Why do Threadripper (WRX80, TRX50, WRX90) not support 2DPC

Every platform out there supports 2DPC including AMD’s server SP3 SP5 SP6, AMD’s consumer AM4, AM5, and even the old threadrippers X399. TRX40 and intel’s competition to all those platforms, including HEDT (W790)

But the only platforms that do not support 2DPC are the threadrippers (WRX80, TRX50, WRX90)

From The consumer’s perspective, that sucks because of the cost of DIMMs does not scale linearly with capacity (especially if you go to LRDIMMs/3DS) and/or it does not allow for delaying the purchase of more DIMMs

Why would they not support 2DPC? Is it just market segmentation? SP5 has more channels, is that not enough segmentation?

edit: SSI-EEB (which is the size of most WRX90 motherboards) has enough space for 24 DIMMs

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The answer here is they could have produced WRX80, TRX50 and WRX90 motherboards with 2DPC but they did not because this is how they want to market the products. WRX80 and WRX90 already have 8 memory channels for performance. With 2DPC that would mean 16 DIMM slots per motherboard and that is simply not going to happen in a HEDT space.

If you want 12 or 16 slots per processor use a server motherboard. Threadripper is already a niche as it is. that is why we get new processors for them the last. First servers, then consumer platforms and then Threadripper. They are not going to segment the niche further.

And TRX40 and TRX50 is definitely not going to get 2DPC, because they would lessen the incentive to go with a more expensive 8 DIMM platform.

At the current time we can be more then happy that AMD is even serving the HEDT market for the few of us who want a platform like that. It is a small market and the revenue is done in servers, a little with consumer chips but HEDT is basically a service to the users and looks nice on marketing material.

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This is based on my understanding of how memory channels work after doing some quick googling. Based on this article that talks about the compromises of 2DPC at high channel counts, and this article that gives a basic overview of how CPUs actually use that memory, I see what AMD has done as an upgrade, actually, and not a downgrade.

Let’s go back to AMD’s older Threadripper platforms. Having used X399 myself, I know for a fact that it is quad-channel and supports up to DDR4-2933 speeds officially. You can fill up all 8 slots for extra memory, but your bandwidth is going to suffer a bit. Remember that this is 4 channels of memory spread across 8 DIMMs. So you can pick either maximum capacity or maximum speed… although for lower slot counts, you can generally have your cake and eat it too (see the first article).

If you want capacity and speed, then ideally you would want 1DPC. But then you run into the project management triangle, and you get sky-high costs for developing a memory controller that can handle 16 channels per CPU (for each of the 16 slots), for example, instead of 8 channels. Admittedly, AMD’s Epyc 9005 CPUs have 12 memory channels and support 2DPC, but you are definitely not running the fastest memory possible if you wish to have 6 TB of RAM in there across 24 slots. Back to Threadripper, AMD giving us 1DPC is actually an improvement, given that they didn’t increase the number of available slots; WRX90 still has 8 slots, while TRX50 has been downgraded to 4. Even if we set aside the fact that DDR5 is faster than DDR4, having 8 channels of memory for 8 slots instead of 4 channels of memory for 8 slots means you can have both high capacity and higher bandwidth (memory speeds = bandwidth), all else being equal.

You can think about this similarly to how process node shrinks improve CPU performance: for the same power, you can get an uplift in performance; or, you can use less power for the same level of performance. Just swap power for number of memory slots, and node shrinks for an increase in number of channels (performance having the same semantics but bandwidth instead of IPC if you want to be technical about it).

Looking at Ryzen, which does have 2DPC, most boards only have 4 slots (and some only 2!). The impact of running 2DPC is basically negligible, and the cost of having proper quad-channel memory would be high for basically nothing (or actually a waste of time for those mini-ITX boards with 2 slots). On the flipside, I’m sure that because 8-channel memory is common on server boards now, this has brought down the costs enough where it could be profitable to develop it for a desktop/workstation platform and ship a nonzero volume at retail to a certain slice of the market. Hi, it’s me, I’m part of that slice.

@H-i-v-e has already said what I would about marketing and product segmentation so I won’t repeat it here.

TL;DR: For having the same number of memory slots, having 1DPC allows you to have both high speed and high capacity, though at a high cost. The more memory slots you have, the larger the performance hit from running 2DPC. Therefore, WRX80, TRX50, and WRX90 having 1DPC is actually an upgrade compared to older Threadripper platforms.

Hope this helps clear up why AMD went with 1DPC on later (and current) Threadripper platforms.

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Short addendum:

This is why your motherboard manual will tell your populate certain slots if you don’t fill every slot (and some people will say 2x 8 GB sticks can be faster than 1x 16 GB stick, assuming same everything else). The problem scales the more slots and fewer channels you have, which servers have a lot of and consumer (even HEDT/workstation motherboards) do not.

W790 would like to have a word with you:

In all honesty I think the reason AMD didn’t implement 2DPC atleast for WRX90 is that 8 slots is “enough”; 64GB RDIMMs were/are cheaper per gigabyte than 16GB RDIMMs, it wasn’t until you got to 128GB RDIMMs that the costs started going up faster than capacity.

Within each product segment, there was either a downgrade or no change though. Threadripper pro shouldn’t be compared to old threadripper, it should be compared to old threadripper pro. of course ipc and memory speed improvements aren’t being factored in because its assumed that they will increase from generation to generation.

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Even old Threadripper PRO, going all the way back to the 3000 series, had 8-channels. Sure, there’s not been any upgrades since it’s always been 8-channel memory for 8 slots, but the fact that AMD created a higher tier where each slot had its own channel (compared to regular Threadripper 3000 series) demonstrates my point.

I don’t think it’s appropriate to compare threadripper to threadripper pro as you suggest with the 3000 series, they two different product segments. The more pertinent question the OP is after is why have we regressed from 8 slots to 4 slots in threadripper across the most recent generation, and why has threadripper pro not offered 2dpc as the competing hedt platform within it’s segment does.

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While some workloads can be DDR clock agnostic, 2DPC’s major for marketing and often substantial to throughput. Have a look at all the threads here obsessing over 2DPC overclocks to get past DDR5-3600. Those often aren’t evidence based but, in benching even really L3 friendly workloads with dual CCD Ryzen, I’ve gotten pretty strong scaling through 4800ish.

The W790 boards I’ve just checked through that I can actually buy all have eight DIMMs per socket, so Xeon W-3400s run 1SPC. Some the boards run W-2400s 2DPC, with others half the DIMM sockets go unavailable. Practically speaking, Zen 4 Threadripper’s thus mostly better. Channel counts are the same more often than not and DDR5-5200’s supported. Could be AMD, ASRock, and Gigabyte decided the niche of customers who’d run 7960-70-80-90X 2DPC at 3600 instead of buying W-2400 or Threadripper Pro was too small to support (not sure about Asus here).

Other than the X13S you’ve shown, the 16 DIMM LGA4677 boards I’m seeing are positioned as server parts. Might be AMD just decided to leave that combination for EPYC and simplify DDR5 Threadripper to 1SPC.

Right. I’m not saying you can’t get high clocks with 2DPC. I’m saying that this gets much more difficult with more slots. Obviously on dual Ryzen (I assume you mean consumer Ryzen) you can get do this without much issue (I assume hitting DDR5-3600MHz/7200MTs is fairly simple, I don’t really keep up with the consumer platforms). Unless I’m completely misunderstanding your statement, this is expected for systems with 4 DIMM slots.

On my EPYC 9004 board at least (Gigabyte MZ33-AR0) which has 24 DIMM slots, you give up a lot of performance going from 1DPC (DDR5-4800) to 2DPC (DDR5-3600). You even give up speed going from 1R to 2R memory at 1DPC.

I’ll also point out that on my board layout, the DIMM slots really cause issues with the PCIe slots (basically you’ll need risers to put in anything full length).

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  1. yes, 2DPC means that you can get less top speed, but not by much, but for ISO MT/s 2DPCs may be faster due to more rank multiplexing (depends on workload), not enough to make up for the difference but still
  2. obviously, you get less memory capacity ISO cost, with 1DPC instead of 2DPC (do not get me started on the cost of LRDIMMs)

I just realized I’ve never seen or heard of a DDR5 LRDIMM before. I’ve seen high rank count DDR5 RDIMMs cap out at 256GB per module, but never an LRDIMM.

Just to clarify for those following along, the W-3400/W-3500 platform allows for 2DPC/2SPC, both the Supermicro X13SWA-TF and Asrock Rack W790D16HM3 have 2SPC for W-3400/W-3500 CPUs, however only the X13SWA-TF is available on amazon/all the normal retailers.
This is in contrast to threadripper pro, but probably also highlights why TR pro kept to 1DPC. Even when given the choice, most board partners chose not to produce 2DPC motherboards for the “pro” segment of hedt ( W-3400/W-3500).

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Could also be that for the kind of people that need 128GB DIMMs and larger, at 8 available channels, money is probably not the issue. They can probably either buy the larger DIMMs or they can buy an Epyc system instead.

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