Does an Amd epyc 7xxx run with unregistered RAM?

[nero243]

I bought a supermicro h11ssl-i and an Epyc Es. I planned to get a retail 7401 first after seeing a lot of them new on Amazon for ~600€, but of course that price was to good to be true, it's just some messup on Amazons side and they misslabled the 7251s.

But back to the topic, registered ddr4 ram is pretty expensive and I would like to run the system on unregistered ram until I can get my hands on, preferably single ranked, registered memory. Has anyone tried running an Epyc with unregistered RAM?

 

[vanfawx]

Epyc only supports DDR4 ECC/Registered RAM. If you want to run with unbuffered ram, go with Threadripper.

 

[nero243]

A threadripper is too expensive for me.

I guess i'll have to pick up some registered ram then. Are there any benchmarks comparing single ranked VS dual ranked RAM on epyc?

I wonder if getting dual ranked cl19 2666mhz ram is going to have any benefits over single ranked cl17 2400mhz.

On my ryzen 5 1600 single ranked memory with sharp timings outperformed high frequency dual ranked ram by ~10% (in memory intensive tasks). But I guess the infinity fabric with 4 Numa nodes could change this.

 

[vanfawx]

Hmm, I'm not sure, have you checked the following piece that Patrick did? https://www.servethehome.com/amd-epyc-infinity-fabric-latency-ddr4-2400-v-2666-a-snapshot/

Hopefully someone with more experience here will jump in!

 

[nero243]

Yes I've seen this article, but it never specifies if single or dual ranked memory is being used.

 

[nero243]

Sorry for the double post, I wrote the amd customer support yesterday and received an answer this morning.

Response and Service Request History:


Thank you for the email.


Yes, AMD Epyc CPUs does support unregistered RAM.


Thank you for contacting AMD.


In order to update this service request, please respond, leaving the service request reference intact.


Best regards,


Santosh


AMD Global Customer Care

But vanfawx you're telling me it's incompatible, did you actually test it?

 

[vanfawx]

I did not. I guess you have the answer from the horses mouth. Let us know how your experience is so others can benefit.

I was basing it based on AMD's documentation - https://developer.amd.com/wp-content/resources/56301_1.0.pdf which only lists RDIMMs and LRDIMMs as supported.

 

[alex_stief]

Even if Epyc did support UDIMM, you would still need to find an SP3 motherboard that supports it as well. I Can't think of any.
For maximum performance: go wit dual-ranked DDR4-2666. Ryzen with tweaked memory frequency and timings is not a good indicator for Epyc CPUs. That's because you won't be able to tweak anything on such a platform.

For your motherboard with only one CPU socket I would recommend one of the Epyc CPUs with a "P" suffix. They are cheaper than their dual-socket capable counterparts.

 

[nero243]

I have to see if udimms work on my supermicro board, but of course it's not officially supported. I'll try it with my 2 sticks of ram I still have before ordering 8 dimms.

Single ranked memory just performs better on ryzen, that has nothing to do with tweaking. And you can adjust the timings etc on any platform by flashing a new SPD on the memory modules, only the frequency will be locked at 2666mhz.

I picked up a 32 core ES now, I hope it won't be full of glitches.

 

[alex_stief]

Single ranked memory just performs better on ryzen

I see we disagree on that

by flashing a new SPD on the memory modules

If you happen to get memory without write-protection on SPD, maybe. Still sounds like an extremely tedious process without being able to adjust stuff in the bios.

 

[nero243]

So i hadn't any luck with UDIMMs in my Supermicro H8SSL-i...

I bought 4 Kingston 2666Mhz RDIMMs now, i'll buy another 4 next year when i got some more money.

But there are options in the bios for adjusting the RAM Frequency, timings and subtimings. There are also Overclocking presets and shitload of custom Options you'd normally find on a highend Threadripper mainboard. It's all just locked away pretty good.

 

[Trotador]

I did not. I guess you have the answer from the horses mouth. Let us know how your experience is so others can benefit.

I was basing it based on AMD's documentation - https://developer.amd.com/wp-content/resources/56301_1.0.pdf which only lists RDIMMs and LRDIMMs as supported.

Page 9 of the document you linked is confusing for me. The first configuration states that 1DPC DR 2666MHz RDIMM will operate at 2400MHz and and the second says 1DPC SR/DR 2666MHz RDIMM will operate at 2666MHz. Is it like that just depending on where DIMMs are mounted: DIMM0 or DIMM1 sockets?. Sure, I am missing something obvious...
Thanks!