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Dual or single rank RAM for Ryzen rendering workstation?

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A mainstream AM4 motherboard with Ryzen PRO CPU and four sticks of DDR4 ECC memory totaling 64 GB in dual channel - that is the goal. Now the question.

Would 4* dual rank, 16 GB memory sticks offer benefit over 4* single rank, 16 GB memory sticks in rendering programs? Talking about GPU based SPECWorkstation rendering stuff. Anyone did test this? Let's say you are working with a GPU that has 8 GB of VRAM and you are running out of GPU VRAM in order to render stuff - then you need system RAM - would dual rank offer any impact in this bottleneck case? We need benchmarks.
 
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Dual rank only matters when you only have 2 sticks.
 
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A mainstream AM4 motherboard with Ryzen PRO CPU and four sticks of DDR4 ECC memory totaling 64 GB in dual channel - that is the goal. Now the question.

Would 4* dual rank, 16 GB memory sticks offer benefit over 4* single rank, 16 GB memory sticks in rendering programs? Talking about GPU based SPECWorkstation rendering stuff. Anyone did test this? Let's say you are working with a GPU that has 8 GB of VRAM and you are running out of GPU VRAM in order to render stuff - then you need system RAM - would dual rank offer any impact in this bottleneck case? We need benchmarks.

What matters is ranks per channel. Assuming the same 8Gb IC, a 4x8GB (single rank DIMMs) setup and 2x16GB (dual rank DIMMs) setup perform similarly because on a dual channel platform you'll still get 2 ranks per channel. A 4x8GB single rank kit and 4x16GB single rank kit wouldn't be any different on performance, still 2 ranks per channel.

But it's ECC memory anyways so you're probably going to be stuck on a JEDEC profile. 3200CL22 or 2666CL19 @ 1.2V etc......basically, thoroughly unimpressive performance for DDR4 regardless, single or dual rank matters little. If the cost in raw memory performance running ECC is something you care about more than ECC, then it sounds like ECC wouldn't be for you. If you have a real need for ECC, no point worrying about performance.

Where you *might* see lower performance would be a 2x16GB kit that uses 16Gb ICs (single rank DIMMs), which are becoming more common these days. And 4x16GB dual rank DIMMs would be harder to drive than 4x16GB single rank DIMMs resulting in lower max achievable freq on your board, but again, ECC won't come in high enough speeds.
 
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If you want 64GB I would say go with 2x32GB.

Generally you want to stick to 2 ranks per channel to keep the memory controller load lower. The alternative would therefore be 4x16 with single rank sticks.
 
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The thinking on this is kinda... all wrong. You want speed but not at the cost of stability. Stop focusing on ranks, it doesn't matter as its noot gonna run at whatever advertised speed anyways at high densities. And then add ECC yea, ugh rinse repeat. I'd also add that ECC is not necessary for a render station.
 
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Doesn't matter whether you are after speed or reliability, lower IMC load is better and single rank loses performance. The optimum therefore (unless you *need* more capacity) is 2 ranks per channel.
 
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First of all yes, ECC is priority over speed, and there will be no overclock on CPU or RAM for that matter. Having said that, i appreciate clearing about ranks per channel. There is indeed a need for 64 GB of RAM (among demanding RAM programs Adobe After Effects will be used for sure), which means, as explained by you, it could be either 2X32 GB in dual rank config or 4X16 GB in single rank config.

Now to clarify: 4X16 GB in dual rank is not recommend, because of higher MC strain? Seriously? But this is not a server, where all the RAM would be used almost all the time. This amount will be used in rare cases, so it can not be bad for memory controller. Surely you are overreacting, right?
 
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Conveniently hidden in the reviewer guides (and not really talked about much any more outside of threadripper and epyc) AMD has these specifications for AM4 (3rd gen) and Threadripper (3rd gen TRX40) respectively

1621521268927.png

1621521314376.png

So even 2 DIMMs per channel with single rank DIMMs drops you from official 3200 supported speeds. 2 DIMMs per channel with dual rank DIMMs drops you down to 2666... Based on what I've seen of the memory controllers, they are plenty capable of running 2 DPC with SR DIMMs at 3800+ stably, but in terms of good practice when looking for stability in a server/workstation system I would always tend towards the 1 DIMM per channel with dual rank DIMMs config, as this balances the lower IMC load with being able to take advantage of rank interleaving for performance.

In terms of servers: Many high density Epyc platforms only run 1 memory slot off each channel, the footprint of the socket+8 DIMM slots is already big, doubling that to 2 DIMMs per channel puts you at 16 which is absolutely enormous. As such, a lot of deployments seem to stick with 1DPC and where necessary capacity wise will just use RDIMMs or LRDIMMs to get additional capacity.
 
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