MP 7,1 Another RAM Q about mixing capacities with same spec

[Gr1f]

Hey folks,
I have read that the general consensus is to remove the shipped apple memory, in this case 4 x 8GB 2933 MHz DDR4 R-DIMM memory modules and replace with the new.

Is there a specific reason for this?

I've 4 new Nemix 32GB 2933MHz DDR4 R-DIMMs and apple's site https://support.apple.com/en-us/HT210103 states that you can mix capacities as long as they're the same spec.

So Apple state "Install the 32GB DIMMs in the first slot of each channel pair.....

I have planned to get another 2x 32GB Nemix DIMMs in a few months but I'd leave the App memory in there if there is negligible performance hit vs 4 X 32GB DIMMs only.

Your insight as always very much appreciated!

 

[Gr1f]

The above layout I added was incorrect, sorry. It's

Seems to work fine but the question still remains. Is there some sort of significant performance penalty?

Edit:
Just ran a Novabench memory test:
RAM Speed: 9059 MB/s

Cant seem to find anything to bench that against tho.
Update...
I did find a few more on Novabench
128GB RAM Speed: 9305 MB/s. (no idea on config)
96GB DDR4 RAM Speed: 17772 MB/s
384GB DDR4 R-DIMM RAM Speed: 8042 MB/s
96GB DDR4 R-DIMM RAM Speed: 8738 MB/s
48GB DDR4 RAM Speed: 17554 MB/s
128GB RAM Speed: 8635 MB/s

Seems to vary quite a bit.

 

[DrEGPU]

I have 288GB of RAM in mine: 6x Kingston 32GB (from a PC build); 4x 32GB Nemix; and 4x Apple 8GB (that originally came with it). It works just fine in both MacOS and Windows bootcamp.

 

[Gr1f]

Yep, I get that it works but just wondering if there's a performance hit by mixing the sizes and if there is does it mean much in the real world.

 

[codehead1]

Yep, I get that it works but just wondering if there's a performance hit by mixing the sizes and if there is does it mean much in the real world.

At a glance, I believe you have your DIMMs in the optimal configuration. And to the basic question, it's pretty simple: With six channels of which you're using four, you'll get 4/6 or 2/3, 67% memory bandwidth. The biggest thing to avoid is 2/6 utilization, at 33% usable bandwidth. 4/6 is a 100% improvement over 2/6, 6/6 is only a 50% improvement over 4/6, so you're doing OK.

Recognize that CPUs avoid going to memory when they can, so 67% memory bandwidth doesn't mean you'll get 67% performance. It's not ideal, but fine till you pick up another couple of DIMMs in the future.

 

[Gr1f]

At a glance, I believe you have your DIMMs in the optimal configuration. And to the basic question, it's pretty simple: With six channels of which you're using four, you'll get 4/6 or 2/3, 67% memory bandwidth. The biggest thing to avoid is 2/6 utilization, at 33% usable bandwidth. 4/6 is a 100% improvement over 2/6, 6/6 is only a 50% improvement over 4/6, so you're doing OK.

Recognize that CPUs avoid going to memory when they can, so 67% memory bandwidth doesn't mean you'll get 67% performance. It's not ideal, but fine till you pick up another couple of DIMMs in the future.

Thanks @codehead1 very helpful!