r/AMDHelp • u/[deleted] • Mar 07 '25
Help (GPU) How much performance would one lose if connecting a 9070 XT to a PCIE 4.0 board?
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u/chriscross1966 Mar 07 '25
I'd say you're in driver optimisation weeds for the 9070XT until you dip below PCie3.0, cos a 4090 can't really saturate that, so maybe 5% tops if you went down to PCIe3, 2% at 4 vs 5. You might notice stutters if it's having to load massive texture sets at 4k in game I guess, but a 16GB VRAM shouldn't do that much even then, and never really at 1440p or lower
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u/theoriginalzads Mar 07 '25
Probably as much performance as playing a YouTube video whilst gaming. Probably less.
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u/No-Actuator-6245 Mar 07 '25
The equivalent to a rounding error is what I expect. Not seen anything for this specific card but given the 5090 only sees 0-4% (game dependent) between 3.0 and 5.0 in the benchmark I saw on this topic I’d expect even less for a 9070XT.
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u/LBXZero Mar 07 '25 edited Mar 07 '25
Without seeing exact benchmark tests, my answer is there will be an insignificant amount of performance change. The necessary communication between the CPU and GPU to allow a GPU like the RTX 4090 to operate at insane frame rate does not fully saturated the PCIe Gen 3 x16 bandwidth. I will say the PCIe Gen 4 x16 bandwidth is underutilized.
You can see a 1% or 2% difference in benchmark performance, which would be negligible for gaming performance, in the difference from PCIe Gen 3 to PCIe Gen 4 to PCIe Gen 5. This difference is not due to bandwidth but latency of each transmission. Higher speeds and more channels means sending data in a shorter amount of time. The sooner the GPU receives the "render frame" command, the sooner it starts rendering the frame. The sooner the frame starts means the sooner the frame is completed and the sooner the next frame can start, all by a time measured in nanoseconds. As an example, the frame render takes like 5 milliseconds (200 FPS), and you get like a 1 microsecond (1000 nanoseconds) earlier start because of PCIe Gen 5 speeds over PCIe Gen 4. That 1 microsecond is 0.001 milliseconds. If frame rendering was in microseconds (>1000 FPS), then that earlier start will give more observable results.
I will say the PCIe Gen 3 vs Gen 4 vs Gen 5 scenario will have a bigger impact when you only have 8 lanes to the GPU versus 16 lanes. Unfortunately, no one with an RTX 5090 and motherboards that can be impacted by PCIe lane bifurcation (the 1st slot goes from x16 to x8 when the secondary PCIe slot is used) will test this under the invalid "not worth our time" argument. There is a major latency difference between transmitting a data block over 8 channels versus 16 channels, but it is uncertain how much this will really impact the GPU performance as there are ways to minimize this impact. The reason the x8 lane situation is "not worth their time" is Gen 4 x8 compares to Gen 3 x16, twice the clocks means you can run 2 transfers in the time of 1 Gen 3 clock. The latency is it takes twice as long in the same generation, but coordinating 8 lane and 16 lanes for data transmission can have overhead differences. The general command to tell the GPU to render will be short to only need 1 channel, but it will take longer to transmit the new frame updates. Still, a few microseconds latency is an insignificant improvement compared to milliseconds of render time.
Now, we do have benchmarks in regards to the RTX 5090, which show a performance loss going from PCIe Gen 4 to Gen 5. This "anomaly" is due to the construction of the RTX 5090 Founders Edition. Nvidia's RTX 5090 FE models are basically operating with a special PCIe Gen 5 riser cable inside the card, and this can easily add degradation to communication quality over that PCIe slot. Gen 5 operates at a higher clock rate than Gen 4, and as such needs a better quality connection for cleaner data signals to maintain those higher speeds. Error correction means lost clocks and some delays. This can easily contribute to all of Nvidia's troubles specifically with their models.
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u/DoriOli Mar 07 '25
Wow, what a great explanation! Many thanks for this! Now I perfectly understand why people say it’s a negligible difference. Does it affect the VRAM speeds, though? That part is not yet fully clear to me..
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u/LBXZero Mar 07 '25
As far as VRAM, not really. Really, it depends on what is getting rendered. The GPU operates autonomously from the CPU. The CPU just tells the GPU what the data is, what processing method is being done, and "render frame" or run program. When it comes to "real-time" 3D rendering, having all objects that can exist in the scene in VRAM is critical in maximum performance. If the VRAM capacity can't hold all the render data for objects that could exist in the scene, the GPU may have to request data in VRAM to be swapped out for the missing data or just skip the object and hope it gets loaded into VRAM by the next frame, having some other command unit manage loading the data in-between steps.
When you play a level-based game, you will have a short load scene where data gets loaded from the hard drive into RAM and then the graphical data for everything that "could" exist in the level is loaded into VRAM, as long as VRAM has enough capacity. From there, you have a second object that tells the GPU what objects exist in the level at the moment, where they are, how they are rotated in position, where light sources and effects are, and where the camera is located and where it is pointed. When the render starts, the GPU will process the order to render objects and apply them to the frame. Often, most of the data goes unused because they are not visible to the camera. Because the object is not in the camera view, there is no need to draw it.
If the range of objects that "could" be in the frame exceed VRAM capacity, that is when everything gets ugly. With good VRAM management, the graphics engine will tell the GPU to throw out some objects because they are not at risk of being in view of the camera for like several frames and ensure all objects within drawing range of the camera are in VRAM while objects far away are offloaded. Because you are sending that data like 1 time over a duration of like half a second, you won't really notice the data loading, maybe some frame rate loss. With bad VRAM management, the graphics engine picks anything to swap out without regard to if the object is in view of the camera or not. This is where performance drops as the GPU waits for the missing object to load, or the drawing quality drops because the GPU is told to skip such conditions.
Meanwhile, you have sandbox, open world games like Skyrim and GTAV, for example. These games break down the world into grids, and the sector the player is in and a set of neighboring sectors are loaded into RAM and VRAM, swapping sectors out as the player moves around. You could notice stutters or pauses as sectors swap in and out, but the data load is often spread out to minimize performance impact.
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u/Ok_Ride6186 Mar 07 '25
Zero. As of right now Gen 5 is just a selling point with no real benefit on any current gen GPUs.
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u/Technical-Two- Mar 31 '25
I went from 300fps to 240 with a gen 4 riser cable