[Duke711]

Testcase: Fluent 2,2 Mio.



AMD 7900x "suck up"



Intels E-cores works for CFD. 13900 has only 8 P (Power Cores) an speed up with more select Cores in the Setup.




https://www.dropbox.com/s/on91aqe5zi...luent.jpg?dl=0

[Habib-CFD]

Hi, something is going wrong with your benchmark:
On 8 cores the 13900k result should be at least twice as fast as the ancient 2695 v2 regardless of the memory bandwidth limit!

[Duke711]

Originally Posted by Habib-CFD

Hi, something is going wrong with your benchmark:
On 8 cores the 13900k result should be at least twice as fast as the ancient 2695 v2 regardless of the memory bandwidth limit!

No, the 13900k has only two Memory Channel. The Memory Bandwith of the 2695 v2 with eight Memory Channel and 110 GB/s is higher than 85 GB/s of the 13900 k. The 13900 K has to small Memory Bandwith.

[flotus1]

Please disable the E-cores in Bios, disable Hyperthreading, and run the test again with 8 threads.

[Duke711]

SMT has no very effect of Solution Time; ~ 5% - 10%. But Fluent is by select too many Cores on too small Memory Bandwidth very buggy. Solution Speed can by solving very slow down, with and without SMT. By a repeat the results, by 12 and 16 selected Cores, were a same. Only a Bug on 8 selected Cores. Now i think the Solution Time on 8, 12 and 16 selected Cores are the same. The difference between 291, 322 and 286 seconds are probably measuring tolerance. The 13900 K has to few Memory Channel to find out that E-Cores to perform on CFD. I think that is possible.




SMT on, E-Cores on


https://www.dropbox.com/s/xl8np06bcs...uent1.jpg?dl=0
https://www.dropbox.com/s/euthm0u6mz...uent2.jpg?dl=0
https://www.dropbox.com/s/iik0mavptq...uent3.jpg?dl=0

[flotus1]

The reason I brought this up: I highly suspect that the results you got initially were mostly influenced by scheduler issues. I.e. the operating system not being clever enough to pin the threads exclusively to performance cores.
Now you could try to manually pin threads and monitor how that goes... or much easier, just disable SMT and E-cores.
If I am reading this right, your second batch of results confirms my suspicion. Since you were able to get pretty much maximum performance on 8 threads.

[Duke711]

SMT and E Cores off has no effect-


SMT and E Cores on:


4 Cores:-> 353 seconds
6 Cores:-> 316 seconds
8 Cores:-> 291 seconds


SMT and E Cores off:

4 Cores:-> 380 seconds
6 Cores:-> 325 seconds
8 Cores:-> 270 seconds

[flotus1]

Then what happened with the results in the first post here?

Anyway, I think the hypothesis stated in the first post -about E-cores in current-gen desktop CPUs being useful for CFD- has been thoroughly debunked. Not that I had much doubt about that, but it can't hurt to check from time to time.

[Duke711]

with only one P Core:


E Cores only runs with 4300 Mhz (P Core / 5500 Mhz)




SMT on, all E Cores on, seven P Cores deaktivated:

4 Cores:-> 600 seconds
6 Cores:-> 579 seconds
8 Cores:-> 440 seconds



7900X ; DDR5 3600 Mhz

8 Cores:-> 443 seconds



2x E5 2695 v2

4 Cores:-> 967 seconds
6 Cores:-> 743 seconds
8 Cores:-> 534 seconds




E Cores works for CFD. Slower (only -15% performance lost -> 5500 / 4300 or 7900X ) but with very high efficiency and very low power consumption.

[wkernkamp]

Why did you not run the 7900x at DDR5-5200?

[flotus1]

E Cores works for CFD. Slower (only -15% performance lost -> 5500 / 4300 or 7900X ) but with very high efficiency and very low power consumption.

Let's stick with comparing apples to apples. The P-cores on the I9-13900k are 63% faster than its E-cores, according to your results.
Comparing it to a knee-capped, different CPU is not the point.

The point about E-cores being useless for CFD is this: You can not run a simulation across both P- and E-cores. It will limit execution speed to whatever the slower E-cores can handle. Now you could start to get creative with load balancing, but: the 8 P-cores already provide enough FP performance to saturate the memory subsystem. It gets even worse when per-core licenses are involved.