Few days ago I found out that my 24/7 system with Ryzen 9 3900X fails prime95 torture test when calculating FFTs with a length of 16K or 20K with AVX2 enabled at stock settings.

This seemed a bit strange to me, because I actually never overclocked this CPU, so the degradation can’t really be a problem here. Said tests fail on BIOS defaults (just after clearing CMOS) as well as with my 24/7 profile with infinity fabric clock and memory clock set to 1866/3733, but the CPU is fully stock (no PBO, no voltage offset). Other than this very specific case, system seems to be 100% stable. It passes OCCT Small data set AVX2 test for over 9 hours, it passes prime95 AVX2 with all other FFT lengths in the 1-64K range for hours, but 16K and 20K fails within few minutes. The fails only occur on worker threads 17,18,21 and 22, as far as I know prime95 always assigns same workers to same CPUs, which would suggest that only 2 out of 12 cores are failing. I run prime95 with SUM(INPUTS) error and Rounding checking enabled. I read about other people having similar problems with their Ryzen 3000 series CPUs1, but those reports were mostly about 3950X.

I am not sure if this issue actually occurred with my CPU from the first day or is it something which started only recently. As mentioned before I only observed this behavior few days ago, as I do not run prime95 with smallFFT preset (which includes 16K and 20K) that often. I definitely run it just after I got my new setup working (summer last year), but back then all BIOSes available for my motherboard had awfully broken PPT/TDC/EDC reporting and that made CPU boost behave in very funky way, because of that I cannot really compare situation back then with present.

My setup consists of:

  • Ryzen 9 3900X
  • ASRock X570 Taichi
  • GSkill F4-3200C14D-32GTZKW (2x16GB B-Die)
  • Samsung EVO 850 500GB
  • Palit Gamerock Premium GTX1080
  • Noctua NH-D15S
  • Corsair AX860

So far I tried following things to fix this:

  • clearing CMOS - does not help;
  • latest AMD chipset drivers (2.07.14.327) - nothing changes;
  • different BIOSes for my motherboard - 3.00; 3.20 and 3.40 - all behave the same way;
  • running with just 1 stick of RAM - issue still occurs;
  • enabling PBO - seems to help a bit (the error occurs bit later), this is most likely due to higher VCore being applied to the CPU, but I cannot make it pass the 16K test with PBO alone (maybe I need to test more combinations of PBO params);
  • enabling Vcore offset of +50mV - this seems to fix the issue, but it makes the CPU runs slightly slower (higher Vcore, higher power usage - boost algorithm slows down the CPU faster);
  • disabling AVX2 in prime95 - it is not really a fix, but the issue was not observed with AVX2 disabled.

I tried contacting both motherboard manufacturer (ASRock) and AMD, but I did not get any significant tips. However AMD support acknowledged that it MIGHT BE CPU defect. Both parties suggested trying CPU in different rig, but I can’t do it at the moment.

In my opinion it seems that CPU just can’t cope with prime95 16K FFT AVX2 test, because the voltage/frequency curve on which it operates is just too aggressive. On default settings it should be able to pass everything prime95 is throwing at it. The whole issue is not a big thing, but it is kind of annoying knowing that not so cheap CPU is not fully stable at default settings. My CPU is from early batches as I got it in summer 2019 and from what I heard the silicon quality got better over time, so hopefully new CPUs are totally stable even under prime95 and I am just unlucky with my chip.

I am still investigating this issue and I will update this post when/if I find some solution.

prime95 16k fail screenshot