4090 Performance in 8x PCIe Slot

FRZ

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I installed an EK Waterblock on my 4090, but the active backplate prevents it from fitting into the 16x PCIe slot on my EVGA Z690.

It's kind of a bummer having a 4090 and being forced to use an 8x slot, but I was curious how much performance, if any, I am losing compared to if I was able to use the top slot.

My CPU is a 13900k
 
it will be fine, you wont even miss the 2-4fps.

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There are a lot of additional variables at play here.

The 4090 uses PCIe 4.0. 8x PCIe 4.0 has the same bandwidth as 16x PCIe 3.0. So as long as the card is using PCIe 4.0, you will be okay, but there is still more to it than that.

The 8x slot - do you know if it is 8x lanes from the CPU or 8x lanes going through the chipset? See this for a reference on where the PCIe lanes might come from: https://www.tweaktown.com/news/8154...6-pcie-5-0-lanes-for-next-gen-gpus/index.html

Lanes directly from the CPU would be ideal. If it's lanes from the chipset, they might be PCIe 4.0 or PCIe 3.0. 8x PCIe 3.0 lanes via the chipset would be a worst-case scenario among the potential possibilities. There is even a chance that the slot might only be 4 lanes, even if it's a physical 16x/8x slot.

You could check your motherboard manual, or just install the 4090 into the 8x slot and then fire-up GPU-Z, which should tell you how many lanes you are using and what version of PCIe they are.
 
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What is this sorcery?
youve never seen a riser cable? its just and extender for the slot. cool kids are using it to vert mount their gpus. dont think it would work in your case without modding though...
 
What is this sorcery?
Move your card anywhere you want, that is if it can fit. Some already used this cable in the reviews on Amazon with a 4090 besides other pcie4 16x cards.
 
But beware. Using a PCIe extension cable WILL cause additional latency.
Electrons do indeed have to physically move through the medium, which takes time.
Luckily, not much time, but it still does take time.
 
But beware. Using a PCIe extension cable WILL cause additional latency.
Electrons do indeed have to physically move through the medium, which takes time.
Luckily, not much time, but it still does take time.
Electrical signals propagate through copper wire at a rate of about 30cm/ns. Given that all these are less than 30cm, it is less than 1ns of latency. If you think you'll notice 1ns of latency, well ok...
 
Electrical signals propagate through copper wire at a rate of about 30cm/ns. Given that all these are less than 30cm, it is less than 1ns of latency. If you think you'll notice 1ns of latency, well ok...

Ha, I don't at all expect to notice it, but it is still technically there and must be accounted for, considering how different bottlenecks can add up.

When dealing with these expensive cards, we have to take into account the fact that OP has gained additional cooling by using a watercooler, but if OP is forced to move the card to a slower slot or use an extension, then what was the point of the fancy cooling if the card is given additional weaknesses?

Not that an extension cable is a bad idea. 1 or 2 nanosecs of latency isn't as bad as 8x PCIe, so in the end, it's not as if buying the cable is out of the question. Just best to know that unless you test and measure both with the cable, and without, in a full x16 PCIe slot, you won't really know the actual real-world difference, which might be nearly nothing, or perhaps the extra nanoseconds in regard to input and output cause more difference than just the absolute speed-of-electrons-in-a-wire latency.

And all this hinges greatly on how expensive the card was, right? The idea that some cheapo GPU has a little more latency isn't as fundamentally painful as feeling like your brand new super expensive RTX 4090 is somehow being hindered. When big money is on the table, I think the stakes change and each nanosecond becomes worth more than mere pennies.
 
There are a lot of additional variables at play here.

The 4090 uses PCIe 4.0. 8x PCIe 4.0 has the same bandwidth as 16x PCIe 3.0. So as long as the card is using PCIe 4.0, you will be okay, but there is still more to it than that.

The 8x slot - do you know if it is 8x lanes from the CPU or 8x lanes going through the chipset? See this for a reference on where the PCIe lanes might come from: https://www.tweaktown.com/news/8154...6-pcie-5-0-lanes-for-next-gen-gpus/index.html

Lanes directly from the CPU would be ideal. If it's lanes from the chipset, they might be PCIe 4.0 or PCIe 3.0. 8x PCIe 3.0 lanes via the chipset would be a worst-case scenario among the potential possibilities. There is even a chance that the slot might only be 4 lanes, even if it's a physical 16x/8x slot.

You could check your motherboard manual, or just install the 4090 into the 8x slot and then fire-up GPU-Z, which should tell you how many lanes you are using and what version of PCIe they are.
His mobo only has 2 pcie 5.0 slots, both physically fit x16. So if he is using x8, it just means the second slot and 8 lanes. PCIE 5.0 so those are twice as fast as pcie 4.0, but I doubt the 8 pcie 5 lanes run at the full speed, but at least he is getting full pcie 4 speed from 8 lanes. So equivalent to x16 PCIe 3.0.



Watch that and your performance would be at the PCIe Gen 3 results, so you can see for yourself the hit it is taking when running PCIe Gen4 x8.
 
You can also just not use the ABP, the RAM is all on one side of the board so 4090's don't really benefit much if at all from ABPs.
 
Cut off the plastic / metal from motherboard cooling nonsense to fit the card. Or buy a new motherboard. Or remove the useless active backplate.
4090 shouldn't be contained so disrespectfully in an X8 slot. Oh the horror!
 
The 4090 only supports PCIe 4.0 so anything having to do with PCIe 5.0 is 100% irrelevant.
If you watched the video, you would know that pcie gen5 and pcie gen 4 results were identical. However, PCIe Gen 3 results were included.

PCIe Gen 4 8 lanes (this is the situation in question) = PCIe Gen3 16 lanes.
So he can compare PCIe Gen 4 to PCIe Gen3 and see exactly what impact it will have. Plus, Mobo being Gen4 or higher, means the lanes, however many there are, run at full Gen 4 speed. This is all relevant information.
 
As others have said, if it really bothers you, try a riser.

That said you are unlikely to notice much of a difference, if any difference at all, as long as you are on a Gen4 system. 8x Gen 4 should be fine. Any framerate difference will likely be in the region of a statistical anomaly.

The bus speed isnt hugely important for framerate. It's mostly used for transferring data to VRAM during level loads and things like that. You may have slightly longer level load times, or get an occasional stutter in a large open world title when it tries to transfer models and textures from a new region to GPU ram while rendering, but you probably wont.

Back in 2010 I built a "DIY Vidock", essentially, an external GPU using an expresscard to PCIe adapter and external power supply.

So I was essentially running a Radeon 6850, a mid range Gen2 16x video card at Gen1 1x speeds.

And guess what? It ran just fine, with only minor performance loss.
Now, a lot of time has passed since then, and the 4090 is hardly a mid range GPU, but you are MUCH closer to max PCIe bus speeds running a Gen4 16x 4090 at gen4 8x, than I was running a Gen2 16x GPU at Gen1 1x.

While I still try to get 16x current gen PCIe speeds for the GPU whenever I can, I really only do that out of a "peace of mind" perspective. I think you'll be perfectly fine at 8x, and won't notice a difference.

If you want, I'd be happy to compare 1:1 benchmarks to see if you are missing anything. (though we probably don't have the same CPU)
 
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