additional DRAM in the system, and then return to the CPU.

AMD said the 3D cache is a boon to a wide range of workloads in data centers, such as artificial intelligence, where it pays dividends to keep data as close to the processor as possible. But where Milan-X stands out is in computationally heavy workloads, such as modeling the structural integrity of a bridge, replicating the physics of an automotive test crash, and simulating air currents cascading around the wing of an airplane.

Another workload AMD is trying to address with Milan-X CPUs is semiconductor design, since the V-Cache guarantees “critical data” used in electronic design automation (EDA) is located closer to the CPU’s cores.

It is impossible for even the most skilled engineers to test every single detail in a final chip design by hand. Chip firms run thousands of simulations to verify the performance in chip designs before the final blueprints are manufactured. To save time, they run simulations at the same time on separate cores in the same CPU. But because the cores are all fighting over limited memory cache and bandwidth, performance takes a hit.

But using V-Cache to upgrade the amount of shared L3 cache means AMD’s Milan-X CPUs can keep even more information close to the CPU cores, reducing latency that can sap the performance of EDA workloads.

AMD said that a 16-core variant of Milan-X can do verification runs on semiconductor designs in Synopsys’s VCS tool around 66% faster than a third-generation, 16-core EPYC CPU without the new 3D V-Cache. Dan McNamara, senior vice president and general manager of AMD’s server business, said Milan-X makes it possible for chip firms to test designs faster or run more tests at the same time, reducing time-to-market.

Microsoft said cloud services based on Milan-X processors are also up to 50% faster for automobile crash test modeling and up to 80% higher performance for aerospace workloads versus rivals’ cloud services.

AMD said it has partnered with many of the major players in EDA and other system design tools, including Altair, Ansys, Cadence, Siemens, Synopsys, among others, to improve how their software runs on Milan-X.

Top manufacturers of data center gear plan to roll out servers with Milan-X inside, including HPE, Dell, Cisco, Supermicro, and Lenovo, among others. AMD said Milan-X chips will be available by the first quarter of 2022.

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