Question EPYC vs RAM

[csp-guy]

Hi Guys,

I need some advice.

Is it possible to use DDR4 2400 LRDIMM ECC memory (HMAA8GL7AMR4N-UH 64GB) module with Epyc GEN1-GEN2 CPU?

I read about gen1, it supports DDR4-2400, but I didnt found my ram module in supported memory list of manufacturer.

Thanks in advance!

 

[alex_stief]

In general, LRDIMM is supported up to DDR4-2666 on Epyc Naples. Higher frequencies on Epyc Rome. Your memory modules should work, but of course, since they are not in the QVL, there is a slim chance that they won't work.
If you want first-hand confirmation from someone who used the exact same DIMMs with Epyc CPUs, you need disclose the mainboard you are using.

 

[diogin]

By the way, does Epyc support Unbuffered, non-ECC DIMMs?

 

[IamSpartacus]

By the way, does Epyc support Unbuffered, non-ECC DIMMs?

No.

 

[diogin]

No.

Thank you!

But I'm curious, how do ThreadRipper 1950x/2950x support Unbuffered & non-ECC DIMMs while their chiplets/CCDs are same with Epyc 7001 models? By using different IMC? Or by configuring IMC with different AGESA settings?

And what about Zen 2 version of ThreadRipper and Epyc?

 

[alex_stief]

Supporting RDIMM with a CPU/IMC that can handle RDIMM is mainly up to the motherboard manufacturer. Since there are no Epyc motherboards targeted at consumers, why bother with UDIMM support? It does not add any value. You loose maximum capacity, and the higher frequency of some UDIMMs would be lost with those locked down Epyc CPUs.
Not sure if there is a way to find out, but support for RDIMM might just be removed from Threadripper CPUs.

 

[diogin]

I fully agree that supporting UDIMM in Epyc is meaningless since no serious people would use UDIMM in Epyc.
What I was interested is, how does IMC or motherboards work with difference types of DIMMs, and whose responsibility is this? Any details on this aspect are highly appreciated.

 

[diogin]

Have learned something about RDIMM and UDIMM these days.

Epyc IMC on the IOD should have the ability to recognize and operate RDIMM, UDIMM, and LRDIMM (unless AMD choose to not implement UDIMM support for IMC on Epyc IOD, compared to IMC on Ryzen/ThreadRipper IOD which support UDIMM).

However, the connecting electrical wires on the motherboard between IMC and DIMMs are different and not compatible between RDIMM, UDIMM, and LRDIMM, so if the motherboard manufacturer don't choose to "connect" the wires for UDIMM support, then the motherboard doesn't support UDIMM, either.

 

[alex_stief]

That is what I meant when I wrote: "Supporting RDIMM with a CPU/IMC that can handle RDIMM is mainly up to the motherboard manufacturer."

 

[Aluminum]

At a hardware level, all Zen cpus probably support both kinds of ram (and ECC). I doubt they actually made a bunch of completely different IMC designs, even with the Zen 2 off-die concept. If anything, Zen 2 looks like they made one very symmetrical design intended to be split/combined to support N channels/dies as needed.

I suspect memory support is merely limited by firmware, and motherboard traces where applicable. The X399 and Epyc v1 are so close to being compatible, I think AMD could make it so if they wanted. Also TRX80 sounds like it might really just be repackaged Rome with new SKUs, time will tell. Signed firmware can keep the platforms apart in the market as much as they need to, but at a engineering level its smarter to share as much as possible.

As a previous example, X99 boards with certain Xeons could support either kind of ram with the same cpu. No mixing at the same time of course.

 

[diogin]

That is what I meant when I wrote: "Supporting RDIMM with a CPU/IMC that can handle RDIMM is mainly up to the motherboard manufacturer."

Yeah. But if AMD intentionally not implement UDIMM support in IMC of Epyc IOD, then motherboard manufacturer can do nothing on adding UDIMM support even if he would like to. Unfortunately I cannot find any information/specification/datasheet on Epyc IMC/IOD so all I can do is guessing...

 

[alex_stief]

May I ask why you need to know this? In the end, there simply are no motherboards for Epyc CPUs that support UDIMM. So what will you do with the information that Epyc CPUs could theoretically handle UDIMM?

 

[diogin]

May I ask why you need to know this? In the end, there simply are no motherboards for Epyc CPUs that support UDIMM. So what will you do with the information that Epyc CPUs could theoretically handle UDIMM?

Well, just be curious, and would like to know more details about IMC and different DIMMs and how they work.

After the learning, I know UDIMM place more electrical load on IMC and can only support modest RAM size, while RDIMM place less load on IMC by buffering address in address bus so it supports more RAM and more DIMMs per channel then UDIMM, and LRDIMM place even less load on IMC by buffering data in data bus and supports even more RAM then RDIMM.

Personally I think this is much meaningful than just installing RDIMMs on Epyc motherboard like everyone does.

 

[alex_stief]

The what now?