RAM modules - memory ranks

[asheenlevrai]

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 (1RxSomething, on the label) or dual rank (2RxSomething, 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-

[Vishera]

25 minutes ago, asheenlevrai said:

Now I don't know what configuration is optimal.

 

Single rank is better.

25 minutes ago, asheenlevrai said:

I also wonder about the use of mismatched modules (mixing single rank modules with dual rank modules).

 

Mismatching RAM can have compatibility issues.

 

[Chris Pratt]

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.

[Chris Pratt]

2 minutes ago, Vishera said:

Single rank is better.

Incorrect. Dual rank is better. Though, you get mostly the same effect from two single rank sticks.

[Vishera]

1 minute ago, Chris Pratt said:

Incorrect. Dual rank is better. Though, you get mostly the same effect from two single rank sticks.

Considering that the standard is at least two sticks - I am not wrong.

[asheenlevrai]

5 minutes ago, Vishera said:

Single rank is better.

 

3 minutes ago, Chris Pratt said:

Incorrect. Dual rank is better. Though, you get mostly the same effect from two single rank sticks.

confusion is maintained

[Chris Pratt]

Just now, Vishera said:

Considering that the standard is at least two sticks - I am not wrong.

Don't even know what point you're making there, but even if you go with that line of logic, two sticks of dual rank gives you four ranks total, which is the optimum. So dual rank would definitely be better than single rank if you only have two sticks.

[asheenlevrai]

2 minutes ago, Vishera said:

Considering that the standard is at least two sticks - I am not wrong.

but 2 sticks of single rank RAM would still be single rank (but dual channel), right?

[Vishera]

Just now, Chris Pratt said:

Don't even know what point you're making there, but even if you go with that line of logic, two sticks of dual rank gives you four ranks total, which is the optimum. So dual rank would definitely be better than single rank if you only have two sticks.

The problem is that with 2 sticks of dual rank adding more in the future can be rather problematic.

[Vishera]

1 minute ago, asheenlevrai said:

but 2 sticks of single rank RAM would still be single rank (but dual channel), right?

Single rank: 2 sticks = 2 ranks

Dual rank: 2 sticks  = 4 ranks

[svmlegacy]

5 minutes ago, Vishera said:

Considering that the standard is at least two sticks - I am not wrong.

You need two sticks per channel to take advantage of single rank sticks.

 

Moral of the story: it doesn't matter much. Dual channel is first priority, then speed, then latency, then ranking.

 

Two "single rank" dimms in different controllers doesn't give the benefits of multiple ranks, only that of dual channel.

[Chris Pratt]

1 minute ago, Vishera said:

The problem is that with 2 sticks of dual rank adding more in the future can be rather problematic.

Granted, but that then just means you're crippling your performance on the off chance that you might add more capacity later and don't just decide to flip your existing kit and buy a new higher capacity kit of two, which would be the better approach anyways. Two sticks is the best config anyways. The only reason mobos even have four slots in some cases is that it gives people the warm feels that they can upgrade later. The majority of the time, they go unused.

[freeagent]

The main limitation when overclocking a dual rank setup is not so much the IMC, but in the motherboard itself, mainly its electrical design in the memory section.  Just my 2c. 

 

My 5600X doesn't OC far with 4 sticks, only up to 1866 stable. but with 2 sticks it rips up to 2000 stable, and can play at 2100 1:1. Performance is close when compared 2vs4 with that CPU, but you really have to lean on the system. Its doubtful many will take the time to learn how. OTOH my 5900X handle 4 sticks very well up to 1900. Running 2 sticks does not help stabilize fclk over 1900, but it has been to 2K. I think CCD2 is a lame duck on my sample.

 

Also I saw MSI lists compatible speeds when running single and dual rank. I haven't seen anyone else do that which I found interesting.