PC Graphics

[phxrider]

Is there a way to measure PCIe bus utilization with an AMD GPU?

I need to see if a performance issue with a game is bus related or CPU limited. (it's on a mini-PC with Oculink).

[FunkZ]

I would not suspect the PCIe bus unless it's restricted by number of lanes or an older generation.

Check out Intel PresentMon - it will show you which component is affecting the frametime and latency.

[phxrider]

As I said, it's on oculink, so it's a possibility. Oculink is an external connector to a PCIe 4 x4 bus. It's not in an x16 slot.

[FunkZ]

What card are you running on a PCIe 4.0 x4 bus?

Obviously if it's something like a 6500XT it's made to run on that.

Otherwise it's restricted to less than 8GB/s.

[phxrider]

7900xt.

I'm actually thinking it's CPU limited, it's running on a 7840HS CPU, which looks to be a lower power version of the 8700G from all the specs. I knew something was a bottleneck when I swapped out the 6800xt I had in my Oculink enclosure to the 7900xt and the Far Cry 6 benchmark (and gameplay) showed no difference between the GPUs. For the record, it's not like this with every game - FC6 is well known for being CPU limited. Starfield got a huge uplift (which also points to CPU as Starfield doesn't care much about CPU in benchmarks, until you get to the really old ones. And looking at AMD APU benchmarks with discrete GPUs, they don't do as well as the 7000 series, likely due to having half the cache.

Another clue:
1440p:

4K:

There's effectively no difference (actually the 4K run had better avg and low by a couple FPS, but that's within the margin of different runs), but the GPU runs close to 99% at 4K and only around 70% at 1440. 

[FunkZ]

For comparison, my 7900XT with 5700X @ 1440

 

[phxrider]

DXR on or off? I have all DXR options on. Also, I'm assuming no FSR with a 7900xt?

I may just build a small form factor PC and put the 7900xt in that, since it wouldn't be much bigger than the eGPU enclosure anyway. I have a 7800x3d and memory already. Just a matter of cost justification - this is not my primary PC and I'd still end up sinking $500.

[FunkZ]

Yes, all quality settings maxxed out, FSR is off. I noticed my VRAM usage was lower than yours, turns out I didn't have the HD texture pack installed. Strangely my FPS improved after I downloaded and enabled that, VRAM still isn't as high as yours though - your card may be using more VRAM due to the restricted x4 PCIe link? If true that's cool it's smart enough to recognize a slow link and automatically buffer more in memory.

[phxrider]

That's a pretty good boost in average FPS over what I'm getting. Unfortunately, I still can't tell if it's CPU or bus.

I did find a way to make lemonade out of this though - I can crank up the resolution until it becomes GPU limited and get a visual quality and sharpness boost. Since I can render it at 4K (using VSR) and get the same performance as 1440, I can do that and at that setting, it also happens to work like a champ with AFMF 2 to double the FPS. So it's not the worst thing that could happen...

As for the VRAM, IDK for sure, but keep in mind I had been screwing around with seeing how it performs at different resolutions, and it could just be that FC6 just doesn't clean up the memory all that well when you do that.

Also, HD textures don't take effect until you restart the game, so if you just checked the box and moved on, it wasn't actually using them.

[FunkZ]

No it was greyed out, I had to quit the game, go to the Steam store, download the HD pack, restart the game, enable them, exit the game and restart it.

I doubt there's 30FPS difference between a 7840HS and a 5700X. Yes the 7840HS has less cache but it also has a higher clock speed.

[phxrider]

Maybe... But being able to crank the resolution up and maintain the same FPS is a telltale sign of being CPU limited and having plenty of GPU.

Are you able to set the PCIe bus speed in your BIOS? If you can, would you mind running the benchmark at 4x? That would be identical to my setup with Oculink and would pretty conclusively rule bus speed in or out as the culprit if you get the about same FPS.

[FunkZ]

With PCIe restricted to 4.0 x4 there surprisingly isn't much performance hit, at least not at 1440p.

 

[phxrider]

With the 7800x3d, definitely a lot better:

[phxrider]

Thanks for doing that.... But wow, not what I expected... Which leaves the question, what the hell is bottlenecking on my system?

Given that we removed PCI bus speed by making them equal, and I have a generation newer RAM running at 5600, and we have the same GPU, all that's left is some kind of CPU/cache bottleneck. It seems odd though that a 5700x would outperform a Zen 4 chip by ~27 FPS. I'm baffled.

However, this won't matter much in the near future, I decided since I have a 7800x3d and RAM sitting here, I might as well get a cheap ITX MB and case and SSD and build a small system, so I'll put the 7900XT in that and use the mini as a server.

[FunkZ]

Completely agree, given that past PCIe 3.0 CrossFire boards would typically split 1x16x into 2x8x I would anticipate a small but minor drop in performance, but not restricting to 4x! Seems even 8GB/s is enough on a PCIe 4.0 bus.

[phxrider]

That was not a great run - the MB turned out to be bad and I was fighting with it most of the night and just copied the game over, set graphics and ran the benchmark before quitting for the night.... I swapped the MB tonight with the replacement from Amazon (GOD I love their return/replacement service!) and this time everything went smoothly so I could just focus on setup and not trying to figure out why the Wi-Fi wouldn't work after a reboot (probably bad Wi-Fi card which I could swap myself, but I'm not going to risk voiding the warranty on a brand-new board)....

Anyway, there are a few steps I forgot, like turning off AFMF which is enabled by default, and I updated to the patch that came out that boosts AMD performance.

This was a fun little project, my first mini-ITX build, I went with the Asus B650E-I board and the Sapphire 7900XT GPU from the Oculink case. The mini-PC will be getting repurposed to be a server. I will also say I wouldn't have done this when gaming was perfectly acceptable on the mini (80 FPS is not horrible), but I had a 7800X3D and RAM sitting here begging to be used for something, so........