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Old   March 11, 2020, 05:00
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Charlie Lloyd
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Hi all,

I'm currently in the process of putting together a large desktop for carrying out CFD tasks and processing of laboratory data. Our current CPU choice is the

AMD Threadripper 3990X 2.9 GHz 64-Core Processor

But I am having doubts about bandwidth requirements of CFD. We will be using this for OpenFOAM and Nek5000 simulations, and possibly Fluent. Does anyone have any experience with this processor, or any thoughts on how it might perform for CFD tasks?

Thanks in advance!
Charlie
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Old   March 11, 2020, 05:24
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That's a definite "hell no" from me

Same as the TR 2990WX, the TR3990X is a horrible value for anything remotely CFD-related. Scaling for parallel CFD on these CPUs stops at around 8-12 cores, thanks to the narrow memory interface. You still pay for the remaining ~50 cores, but you can not use them.

A much better choice would be a system with 2 Epyc Rome CPUs. Already the "entry-level" Epyc 7302 (16 cores) will run circles around a 3990X. Depending on your total budget, you can also get CPUs with 24 cores (Epyc 7352) or 32 cores (Epyc 7452) each.
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Old   March 11, 2020, 05:35
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Thanks for the quick response - I did have doubts about it but I wasn't expecting it to be as bad as that! I will have a look at your suggested options.
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Old   March 11, 2020, 05:49
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From what I've read so far, memory bandwidth seems to be the limiting factor when it comes to CFD calculations.

So 8 memory channels are a lot better than 4. In combination with 3200 MHz RAM that is now supported by the 7XX2 generation that should not be a problem anymore.
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Old   March 11, 2020, 09:55
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Quote:
Originally Posted by EagerToLearn View Post
So 8 memory channels are .... 7XX2
Note that the 72X2 series only supports 4 channels. The 73X2 and 74X2 series support 8 channels.
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Last edited by Habib-CFD; March 11, 2020 at 17:04.
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Old   March 12, 2020, 12:29
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Quote:
Originally Posted by Habib-CFD View Post
Note that the 72X2 series only supports 4 channels. The 73X2 and 74X2 series support 8 channels.
WOW!!! Good catch! I didn't realize that as everything says 8 memory channels!
Looks like it can only support 4 at a time though and it doesn't increase with more than 4 channels.

That's really misleading!
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Old   March 15, 2020, 04:51
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This is not correct. The Epyc 7262 got the same memory bandwidth as the 7302. The CPU's with 'half' bandwith are CPU's with 64 MB cache. Awoid them for CFD and prefer CPU's with at least 128 MB cache. It is not the cache itself that maters, but the 64 MB parts use just 2 chiplets, whereas the 128 MB parts use 4 chiplets, and the bandwidth between the in/out chip and the chiplets become the limiting factor for CPU's with just two chiplets. All Epyc CPU's support 8 memory channels, even though the 64 MB parts can not pass the bandwidth all the way to the chiplets.
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Old   March 15, 2020, 06:01
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Quote:
Originally Posted by ErikAdr View Post
This is not correct. The Epyc 7262 got the same memory bandwidth as the 7302. The CPU's with 'half' bandwith are CPU's with 64 MB cache. Awoid them for CFD and prefer CPU's with at least 128 MB cache. It is not the cache itself that maters, but the 64 MB parts use just 2 chiplets, whereas the 128 MB parts use 4 chiplets, and the bandwidth between the in/out chip and the chiplets become the limiting factor for CPU's with just two chiplets. All Epyc CPU's support 8 memory channels, even though the 64 MB parts can not pass the bandwidth all the way to the chiplets.

You mentioned the only exception to the 72X2 series, but don't forget that the 7262 has 8 cores and of course cannot be between the candidates.


AMD direct references:
"Performance optimized to 4 channels with 2667 MHz speed DIMMs for the following models: AMD EPYC™ 7282, AMD EPYC™ 7272, AMD EPYC™ 7252, and AMD EPYC™ 7232P processors. Additional memory channels will not increase overall memory bandwidth."


In a simple words, the 8 memory channels with 85.3 GB/s of limitation means 8 DIMM's!
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