[SOLVED] PCI Express Lanes Question

[ManOfArc]

Hypothetical...
Assuming a sufficient CPU and other components, I was wondering at what point on the graphic card hierarchy we would find a card that can saturate the PCIe 3.0 buss @ x8 lanes? Would, say a RTX 2070 or maybe an RX 5700 XT, saturate the PCIe buss if stuck at only x8 lanes?
If I'm asking too naive a question, it's because I'm still learning.

 

[Phaaze88]

There's already 2 cards that actually do this at Gen 3 x8 that I know of - the performance hit is really minor, but...
FYI, the 2 cards I know of are the 2080Ti and 3090. The 3080 and 6900XT probably do it too, considering where the other 2 cards stand.

 

[hotaru.hino]

As long as the graphics card's VRAM is not full, then even an RTX 3080 suffers maybe 10% performance loss on average. I don't know if you can call this "saturated" though since the tests I've seen don't report PCIe bus activity which is exposed for monitoring tools to pick up.

However, if you fill the VRAM, then we can look at the Radeon RX 5500, which is an 8 lane card. If you use a PCIe 3.0 bus instead of the 4.0 bus it was designed for, performance starts to tank once you fill its VRAM.

 

[InvalidError]

I agree with hino, PCIe bandwidth is not a significant problem until the GPU runs out of VRAM. 4GB on 3.0x8 is starting to hurt quite bad at the low end.

 

[ManOfArc]

Thanks for the replies. I guess it involves more than just PCIe lanes, then huh?

I in fact have an RX 5500 XT with 8GB GDDR6. When I later realized I had bought an 8-lane card, I was a bit dismayed. I began wondering why AMD would cripple its lane-width while still giving it 8GB VRAM.

Case in point: Its big brother, the RX-5600 XT @ 16 lanes only has 6GB VRAM. Something feels off with that arrangement to me. Seems the 5600 should have had 8GB and the 5500 6GB. I was trying to figure out the logic. Something to do with the fact that the 5500 has a 128 bit wide memory buss while the 5600 is 192 bit?

 

[InvalidError]

Case in point: Its big brother, the RX-5600 XT @ 16 lanes only has 6GB VRAM. Something feels off with that arrangement to me. Seems the 5600 should have had 8GB and the 5500 6GB. I was trying to figure out the logic. Something to do with the fact that the 5500 has a 128 bit wide memory buss while the 5600 is 192 bit?

Yes. Memory interface width pretty much dictates memory size increments since GDDR5/6 is one chip per channel and memory chips scale in powers of two. If you have 4x32bits channels (128bits), you get the choices of 2GB, 4GB or 8GB depending on whether you pick 4Gbits, 8Gbits or 16Gbits GDDR6 chips. Got 10x32bits channels (320bits), your options are 5GB, 10GB or 20GB.

 

[hotaru.hino]

Thanks for the replies. I guess it involves more than just PCIe lanes, then huh?

I in fact have an RX 5500 XT with 8GB GDDR6. When I later realized I had bought an 8-lane card, I was a bit dismayed. I began wondering why AMD would cripple its lane-width while still giving it 8GB VRAM.

Case in point: Its big brother, the RX-5600 XT @ 16 lanes only has 6GB VRAM. Something feels off with that arrangement to me. Seems the 5600 should have had 8GB and the 5500 6GB. I was trying to figure out the logic. Something to do with the fact that the 5500 has a 128 bit wide memory buss while the 5600 is 192 bit?

Memory bus size does not dictate how many PCIe lanes the card has. There are plenty of cards with 128 or even 96 bit wide buses using 16 lanes.

Only AMD will ever know. It may have been a cost cutting measure. It may have been a dumb way to promote PCIe 4.0. Either way, they were caught with their pants down.

 

[Zerk2012]

Hypothetical...
Assuming a sufficient CPU and other components, I was wondering at what point on the graphic card hierarchy we would find a card that can saturate the PCIe 3.0 buss @ x8 lanes? Would, say a RTX 2070 or maybe an RX 5700 XT, saturate the PCIe buss if stuck at only x8 lanes?
If I'm asking too naive a question, it's because I'm still learning.

Neither of the cards you listed the 2080ti hit the boarder line with a bit of differeance.
https://tpucdn.com/review/nvidia-ge...ing/images/relative-performance_2560-1440.png

https://www.techpowerup.com/review/nvidia-geforce-rtx-2080-ti-pci-express-scaling/6.html

 

[ManOfArc]

Neither of the cards you listed the 2080ti hit the boarder line with a bit of differeance.
https://tpucdn.com/review/nvidia-ge...ing/images/relative-performance_2560-1440.png

https://www.techpowerup.com/review/nvidia-geforce-rtx-2080-ti-pci-express-scaling/6.html

Thank you. That was eye-opening. I am definitely bookmarking that site for future reference.

 

[Zerk2012]

Thank you. That was eye-opening. I am definitely bookmarking that site for future reference.

Yes people way overthink this for some reason. Just for a extra reference a 2.0 X16 is a bit faster than 3.0 X8
In general if your not using a top end card no biggie I would put the new RTX 3070 just below the 2080ti so if your not buying a RTX 3080 then the difference would be so small their no big deal. Even then not really enough to stress over. But not sure I would spend the extra for a 3080 and not be able to get the performance from it that would be like wasting money.

When you get into 2 or 3% difference based at 100 FPS your going from 100 FPS to 98 FPS or 100 FPS to 102 FPS nothing that is game stopping.

EDIT the 1% low FPS could be a bit more I have not researched that.

 

[InvalidError]

In general if your not using a top end card no biggie I would put the new RTX 3070 just below the 2080ti so if your not buying a RTX 3080 then the difference would be so small their no big deal.

If you are using a 4GB RX5500 though, 3.0x8 vs 4.0x8 can give you 50+% higher performance, which is kind of significant when the 4GB and 8GB models are ~$300 apart in retail prices for people who are desperate enough to buy a GPU today.

 

[ManOfArc]

If you are using a 4GB RX5500 though, 3.0x8 vs 4.0x8 can give you 50+% higher performance, which is kind of significant when the 4GB and 8GB models are ~$300 apart in retail prices for people who are desperate enough to buy a GPU today.

What if that RX 5500 XT were an 8GB card instead?

 

[InvalidError]

What if that RX 5500 XT were an 8GB card instead?

The difference with the 8GB model on 4.0 vs 3.0 is negligible, it is only the 4GB model that shows a massive benefit almost closing the gaps with the 8GB model.