Epyc Rome (7002) 6 DIMM or 4 DIMM?

Hey all. I am using an AMD Epyc Rome 7742 CPU in an AsRock Rack ROMED6U-2L2T motherboard which has six memory slots.

Looking at the memory configuration guidelines, six DIMMs is not a supported configuration, and it looks like 6 channel interleaving is an advertised feature of 7003 (so presumably it was not a feature with 7002).

Should I leave two slots unpopulated?

https://developer.amd.com/wp-content/resources/56502_1.00-PUB.pdf#page10

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Also I’ll follow that up: with 32GB / DIMM I can either populate with 1Rx4 or 2Rx4 modules. Would it be better for latency to go with 1R modules?

Unless the firmware for the mainboard specifically prevents it, Rome does support 6 channels, but not in a single interleave set, so performance will be very degraded.

Best information about this I found was under “Configuration of 6 DIMMs - unbalanced” in some Lenovo documentation: https://lenovopress.com/lp1268.pdf

Quoting here:

Six memory DIMMs can be populated in a 1:1:1:1:0:0:1:1 configuration as shown in
Figure 11. This configuration follows balanced memory guideline 1, but not guideline 2. This
results in an unbalanced configuration. It results in one 4-channel interleave set and one
2-channel interleave set. Average bandwidth for this configuration was measured at 40%.
Worst case memory bandwidth was measured at 28%

EPYC Gen 3 added support to 6-channel memory interleaving when memory is populated in
the 1:1:0:1:1:0:1:1 configuration as shown in Figure 12. While technically unbalanced when
memory is populated in this specific configuration, a 6-channel interleave set can be formed, and memory bandwidth was measured at 71%.

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I am running that motherboard with 6 DIMMS (2x32GB, 4x16GB) and a 7402P … all working flawlessly, except that the BIOS doesn’t ‘see’ my 16GB dimms (not even when only one pair of them is in) …
The motherboard manual doesn’t mention anything other than 6 channels being supported, and doesn’t list Milan or Rome specifically …

So, from the sound of it, four channels would be the better configuration because at least I would be getting “50%” bandwidth?

To be sure, my workloads run mostly in cache, but if I can save on two fewer modules and get better performance I’d be glad to!

Hmm, in theory no, they are both DDR4-3200 PC4-25600 2Rx8 RDIMM ECC 64 GB kits (Nemix) one is 4x16, the other is 2x32

I wouldn’t know, my ‘workloads’ are a 32GB OSX VM that is my work daily driver, a 16GB Windows 10 VM that is my gaming rig, never really tested the performance out of it, other than using it 10-14 hours a day, both being very fast and responsive, and the thing seldom crashing on me (almost always for something stupid I did, like fiddle in the case when open and bump a PCI-E card …)

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