Ryzen 9 3950x Benchmark Machine: What should I test for you?

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Re: Ryzen 9 3950x Benchmark Machine: What should I test for

Postby MeeLee » Tue Aug 04, 2020 4:46 pm

There's got to be an explanation as to why the 3950x runs lower PPD on between 16-25 threads. It's either running on lower frequency, or something else inside the system is slowing down (IF,...).
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Re: Ryzen 9 3950x Benchmark Machine: What should I test for

Postby bruce » Tue Aug 04, 2020 6:49 pm

FAH makes very heavy use of the FPU to do FP32 operations. Making use of HyperThreading/SMT means there will be additional inter-thread competition for FP32 resources, whether or not there is a change in temperature which alters the effective computation rate. The second "half" of a HT pair contributes less resources than the first "half" because the shared resource is busy much of the time.
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Re: Ryzen 9 3950x Benchmark Machine: What should I test for

Postby _r2w_ben » Tue Aug 04, 2020 10:57 pm

I don't have a high core count machine to test on, but this is my theory:
At 16 and 32 threads, the OS is preempting FAH more often than when there is 1 physical core free.
In the 17-23 range, 1 of 17 (5.8%) to 7 of 23 (30.4%) of threads are bouncing around taking CPU time from 16 threads allocated to a physical core. Scheduling this efficiently is challenging. Since the threads need to synchronize to exchange data regularly, the slowest thread (allocated the least CPU time per second) determines overall performance.
24-31 still has scheduling challenges but since there are more threads, there should be less frequent migration between cores. Most physical cores are juggling two threads so all threads move forward at a more steady pace.
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Re: Ryzen 9 3950x Benchmark Machine: What should I test for

Postby MeeLee » Wed Aug 05, 2020 4:25 am

The 3950x has 8x quadcore chiplets, that in most motherboards at least 4 can run independently of one another in terms of CPU frequency (unless you select all core load in the BIOS).
My experience, with up to 16 cores, the CPU gets enough power from the VRMs, to run high frequencies.
16-25 cores, and the CPU sees a non-100% load, so lowers the CPU frequency. Haven't tested it yet on a non PBO enabled setup, but with PBO, that's what my motherboard does.
Then once you surpass 25 cores, the motherboard probably overrules the CPU TDP of 105W, and gives the CPU all it's got, so all thread get another frequency boost, thanks to the extra power the motherboard feeds the CPU beyond the AMD specifications.

I think this might be what happens, but I can't confirm, since I don't have an AMD system running right now (offline for maintenance).
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