You’ve probably heard of dual-channel memory, but what about dual-rank? Yes, that’s another kind of memory DIMM (module). In simple words, a dual-rank memory DIMM is the equivalent of having two regular “single-rank” memory modules on the same DIMM. As such, a dual-rank memory module should be twice as wide (in terms of bandwidth) as a single-rank module.
A single-rank configuration has a width of 64-bit while a dual-rank module will be 128-bit wide. However, as a memory channel is just 64-bit wide (same as a single-rank module), the memory controller can only address one rank at a time. As you can expect, this should make dual-rank memory slower than traditional single-rank modules, even though they are denser. In truth though, the difference between the latency of single and dual-channel modules is hardly perceivable.
This is due to the fact that dual-rank memory has a higher number of open rows (pages), increasing the hit rate and therefore reducing the chance of a flush. At the same time, accessing the different ranks induces a latency penalty, stalling the pipeline in some cases, thereby degrading performance. Therefore, the overall impact of multi-rank modules varies from application to application. In general, memory bandwidth-sensitive workloads benefit from dual-rank memory.
On consumer PCs, having all four DIMMs installed is akin to having a dual-channel, dual-rank configuration. Generally, it’s been seen that dual-rank configs are 5-10% faster than single-rank PCs, with the benefits being more pronounced on AMD’s Ryzen CPUs.
As you can imagine, there are also quad-rank memory DIMMs as well that can be considered as two dual-rank DIMMs on a single module. Once again, as only one out of the four only one rank is accessible at a time, these modules are a fair bit slower than single-rank DIMMs. For this reason, you won’t find quad-rank modules on mainstream PCs.
Because a rank is 64 or 72 bits, an ECC module made from x4 chips will need eighteen chips for one single rank. An ECC module made from x8 chips needs only nine of them for a rank. A module made from eighteen x8 chips would be dual-ranked. An ECC module that has twice as many x8 chips will be quad-ranked. Here, x4 and x8 represent the number of memory banks per DRAM chip.
Quad-rank DIMMs are usually LRDIMMs, but in practice function as dual-rank modules. This is due to abstraction on the part of the LRDIMM buffer that effectively makes the quad-rank DIMM seem like a dual-rank DIMM to the system.
This helps not only increase the DIMM count on servers but also mask the latency penalty induced by quad-rank modules (to some extend). Most servers support as many as three LRDIMMs per memory channel, significantly boosting memory capacity and speeds over standard quad-rank RDIMMs.
Information regarding memory rank can be usually found on the DIMM label. Single rank is usually labeled as 1Rx (1Rx8/1Rx8/1Rx16), dual rank memory is labeled as 2Rx (2Rx8/2Rx8/2Rx16), and quad rank as 4Rx (4Rx8/4Rx8/4Rx16). It’s important to note that the physical arrangement of DRAM chips on the DIMM doesn’t necessarily indicate the rank. A single-sided module may not always be a single-rank DIMM, and a dual-sided module may not always be a dual-rank DIMM and vise versa.