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I have an RTX 4090 and the ASUS Z90 Maximus Hero (Users Manual). I need to install it into the second PCIe 5.0 slot, but the manual shows that the 1st (PCIEX16(G5)_1) and 2nd (PCIEX16(G5)_2) slot are not equivalent, which I didn't know before, because ASUS stated something different on their website:

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However, the manual shows:

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This is indeed confusing, because they also state:

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I therefore assumed that if I leave the other PCIe 5.0 slot untouched, the other will get x16. However, I can see that the 2nd slot does not have as many contacts as the 1st slot has, so I assume that the 2nd slot is never able to offer more than x8.

Will an RTX 4090 run fine on a PCIe 5.0 x8 slot?

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PCIe generations generally double the speed of the slot over the previous generation. What that means for an end user is that a PCIe 5 x8 slot is roughly equivalent to a PCIe 4 x16 slot so you can assume that if the card worked well on a previous generation board then it should work just as well on a newer board with fewer lanes.

One of the main features of PCIe is also that the number of lanes is established and negotiated at startup so if a port only has x8 available, then both sides will go "oh, I only have 8 lanes available" and will carry on regardless, simply using less lanes that it originally expected.

At worst I might expect to see a difference in FPS measured in single digits, if you could measure any difference at all that wasn't attributable to simple measurement error.

From Wikipedia the transfer rates are as follows.

Version Transfer rate per lane x1 x2 x4 x8 x16
1.0 2.5 GT/s 0.250 GB/s 0.500 GB/s 1.000 GB/s 2.000 GB/s 4.000 GB/s
2.0 5.0 GT/s 0.500 GB/s 1.000 GB/s 2.000 GB/s 4.000 GB/s 8.000 GB/s
3.0 8.0 GT/s 0.985 GB/s 1.969 GB/s 3.938 GB/s 7.877 GB/s 15.754 GB/s
4.0 16.0 GT/s 1.969 GB/s 3.938 GB/s 7.877 GB/s 15.754 GB/s 31.508 GB/s
5.0 32.0 GT/s 3.938 GB/s 7.877 GB/s 15.754 GB/s 31.508 GB/s 63.015 GB/s

So instead of 63GB/s transfer rate you should get 31.5GB/s.

It absolutely should work at x8 and should work well but you will have a barely perceptible time difference in uploading texture and geometry data.

To fully fill a 24GB 4090s memory would take approximately 0.75 seconds instead of 0.425 seconds.

That a somewhat unrealistic workload for gaming and assumes that all the data is held in main RAM, in practice you will be bottlenecked by your storage device as the fastest NVMe drive is somewhere around 7.5GB/s.

For scientific workloads where you are processing data on the card and copying to RAM then you may well notice a difference, albeit very slight.

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  • You say, PCIe 5.0 x8 is roughly equivalent to a PCIe 4.0 x16 slot. But is that true for an RTX 4090 which uses PCIe 4.0? I believe it can only use PCIe 4.0 x8 then, as it seems not to be backwards compatible. Mar 18, 2023 at 12:40
  • @Tintenfisch PCIe has always been backwards compatible. If the 4090 is only PCIe 4 then you will get the gen 4 x8 speed which is still a reasonable 15GB/s
    – Mokubai
    Mar 18, 2023 at 14:01
  • @Tintenfisch PCIe version will make nearly no difference in most games: youtube.com/watch?v=Q9DApbrhDDw The fact that PCIe 3 is at worst only a few FPS lower and most of the time is basically the same shows how little difference there is in actual gameplay.
    – Mokubai
    Mar 18, 2023 at 17:18
  • That is really impressive, didn't expect that. But it's great! Then I am not forced to use the 1st slot. Mar 18, 2023 at 17:22
  • @Tintenfisch found another slightly more relevant (4090) post that actually does more explanation and benchmarking and shows basically no difference. youtube.com/watch?v=v2SuyiHs-O4 so yeah you might see a tiny and imperceptible difference in load times, but most things during actual gameplay the difference will not be much. While you should prefer the first slot, in most gaming workloads you're probably fine whichever slot you use. That guy does a lot more interesting explanation as well.
    – Mokubai
    Mar 18, 2023 at 17:29
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Two answers for the question: Will an RTX 4090 run fine on a PCIe 5.0 x8 slot?

Yes, but depending on what you will be using it for.

For most users, like gaming, it would be difficult to see a difference in performance, maybe just a single digits FPS drop. If you will be doing very demanding computations [like for science] that would require the FULL utilization of your graphics card, then your graphics card will be hitting a serious bottleneck by the x8 lanes. Let me explain.

The other two answers miss one important fundamental technicality. Since the 4090 is based on PCIe v4, it will not be able to utilize PCIe v5 speeds. So if your PCIe v5 x16 slot [max of 64GB/s] is downgraded to x8 lanes [max of 32GB/s] then the graphics card will be running at PCIe 4 x8 speeds [max of 16GB/s] (not PCIe 4 x16 speeds 32GB/s).

Here is one source that goes more in depth on the topic. https://www.youtube.com/watch?v=v2SuyiHs-O4

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I therefore assumed that if I leave the other PCIe 5.0 slot untouched, the other will get x16. However, I can see that the 2nd slot does not have as many contacts as the 1st slot has, so I assume that the 2nd slot is never able to offer more than x8.

You are correct.

PCIEX16(G5)_2 is at most PCIe 5.0 x8.

PCIEX16(G5)_1 is at most PCIe 5.0 x16.

Both together have max x16 support. So it can be either x16/x0 or x8/x8.

Will an RTX 4090 run fine on a PCIe 5.0 x8 slot?

Yes but it will run at half the bandwidth of PCIe 5.0 x8. This is not a problem since 4090 doesn't utilize the full bandwidth anyway. In fact, 4090 doesn't work with Gen 5 directly. It will use PCIe 4.0 on PCIe 5.0 slot as PCIe are back-compatible.

Note: PCIe 5.0 is double of PCIe 4.0. So PCIe 5.0 x8 is same as PCIe 4.0 x16.

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  • So should the author just use the PCIe 4.0 slot and save their PCIe 5 slot for something else?
    – Ramhound
    Mar 18, 2023 at 7:22
  • Typically, it's rare to have something that needs more bandwidth. Regardless, why not just use the first(x16) port for 4090 and later switch if and when you have something else that needs more bandwidth.
    – Anutrix
    Mar 18, 2023 at 10:02
  • @Ramhound: The mainboard only has PCIe 5.0 slots. Mar 18, 2023 at 12:38
  • @Tintenfisch - Your screenshot and the manual says otherwise
    – Ramhound
    Mar 18, 2023 at 15:18
  • Sorry, yes! I referred the the 1st and 2nd slot only. About the 3rd I didn't think about because I'll probably never use it. Mar 18, 2023 at 15:46

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