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Comparing CPUs beyond 2x GHz => 1/2 Time

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Old   March 14, 2019, 08:09
Default Comparing CPUs beyond 2x GHz => 1/2 Time
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So I've been scratching my head on this. I have one Openfoam case that I ran on 3 PCs and recorded the time it took for each PC to reach the same amount of steady state iterations. The versions of Openfoam, Ubuntu, and all other parameters were the same, the only difference between the PCs is their hardware setup. Here's the setups:

PC #1
CPU = Ryzen 1950x (3.4GHz/16 cores)
64 GB of ram (4x 16GB)
4x 8 TB with 256 MB of cash on RAID0 (via the motherboard)

CPU = Intel i5-2400 CPU (3.10GHz/4 cores)
8 GB of ram (2x 4GB)
2x 500 GB + 1 TB USB HD on a software RAID 0

CPU = Intel i7-47900 CPU (3.60GHz/4 cores)
16 GB of ram (2x 8 GB)
2x 500 GB on RAID 0 (via the motherboard)

Time wise, I used PC#1 as my baseline, so 100%, yet PC#2 was 41% and PC#3 was 58%.

So what I'm scratching my head with is, if I use the example of #1 vs #3, which have similar GHz/core, why wasn't PC#3 closer to 25% and PC#2 less than 25%?

I'm trying to estimate how much of a performance gain a future system would have, and I would like to compare options between dual Epyc and dual Xeons, however, my half baked time study seems to tell me that comparing GHz/core between Intel CPUs and AMDs isn't like comparing apples to apples.

Any help is appreciated.
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Old   March 14, 2019, 13:33
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however, my half baked time study seems to tell me that comparing GHz/core between Intel CPUs and AMDs isn't like comparing apples to apples
True, but only part of the issue here.
The same applies to comparing a 2nd-gen Intel Core CPU (probably with slower RAM) to 4th gen. They are not exactly close.
But the main point is: you are trying to work out CFD performance by using aggregate CPU frequency aka Frequency x number of cores. It is a useless metric for many applications: bad scaling or other bottlenecks like memory bandwidth will throw off any estimate.

There are quite a few performance numbers for different platforms in the pinned thread of this subforum. They will give you a better idea of how a platform could perform.
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amd epyc, cpu time, intel processor, openfoam 4.1

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