[CeesH]

Hi all,

I am trying to set up budget estimation for a new CFD workstation and could use some advice for processors, considering the wide range of available options. I work with FLUENT, HPC pack allowing max. 32 cores per simulation. What processor would you advice to make good use out of the licensing options, with good performance for money?

Thanks in advance!
Cees

[flotus1]

You have two different optimization goals here:
1) maximum performance per dollar: 2x AMD Epyc 7351
2) maximum performance per core: 4x Intel Xeon Gold 6144

Option 1 will cost you somewhere around 8000$ upwards depending on the rest of your hardware requirements. Option 2 is in the price range of 20000$ and will be faster overall, somewhere in the ballpark of 50% with 32 cores used.

[CeesH]

Hi Alex,

Thanks for the reply - I didn't word it very clearly indeed, my apologies.
I didn't necessarily literally mean top dollar/performance or top absolute performance, but a good balance between the two. I think a price range of 4-8kE for the processors alone would be manageable; it's still quite broad as we're in a very exploratory phase.

In any case, what would be the advantage of using 4x 8 over 2x 16 cores? Is it because there are no 16-core processors with similar per-core specs on the market? Or other specific benefits?

Best,
Cees

[flotus1]

There are three factors that affect parallel CFD performance.
1) available memory bandwidth
2) CPU clock speed
3) CPU performance normalized by clock speed, sometimes broken down to the simple term "IPC"

In terms of (3) AMD Epyc is slightly better than Intels Skylake-SP.
As far as clock speed is concerned, the more expensive versions of Xeon Gold and Platinum have higher clock speed than Epyc processors, which more than compensates for lower IPC in case of the high clock speeds a Gold 6144 offers.
So it comes down to memory bandwidth: Skylake-SP has 6 memory channels DDR4-2666, AMD Epyc has 8 channels DDR4-2666. 24 channels total vs. 16 channels total.

Why not take a more expensive Intel 16-core model? You pay more and get less compared to AMD Epyc 16-core CPUs: 12 memory channels total vs 16. High clock speeds can not compensate a lack of memory bandwidth for parallel CFD.

[ErikAdr]

A compromise in terms of price and performance could be 4 x AMD EPYC 7251. In this setup the number of memory channels equals the number of cores, and the memory bandwidth will be more than sufficient for the cpu performance, even for cfd. I think you will need a fast interconnect i.e infiniband between two dual processor boards.

[Micael]

Just so you know, you might actually have access to 36-core licence. See release note R19 (attached picture), you might have a 4-core HPC that can now added to HPC pack. I did test it with 12-core on FLUENT and it indeed consummed only 1 HPC pack (8-core from 1st HPC pack + 4-core base allocation).

[CeesH]

Thanks both; I'll have our IT support look into the epyc options. I guess hyperthreading can be turned of as desired?

Best,
Cees

[CeesH]

Hi Micael,

You are correct - just checked on the current system, thanks. I'll take it into account.

Cees

[flotus1]

Originally Posted by CeesH

Thanks both; I'll have our IT support look into the epyc options. I guess hyperthreading can be turned of as desired?

Best,
Cees

Yes, SMT can be turned off with Intel and AMD processors. And I would still recommend it.

[canopus]

Sorry to barge in.

Will AMD EPYC™ 7451 (Base speed 2.3 GHz) be worth over AMD EPYC™ 7401 (2GHz)?

The price difference is $600 and I am going for Dual socket system

To offset the cost will have to halve the RAM to 128GB.

If the difference in clock speed will mater then off course I can upgrade RAM later.

The intended simulations are LES combustion.

Lastly, Is on board graphics card enough?

Thanks

[flotus1]

Take a look at AMDs data sheet: https://www.amd.com/system/files/201...Data-Sheet.pdf
7401 has an all-core turbo of 2.8GHz, 7451 gets 2.9GHz. The only question mark that remains here is if these 24-core processors are capable of sustaining turbo frequencies over prolonged periods of time. I honestly don't know.
Personally I consider none of the 24-core CPUs worth the price increase over the 16-core models.
However: buying 128GB now with an upgrade path to 256GB would mean that you do not populate all memory channels, at least on most dual-socket SP3 motherboards that only have 16 slots. And if you buy a motherboard with 32 slots and populate all of them there is a high chance you won't be able to run maximum memory frequencies unless you get LRDIMM which has higher latencies.
So my advice would be: get as much memory as you actually need, distributed across 16 DIMMs. This is more important than choosing between 16 or 24 cores.

Edit: Onboard graphics on SP3 boards is not sufficient for most things you do outside of a command shell. Get at least a GTX 1050TI if you want to do some pre- and post-processing with a GUI tool.