Massive modules for massive servers

Micron announced this week that it has begun sampling 256 GB multiplexer combo (MCR) DIMMs, the company’s highest-capacity memory modules to date. These new DDR5-based MCRDIMMs are targeted at next-generation servers, specifically those powered by Intel Xeon Scalable “Granite Rapids” processors, which are set to support 12 or 24 memory slots per socket. Using these modules enables data center machines with 3 TB or 6 TB of memory, which when combined can achieve effective data rates of DDR5-8800.

“We have also begun sampling 256 GB MCRDIMM modules, which further improves performance and increases the DRAM content of each server,” Micron CEO Sanjay Mehrotra said in prepared remarks for the company’s earnings call this week.

In addition to announcing the availability of samples of these modules, Micron also gave demonstrations at NVIDIA’s GTC conference, where server vendors and customers are enthusiastic about building new servers for next-generation AI accelerators. Our colleagues at Tom’s Hardware managed to get a few photos of Micron’s 256 GB DDR5-8800 MCR DIMM.

Image Credit: Tom’s Hardware

Apparently, Micron’s 256 GB DDR5-8800 MCRDIMM comes in two variants: a taller module with 80 DRAM chips spread out on both sides, and a standard height module using a 2Hi stacked package. Both are based on a single 32Gb DDR5 IC and are designed to cater to different server configurations with standard height MCRDIMMs suitable for 1U servers. The higher version consumes around 20W, in line with the expected RDIMM for 128 GB DDR5-8000 which consumes around 10W in DDR5-4800 mode. I don’t know the power consumption of the version using the 2Hi package, but expect it to be hotter and harder to cool.

Image Credit: Tom’s Hardware

Multiplexer Composite Rank (MCR) DIMMs are dual-rank memory modules with dedicated buffers that allow both ranks to operate simultaneously. This buffer enables the two physical columns to operate as separate modules working in parallel, allowing 128 bytes of data to be retrieved simultaneously from both columns per clock cycle (compared to 64 bytes per cycle for a conventional memory module ), effectively doubling the performance of a single module. Of course, since the module retains the physical interface of a standard DDR5 module (i.e. 72-bit), the buffer works with the host at a very high data transfer rate to transfer the acquired data to the host CPU. These speeds exceed standard DDR5 specifications, in this case reaching 8800 MT/s.

While MCR DIMMs make memory modules slightly more complex than regular RDIMMs, they increase the performance and capacity of the memory subsystem without increasing the number of memory modules involved, making it easier to build server motherboards. These modules will play a key role in next-generation servers handling increasingly demanding applications, particularly in the field of artificial intelligence.

Source: Tom’s Hardware, Micron

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