Arch Linux

Pulec

Member

Registered: 2014-07-06

Posts: 24

Automatic allocation on Ryzen 7 {7,9}800X3D vs Ryzen 9 {7,9}950X3D

In a simplified way Ryzen 7 {7,9}800X3D is 8 core CPU where all cores have access to the L3 3D V-Cache.
Whereas Ryzen 9 {7,9}950X3D has two CCDs but the other 8 cores (4 cores in case of {7,9}900X3D variant) have regular L3 cache.
In other words, the Ryzen 9 is two Ryzen 7 in one package, but in the case of X3D models, only 8 cores have access to the L3 3D V-cache.

Thus it makes sense to allocate CPU cores with extended L3 3D V-Cache to applications which has use for it, which are mostly CPU-intensive games (Dwarf Fortress and Factorio are often used in benchmarks).
One can read that for example Ryzen 9 7950X3D is often slower in games than Ryzen 7 7800X3D just because CPU cores without L3 3D V-cache are used and most games won't somehow test and pick the fastest cores.

In my case, I use OBS with software encoding to record my gameplay. So it would be relatively simple to use

taskset -c 0-15 %command% 

for each Steam game (as some people on r/linux_gaming advise) and use taskset for the unused cores for OBS or some hungry process. The chance of user error is still huge though, it would be awesome if Kernel could do this for me somehow.

How does core allocation work in the kernel? Is there some experiment to automatize this for these exact CPUs?

I decided to upgrade from Ryzen 7 5800X3D on AM4 to AM5, so 9800X3D is an easy choice, but I am tempted by 9900X3D or 9950X3D releasing in a few weeks.
Currently from my OBS stats, gameplay recording eats about 8-10% CPU, so it's generally only a small bite in performance, but if I can dedicate 8 cores that would otherwise not be used by the game anyway (afaik few games allocate more than 8 cores) that would be ideal. In an optimal scenario, the whole system would run on the CCD with regular L3 and Steam games would use the CCD with L3 3D V-Cache.

BTW: I stopped using GPU encoding since there can be lots of lost frames when games are using 99% GPU. Using some kind of HDMI splitter and recording on another computer is out of the question just because of the complexity, additional delay, and capture cards having issues with 21:9 resolutions.

Maybe Level1Techs video Core Parking On The 7950X3D, Explained! is relevant here (e.g. at 12:00 mark for the OBS case) even though the tested system runs Windows there.

P.S: The character limit on topic name (subject) made it tricky to summarize what I mean, I hope it will be easy to search in the future

Last edited by Pulec (2025-02-17 05:23:11)

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IRC: pulec

Pulec

Member

Registered: 2014-07-06

Posts: 24

Re: Automatic allocation on Ryzen 7 {7,9}800X3D vs Ryzen 9 {7,9}950X3D

I missed Level1Techs video Linux on the 7950x3d, so either do the BIOS tweaks and manual taskset tweaking or just Ryzen 7 *800X3D and don't care for anything? Hmm.

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IRC: pulec

Joshmoooze

Member

Registered: 2025-07-06

Posts: 1

Re: Automatic allocation on Ryzen 7 {7,9}800X3D vs Ryzen 9 {7,9}950X3D

How is the CPU holding up?

Pulec

Member

Registered: 2014-07-06

Posts: 24

Re: Automatic allocation on Ryzen 7 {7,9}800X3D vs Ryzen 9 {7,9}950X3D

How is the CPU holding up?

It has been almost 4 months, and I was lucky to survive the OS unrelated HW issues mentioned here, which were, afaik, related to memory timings, boost clocks or something (too lazy to search for that) that caused spikes of high voltage that could burn the CPU. New AGESA in BIOS lowers the boost clocks, lowers the performance by a tiny bit, but there should be no more voltage spikes.

Besides that, it does happen that the system takes a long time to POST and somehow detects only one memory stick, turn off, wait and turn on usually fixes it, a simple reboot doesn't. But my specific 48G sticks weren't verified with ASRock X870 Taichi Lite, so verified one might work fine.

Anyway, the only thing I tried was changing

/sys/bus/platform/drivers/amd_x3d_vcache/AMDI0101:00/amd_x3d_mode

from frequency to cache, this is described in phoronix AMD's 3D V-Cache Optimizer Driver For Squeezing More Ryzen 9 9950X3D Performance article.

But I didn't do any test myself, things work. I do a lot of gaming and record the gameplay and then reencode to HEVC (ffmpeg -i $1 -c:v libx265 $2), and using multiple processes doesn't slow down the others too much, but that's what you would expect from any 16-core CPU.

As for gaming, again, things just work, and I haven't compared how games run on 9800x3d with Nvidia 4070 super, since everyone uses different settings + obs recording eats a 1 or 2 of performance.

Not really related to this CPU, games like Total War: Rome II run better if limited to 4 CPUs https://www.protondb.com/app/214950?device=pc.

Then Kingdom Come Deliverance I, which is just absolutely terribly optimized, was a true test. I was hoping they would patch it with the release of the sequel, which is amazingly optimized, but I guess the engines are just too many versions apart. With 5800x3d, I had to split recording for 2 CPU cores (4 threads) and the rest for the game, for the game to run ok, but recording was still choppy. With the same 25% split on 9950x3d unsupringly, the recording was fine, and the game finally ran smoothly.

I could have used GPU encoding, but since my GPU usage was already at 99% I used CPU encoding; CPU was barely at 33% usage anyway.

Last thing to note is that with taskset and CPU, e.g. 0-4, you should check

lstpo -p

(hwloc package) where you can see two dies, where the first die is the one with 3d v cache.
But if you want to task select all cores and threads from the first die, it's not cores from 0-15, but rather 0-7 and 17-24. But I rarely do that, since I usually rather rely on the amd_x3d_mode set to cache described above.

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IRC: pulec

squidie

Member

Registered: 2025-04-04

Posts: 9

Re: Automatic allocation on Ryzen 7 {7,9}800X3D vs Ryzen 9 {7,9}950X3D

How is the CPU now plus one year later?

Pulec

Member

Registered: 2014-07-06

Posts: 24

Re: Automatic allocation on Ryzen 7 {7,9}800X3D vs Ryzen 9 {7,9}950X3D

How is the CPU now plus one year later?

All good, using 105W TDP lock to keep the temps below 70 °C, no issues with the updated ASRock X870 Taichi.

I haven't tested the performance difference between 105W TDP lock and full unlock, AFAIK from reviews, the game performance is almost the same. I mostly run ffmpeg -c:v libx265 on my GoPro and Dashcam videos, and performance is good enough. It beats having Coolermaster MasterAir MA824 Stealth going too loud and making the room too hot.

Honestly, PC paired with silly big XFX 9070XT and no FPS limit, trying to reach 150+fps in modern games, the GPU is going near 100% as it should, the whole PC gets a bit silly hot, raising the room temperature a few °C even with decent air circulation.
So I am thinking about getting an optical DisplayPort and USB hub and moving the PC away to a ventilated "loud room".

But when not gaming, it is great so far, though I might have been happy with 9800X3D as well.

I am not doing any micromanagement with which cores should run for which process.

If you want any tests done, hit me on IRC.

Last edited by Pulec (2026-04-14 11:10:51)

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IRC: pulec