MRDIMMs (multi-rank buffered DIMMs) could become the standard for buffered DIMMs by 203x. In addition, AMD (opens in new tab) announced a commitment at MemCon 2023 to help advance JEDEC’s MRDIMM open standard, which offers significant bandwidth improvements over standard DDR5 DIMMs.
As core counts continue to increase, it has always been difficult to provide processors with the necessary memory bandwidth. This is one of the reasons why AMD and Intel have moved to DDR5 memory in mainstream processors like his Ryzen 7000 and Raptor Lake. So you can imagine the challenges in the data center segment where AMD’s EPYC Genoa and Intel’s Sapphire Rapids Xeon chips are pushing up to 96 and 60 cores respectively.
Slotting these multi-core EPYC and Xeon monsters in 2P or even 4P configurations further complicates things. The result is a giant motherboard with a ton of memory slots. Unfortunately, motherboards haven’t gotten much bigger and processors continue to debut with more cores. Existing solutions exist, including proprietary interfaces such as Compute Express Link (CXL) and High Bandwidth Memory (HBM) formats. MRDIMM aims to be another option for the vendor to alleviate the problems associated with scaling his DRAM speed.
The purpose of MRDIMMs is to double the bandwidth over existing DDR5 DIMMs. The concept is simple. Combines two DDR5 DIMMs to deliver double the data rate to the host. Additionally, it is designed to allow access to both ranks simultaneously. For example, combining two DDR5 DIMMs at 4,400 MT/s gives an output of 8,800 MT/s. According to the presentation, a special data buffer or multiplexer combines the transfers from each rank, effectively converting two DDRs (Double Data Rate) into a single QDR (Quad Data Rate).
MRDIMM specifications
| MRDIMMs | data rate |
|---|---|
| Gen1 | 8,800MT/s |
| Gen2 | 12,800MT/s |
| Gen3 | 17,600MT/s |
Generation 1 MRDIMMs offer data transfer rates up to 8,800 MT/s. After that, JEDEC expects his MRDIMM to improve gradually, reaching 12,800 MT/s and then 17,600 MT/s. It’s a long project though, as it’s likely that the 3rd generation of his MRDIMMs won’t come out until 2030 or later.
Intel has worked with SK hynix and Renesas to develop Multiplexer Combined Ranks (MCR) DIMMs based on a concept similar to MRDIMMs.According to a retired engineer Chiakohua (opens in new tab), AMD had prepared an equivalent proposal called HBDIMM. There are some differences. However, there is no published material comparing MCR DIMMs and HBDIMMs.
Korean DRAM makers expect the first MCR DIMMs to offer transfer speeds in excess of 8,000 MT/s, so the performance is comparable to first-generation MRDIMM products. Intel recently demonstrated their Granite Rapids Xeon chips with their new MCR DIMMs. A dual socket system provides a memory bandwidth equivalent to 1.5 TB/s. There were 12 MCR DIMMs clocked at DDR5-8800.
The MRDIMM roadmap is vague as it doesn’t indicate when we can expect 1st generation MRDIMMs. However, AMD EPYC Turin (Zen 5) processors to compete with Granite Rapids are coming in 2024. So Granite Rapids can use his MCR DIMMs, so it’s reasonable to expect MCR DIMMs to be available by then. It’s not an official confirmation, but given AMD’s recent promises, it’s highly likely that Torino will be able to take advantage of MRDIMMs, so MRDIMMs could also make an appearance in his 2024 year.
