[SOLVED] Eight vs Sixteen DIMMs with dual EPYC 7443

[maksins]

I am getting a workstation with dual socket EPYC 7443 processor. The motherboard has 16 DIMMs.
As I understand, I should have at least 1 DIMM/stick per channel for balanced performance. Since it is dual EPYC, I have 16 memory channels, so I was inclined to get all 16 DIMMs filled.
My vendor is telling offering me 8 x 32 GB 3200 MHz dual channel memory sticks. I requested 16 x 16 GB 3200 MHz, but he says if I use all 16 DIMMs, my memory may operate at lower frequencies (e.g., 2666 MHz).
I have three questions:
How much of performance different would be in terms of 8 x 32 GB vs 16 x 16 GB?
Would dual channel RAM mean that effectively I should fill X/2 DIMMs for X memory channels?
Is it true that if I fill all 16 DIMM slots then memory may operate at lower frequencies?

Please share your insights.

 

[kanewolf]

I am getting a workstation with dual socket EPYC 7443 processor. The motherboard has 16 DIMMs.
As I understand, I should have at least 1 DIMM/stick per channel for balanced performance. Since it is dual EPYC, I have 16 memory channels, so I was inclined to get all 16 DIMMs filled.
My vendor is telling offering me 8 x 32 GB 3200 MHz dual channel memory sticks. I requested 16 x 16 GB 3200 MHz, but he says if I use all 16 DIMMs, my memory may operate at lower frequencies (e.g., 2666 MHz).
I have three questions:
How much of performance different would be in terms of 8 x 32 GB vs 16 x 16 GB?
Would dual channel RAM mean that effectively I should fill X/2 DIMMs for X memory channels?
Is it true that if I fill all 16 DIMM slots then memory may operate at lower frequencies?

Please share your insights.

Yes, the 7443 is an 8 channel memory controller CPU -- https://en.wikichip.org/wiki/amd/epyc/7443 So with two of them you want 16 DIMMs for maximum memory bandwidth.

 

[maksins]

Yes, the 7443 is an 8 channel memory controller CPU -- https://en.wikichip.org/wiki/amd/epyc/7443 So with two of them you want 16 DIMMs for maximum memory bandwidth.

Thanks Kanewolf for answering. Any idea of performance difference between 8 x 32 GB vs 16 x 16 GB? Does dual channel RAM utilizes two channels. Is it equivalent to saying that I should use 16 DIMMs with single rank memeory or 8 DIMMs with dual channel memory for effectively utilizing 16 channels of my dual socket CPU?
My apologies if the questions seem convoluted.

 

[kanewolf]

Thanks Kanewolf for answering. Any idea of performance difference between 8 x 32 GB vs 16 x 16 GB? Does dual channel RAM utilizes two channels. Is it equivalent to saying that I should use 16 DIMMs with single rank memeory or 8 DIMMs with dual channel memory for effectively utilizing 16 channels of my dual socket CPU?
My apologies if the questions seem convoluted.

The rank of the DIMMs won't change the number of memory channels. The memory channels are mapped to DIMM sockets. To get 8 channel memory, you need 8 DIMMs. With dual socket you need 16 DIMMs.
How much performance you lose will depend on your application and how dependent it is on memory bandwidth. If you are pulling 8K RAW images to process them, then you probably will benefit from the extra bandwidth. Only benchmarking can give you a definite answer. But trying to keep 96 total threads busy, I would want all the memory bandwidth possible.