RAM bandwidth with 4x vs 2x 48GB

[wibionerd]

Hi, all. I am new to building computers and am currently working on a high-end build for both gaming and prime crunching (with Fermat probability testing). Large amounts of RAM are helpful for a "pre-testing" phase of these Fermat tests with more = better. However, it is my understanding that increasing RAM to four sticks from two nearly invariably reduces speed by a large amount. Are there motherboards that are more optimized for four sticks of RAM? I have my eye on the new 28 CL 2x48GB 6,000 Royal Neo. However, I assume I would not want two of these kits and that when four sticks of RAM are used they should be purchased as a kit of four rather than two kits of two. If it helps, I am likely getting the 9800x3d for my CPU. Thanks!

[GSKILL TECH]

There are 4 channel and 8 channel motherboards that can maintain a high DRAM Frequency with high capacity. For example, Intel W790 or AMD WRX90 TRX50

G.Skill Zeta R5 and R5 Neo will have higher capacity memory kits with high frequency. Check the QVL for each memory kit model to see what motherboards have been officially validated for use:

https://www.gskill.com/products/1/400/R-DIMM-Memory

For 2 channel motherboards, 2 x 48GB will likely be highest in the foreseeable future.

[wibionerd]

There are 4 channel and 8 channel motherboards that can maintain a high DRAM Frequency with high capacity. For example, Intel W790 or AMD WRX90 TRX50

G.Skill Zeta R5 and R5 Neo will have higher capacity memory kits with high frequency. Check the QVL for each memory kit model to see what motherboards have been officially validated for use:

https://www.gskill.com/products/1/400/R-DIMM-Memory

For 2 channel motherboards, 2 x 48GB will likely be highest in the foreseeable future.

Thanks for your response. Apologies for my lack of knowledge. I know what I want my PC to be able to do, and some about the requirements for programs I want to run, but the technical details are beyond my knowledge. Aside from gaming, I will also be using both the CPU and GPU (one primary for gaming, 1-2 additional for primality testing). When the CPU is running, a "pre-phase" of the primality test will occasionally allow the CPU to skip the much more computationally intensive full primality test thereby saving 24+ hours of computation time. For this "pre-phase" to be successful, more RAM is better but the odds of this pre-phase being successful are on average still low. The CPU will be slowed down if there is a decrease in RAM bandwidth. I am eying the MAG 870E Tomahawk for its ability to do multiple GPUs. So far, my proposed system looks something like:

-CPU: AMD Ryzen 7 9800x3d
-GPU AMD Radeon Sapphire NITRO+ RX 7900 XTX Vapor-X (for gaming)
-GPU 2: AMD Radeon VII (for primality testing)
-GPU 3: ?? AMD Radeon VII (for prime testing)
-MOBO: AMD MAG X870E Tomahawk WIFI
-RAM: G Skill Trident Z Royal Neo DDR5 6000 2x48GB CL 28
-SSD: Seagate FireCuda 530R 2TB -PSU: Seasonic VERTEX GX-1000 80+ Gold ATX 3.0

I would definitely consider increasing to 4x48 GB 6000 CL 28 unless this isn't advised due to it necessitating two kits. I don't think the 9800x3d is rated for anything faster than 6000, and I think if I'm reading the specs correct for the X870E, putting 4 sticks of RAM would make the max speed something like 4800 (again, not entirely sure of this).

[emissary42]

Official AMD spec with four modules is as low as DDR5-3600. Anything above that is not guaranteed and considered overclocking. From experience at least DDR5-4400/4800 usually works, with matching modules/kits.



The MSI MAG X870 Tomahawk also has at best two usable PEGs, one at PCIe 5.0 x16 and the other at PCIe 4.0 x4. The third slot is only connected via a single PCIe 3.0 lane. There are a couple of other AM5 motherboards, like the ASUS ProArt B650-Creator, that instead support a x8/x4/x4 configuration.

In general I would agree with GSKILL TECH , your requirements are pushing what is possible with a mainstream platform. The additional PCIe lanes and memory channels make something like AMD sTR5 or Intel 4677 more suitable. Their downside is, not exactly being geared towards gaming performance (compared to a 9800X3D setup).