[Andrew2]

Hi everybody,

I was looking at building a workstation for CFD, although I might do some gaming on it. I plan to run steady and unsteady simulations with mesh sizes from 1m to 25m cells, and some small LES test cases.

After some research, I somewhat concluded that i9-9920x is the best CPU if I want to keep the cost at $2000ish, is this correct? Are there any other better options out there? AMD processors are restricted to two memory channels except for the much more expensive AMD EPYC processors. Is there also any significant benefit to using an Intel Xeon processor? The Intel Xeon W-3235 looks to be the equivalent of the i9-9920x, is the extra $200 worth it?

For the motherboards, are there any guidelines to choosing the correct one for CFD? The EVGA X299 Micro ATX 2 appears to be a good match.

Finally, is it preferential to use a liquid cooler?

[flotus1]

i9-9920x is the best CPU if I want to keep the cost at $2000ish, is this correct?

It is one option, but at around 1250$ it might be hard to put together a well-balanced system in the 2000$ range.

AMD processors are restricted to two memory channels except for the much more expensive AMD EPYC processors.

There is AMD Threadripper to fill the gap. For now, you could get the 16-core 2950x for ~800$. There are of course some trade-offs to be made compared to Intel X299, which affect non-CFD workloads like gaming.
The 3rd generation will be released this year, and it is safe to assume that it will be better than anything Intel has to offer on X299.

Is there also any significant benefit to using an Intel Xeon processor?

"Benefit": ECC support. Downside: no overclocking

For the motherboards, are there any guidelines to choosing the correct one for CFD?

There are no special requirements concerning the motherboard.
But with the µATX motherboard you have in mind, you would need to start with 4x16GB of memory, because it only has 4 DIMM slots.

Finally, is it preferential to use a liquid cooler?

Unless you intend to do some serious overclocking which would require a decent AIO water cooler (360mm or larger), most high-end air coolers in the 80$ price range will do the job. Cheap AIOs are not inherently better than air coolers in the same price range. An I9-9920x will run hot regardless of the cooling solution, due to the cheap thermal interface material used in these CPUs.

[Andrew2]

Thank you for your reply.

Are there any benefits of getting the 2950x vs the 2920x since you're restricted to 4 memory channels and unable to get full usage out of the extra 4 cores? I have seen people suggest 3 cores per DIMM slot.

Maybe I'm missing something, but why would I need to start with 4x16GB of memory? Why would 4x8GB not be allowable with the motherboard?

[flotus1]

In workloads that are not NUMA-aware -like most software outside of the engineering and scientific realm- you can only use half of the cores effectively. So having 8 cores on one NUMA node helps.
Apart from that, considering the cost for the whole workstation, 180$ extra for the 16-core model seem like a no-brainer to me. 2-3 cores per memory channel is a rule of thumb when paying for per-core license fees. You can still get higher performance beyond that, the increase is just non-linear.

Maybe I'm missing something, but why would I need to start with 4x16GB of memory? Why would 4x8GB not be allowable with the motherboard?

Nothing prevents you from using 4x8GB RAM. But there are downsides to it.
If you ever want to upgrade to 64GB total, you have to swap out all of the memory. And 25M cells will require more than 32GB of RAM, at least when using double precision in most solvers. Fluent coupled solver in double precision for example can use over 3GB of RAM per 1million cells.
And there is the issue of internal memory organizaton. 8GB DDR4 UDIMMs are mostly single-ranked, while 16GB UDIMMs are still predominantly dual-ranked. The latter provides higher performance in memory bandwidth intensive workloads like parallel CFD.

Edit: which software are you using? Do you have enough licenses for 12 or 16 cores?

[flotus1]

Some recent developments that are definitely relevant to you: https://www.anandtech.com/show/14925...for-under-1000
tl; dr: Intel brings their next generation of CPUs for the X299 platform with huge price drops. Availability in November.

To avoid buyers remorse, hold your purchase until November if you can.
Seems like they already felt the pressure from next-gen Threadripper CPUs.

[Andrew2]

Yeah, I saw the news on Intel's 10th generation processors today!

I was planning to wait until November/December to make my decision to begin with to take advantage of Christmas vacation to assemble the workstation and to see what AMD is offering with their 3rd generation Threadripper.

[Andrew2]

Hi, a further question on RAM frequency. AMD threadripper and the new Intel i9's have a support for RAM with a frequency of 2933MHz. Reading other posts, processors can generally handle higher frequency RAM but does the processors actually make use of the higher frequency or will it be limited to 2933MHz? If it does, is the benefit worth the extra cost?

[flotus1]

The extra cost is rather low these days, and the benefit is substantial, at least for CFD.
Threadripper 2nd gen is still picky when it comes to memory frequencies beyond 3200MT/s. Choosing the right memory modules becomes important, and some manual tuning might be required.
Intel CPUs in general are better at running high memory frequencies. But the memory controllers used here have their drawbacks, e.g. very low bandwidth per core: https://www.anandtech.com/show/11544...-the-decade/12

Anyway, for the current gen HEDT CPUs I would settle for DDR4-3600. It is the sweetspot in terms of price/performance.
I know this might sound counter-intuitive, but for a CFD workstation, excess budget is usually better spent on memory than on a CPU that is slightly faster on paper.

[Andrew2]

Hi,

With the specs of the new Intel i9's and AMD threadripper mostly revealed, is the high cost of the new Threadripper 3960x worth it? Will only having 4 memory channel hinder 3960x's performance for CFD? For the same price of the 3960x, I could get the EPYC 7402p and trade a lower clock speed for more memory channels. The 10th generation Intel i9's or the older threadripper 2950x appear to be a better deal. Is this a fair assessment?

[flotus1]

At 1400$ for the smallest Threadripper SKU, it is a tough sell for our purpose. I have no doubt that it is faster than any I9 CPU, especially for CFD. But since Intel cut the prices for their HEDT lineup in half, they are actually the better value. If only there was an entry-level Threadripper CPU with 16 cores, and a price that did not reflect the leap in performance between Zen and Zen2
Prices for TRX4 motherboards are not yet final, but I expect them to be higher than entry-level X299.
If you don't need the high clock speeds for FPU-limited or single-threaded tasks, Epyc Rome is the better choice compared to Threadripper 3000 series.

[Simbelmynë]

How about the old 1920X? It has very competitive pricing now (around 300 Euros). It should be slightly faster than a 3700X, with tight memory timings, at a similar total price point.

[Andrew2]

Yes, the 1920x is a possibility but the 2920x and the i9-10920x have a higher boost frequency for a higher cost. I have heard rumors that Intel might reduce their 10th gen more before their launch date of Nov 25th?. I guess this something I have to decide whether I want to more money.


I have also being toying with the idea of getting an AMD Epyc instead. Does the performance of an Eypc suffer if you use it in a single socket configuration? Looking at the benchmarks, I could find two tests for a single Epyc 7351, but the results between the two tests differ greatly. The test on first page shows it only performing marginally better than a threadripper or an i9, and the other test (page 7) shows the 7351 as being twice as fast when using 16 cores. A single Epyc build looks to be $500-700 more than a desktop CPU build while a dual Epyc build looks to be $2000 more.

For the 2nd generation of Epyc, there is the 7282 and 7302. Does doubling the L3 cache significantly effect CFD computations?

[Simbelmynë]

Where I live the 2920x is 50% more expensive than the 1920x. There is no availability of 10920x.


If you are not trying to build on a tight budget then there are many options. EPYC is probably the best value.