RAM units - storage levels
RAM units - storage levels
Hi I know for a long time now that running RAM in dual channel is better (faster) than running it in single channel. However, I recently heard about how RAM modules can have different "ranks". Typically, RAM modules are either single rank ( 1R xSomething, on the label) or dual rank ( 2R xSomething, on the label), while quad rank (and even octa rank) modules are also available as server parts AFAIU. Now, I got a little confused from what Google could tell me. It seems to me that the information I found is conflicting. I am not sure if I should rather use singe rank or dual rank modules. Apparently performance depends on multiple factors, including the number of RAM slots that are populated. IIUC: 4 single rank modules would result in : dual channel, dual rank (2 ranks/channel) 4 dual rank modules would result in : dual channel, quad rank (4 ranks/channel) 2 single rank modules (installed appropriately) would result in : dual channel, single rank (1 rank/channel) 2 dual rank modules (installed appropriately) would result in : dual channel, dual rank (2 ranks/channel) Now I don't know what configuration is optimal. I also wonder about the use of mismatched modules (mixing single rank modules with dual rank modules). Please help me sorting all of this out Thank you very much in advance for your help. Best, -a-
Channel and rank are entirely separate topics, so it's best not to conflate them. Dual channel is better simply because you're actually using the available bandwidth. Running single channel halves your bandwidth. Ranks are somewhat similar, but not dependent on how many slots and/or channels are populated, in principle at least. Each rank can be individually addressed, so operations can be paralleled across multiple ranks, just as operations can be paralleled over multiple channels. However, here, the individual stick doesn't matter as much as the total across all sticks. Two sticks of single rank RAM work equally as well as one stick of dual rank, because you have two total ranks in either case. Where things get sticky is with the IMC. The more sticks you have populated, the harder the IMC has to work. Likewise, the more ranks you have, the harder the IMC has to work. While populating all for slots with dual rank for a total of 8 ranks technically gives you more ability to parallel operations, it also puts the maximum amount of stress on the IMC, which can cause anything from outright failure to boot to just not being able to run the RAM at as high of a clockspeed and/or with as tight of timings. In most cases 4 total ranks works out to be the best balance of performance and stability, which as stated previously, can be achieved with two sticks of dual rank or four sticks of single rank. The effect is the same in either case. The final piece of the puzzle is the memory density which is the part after the x in something like 2Rx8. The density determines the number of chips that make up the stick of RAM. You need more x8 chips than x16, for example to make up 16GB. There's also somewhat a parallelization factor here but it's more in retrieving the data. Linus used an excellent example in one of his videos comparing each chip as a stack of books in a library. One librarian can search one stack of books. The large the stack is (higher the density) the longer it takes that librarian to potentially find the book. The smaller the stack, the easier/quicker it is. That said, clockspeed and timings still account for the majority of difference in performance between one kit of RAM and the next. The rest is largely determined by capacity, anyways, so you don't always even necessarily have a choice. For example, 32GB sticks are all going to be dual rank, and probably all use x16. There's only so much room on the RAM module. All 8GB sticks will be single rank, and primarily use x8. 16GB sticks are the wildcards right now, with some being single and some being dual rank, and some using x8 and some using x16.
Dual rank performs better. However, you mainly achieve a similar result with two single-rank sticks.
It seems like you're questioning the reasoning behind dual rank versus single rank. Even with a straightforward approach, using two sticks of dual rank provides four ranks, which appears optimal. Dual rank clearly offers an advantage over single rank when you only possess two sticks.
Yes, two sticks of single-rank RAM remain single-rank, though they operate in dual-channel mode.
The issue arises because using two sticks of dual rank may lead to complications later on.
Single rank: Two sticks equal two ranks Dual rank: Two sticks equal four ranks